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introduction to programming
Chapter
1.
2.
3.

COMPUTER PROGRAMMING FUNDAMENTALS

Computer Fundamentals
Programming Fundamentals
Elementary Programming Techniques
ON-LINE OPERATIONS

9'1"

System Description and Operation
Loading, Editing, and Debugging
ADVANCED PROGRAMMING TECHNIQUES

9.“?

Input/ Output Programming
DECtape Programming
Floating-Point Packages

Eflfifllfifl

'~
.

programming
‘

prepared
by
small systems soFtware

,
-

documentation group

digital equipment corporation

pdp-s handbook series

First Edition, January 1969
Second Printing, July 1969

Second Edition, September 1970
Third Edition, May 1972

Fourth Edition, September 1973
Fifth Edition, April 1975

The

and

availability of the software products de—
scribed in this manual are subject to change without notice. The
availability or performance of some features of the software prod—
ucts may depend on a specific configuration of‘equipment. Consequently, DEC makes no claim and shall not be liable for the
accuracy of the software products. Distribution of software products shall be in accordance with the then standard policy for each
such software product.
description

‘

Copyright © 1970, 1971, 1972, 1973,1975
Digital Equipment Corporation
following are registered trademarks of Digital Equipment
Corporation, Maynard, Massachusetts:
The

DIGITAL

CDP

KAI 0

COMPUTER DNC
EDGRIN
LAB

LAB-8

PS/ 8
QUICKPOINT

‘LAB-S/e

RAD-8
RSTS

COMTEX

EDUSYSTEM

LAB-K

COMSYST

FLIP CHIP

OMNIBUS

RSX

08/8

RTM

DDT

FOCAL

DEC

GLC—S

DECCOMM

IDAC

DECTAPE

IDACS

DIBOL

INDAC

SABR

PDP
7,;

PHA

’I‘XPESET 8.
UNIBUS

Foweord
As few as five years ago, the suggestion that a computer or com-

puter—based system could be readily available to users at all levels
of technical knowledge and ability still evoked surprise and concern among many. To help convey our position that programming
a minicomputer was not a restricted undertaking, we introduced
in January of 1969 our first major handbook dealing specifically
with the fundamentals of machine and assembly language program—
ming on a minicomputer—Jntroduction to Programming. Since

that time, the demand for this handbook by users in every field
and occupation, experienced programmer and novice alike, has

clearly proven the value of such a book.
-

In addition to Introduction 1‘0 Programming, we include several

other volumes in our PDP—S handbook series. The Small Com—
puter Handbook provides extensiVe technical information concern-

ing hardware options, interfacing, system operation and installation planning; this handbook is invaluable to those who will develop
and maintain a minicomputer installation. The EduSySrem Handbook contains a complete self-instruction course in the use of the
BASIC programming language, plus user guides to each of the
existing EduSystems—systems designed specifically for classroom
use. Finally, the. forthcoming OS/ 8 Handbook will present comprehensive information dealing with DEC’s complete computer
system for the PDP—S—OS/ 8. OS/ 8 provides the programmer
with an extended library of system programs, including a text
editor, octal debugging program, assemblers, loaders, and FOR-

TRAN IV.

Once again, I wish to thank all programmers, writers, teachers
and students who have contributed to
,

our

handbooks

Through

continue producing extensive low- cost

your support
gramming information for PDP-8 computers.
we can

pro—

MK/%
Kenneth H. Olsen

President,

Digital Equipment Corporation
'

iii

ERROR REPORTING

If you find any

questions

errors

or comments

in this handbook,

if you have

anyfl

concerning the clarity or completeness-of

this handbook, please direct your remarks to:

Digital Equipment Corporation
Software Communications, Parker Street

Maynard, Massachusetts 01754

IntroduCtion to Programming is DEC’s introductory computer
programming handbook. The first five chapters provide a thorough

explanation of computer mathematics and an introdiJction to
machine language and assembly language programming and to
the basics of program loading and error correction (debugging).
The experienced programmer may choose. to skip the first five
chapters and begin .reading the Advanced Programming Techniques
described in Chapters 6, 7, and 8. These chapters describe input/
output (I/O) programming techniques, DECtape programming
techniques, and DEC’s floating-point packages.
BINARY ARITHMETIC
The binary number system is the basic concept behind digital

computers. The.numbers 0 and 1 can easily be substituted for the
two physical states associated with computer hardware: a switch
is either on (1) or ofi (0), an area in the computer’s memory is.
either magnetized (1) or not magnetized (0). In computer pro-

gramming terminolOgy, the binary digits 0 and 1 are sometimes
called bits (binary digits). Binary numbers are the machine language of computers. All computations, no matter how complex,
performed by digital computers are made in binary arithmetic;
therefore, students of computer programming must be thoroughly
familiar with the binary system before they can begin to write pro-

Chapter 1 and part of Chapter 2 provide an excellent
introduction to binary arithmetic.
grams.

ASSEMBLY LANGUAGE PROGRAMMING
Since it is

quite burdensome to write programs in strings of
12-bit binary numbers (called words) an assembly language was
developed. Assembly language enables the programmer to substi—
tute English-like mnemonics for the binary numbers. For example,
V

the mnemonic J MP (for jump) is interpreted by the computer as
the 12-bit binary number 101 000 000 000. The three bits in
each of the four groups

are

given the octal values 4, 2, and 1,.

reading from left to right, so that if all three bits of a group were
set to 1 (111), the octal value would be 7- (4+2+1=7). Thus
the
octal value for JMP is 5000

(101‘ binary =4+0+1 octal

101

000

5).

Assembly language programming is discussed in Chapters 2, 3,
and 5.

PDP-8/ E SYSTEM DESCRIPTION AND USE
The programs presented in this handbook are designed to run
on a PDP-S/E computer. The PDP-8/ E and the
peripheral equipment which comprises the PDP—S/E' system are described in detail
in Chapter 4.
The program commands which cause the PDP—8/E to accept
data from one of its peripherals or to send data to one of its

peripherals are called input/ output commands. Input/ output com—
mands are discussed in Chapter 6.
One of the most important peripheral devices is the DECtapev

unit. The DECtape unit serves as an auxiliary magnetic tape data
storage facility. DECtape is easier to use than ordinary computer

magnetic tape because it allowsthe user to store information atw
fixed positions which may be directly addressed. DECtape pro—,
gramming is presented in Chapter 7.
DEC’s floating-point packages provide for easy performance of

operations such as addition, subtraction, multi-V
plication, and division,- while maintaining a high? degree of precision. Floating-point numbers represent quantities in the form of a
fractional number multiplied by the number base 10 (for decimal)
with an exponent (e.g., 12
.12x102). Floating—point representation is a great help to computer programmers because it allows
them to use and save very large and very small numbers by saving
only the significant digits and computing an exponent to account:
basic arithmetic

trailing zeroes. The floating-point packages are
fully explained in Chapter 8.

‘for leading and

COMMON PROGRAMMING TERMS
Such words as loop, jump, nesting, and array have special mean—
ings to computer programmers. Familiarity with these terms is prerequisite to an understanding of the information presented in this
handbook. The Index/Glossary at the end of the handbook defines
many of the commonly used computer programming terms.

vii

CHAPTER 1
Introduction

COMPUTER FUNDAMENTALS
‘1-1

........................................

The Computer Challenge.

1-1

................................................

Computer Applications
Computer Capabilities and Limitations
Number System Priiner

1-2

......

1—4

..........................

.1-5

....................

Binary Number System
Octal Number System

..,

1—6

.................................................

Arithmetic

Operations

Logic Operation Primer
The AND Operation
The OR Operation

...........

1-29

..........................................

1-29

..... .......

.................................

........................................

....................................

..............................................................

.......

I. .................................

..................................................................

........................................................................

.............................

Computer Data Formats
Characters

....................................

......................................................

....................................................

Number Representations

1-30

‘

.......................

Memory Unit
Input Unit
Output Unit

1-23

_. ................................

General Organiza'tion of the PDP-8

Control Unit

................................

..............................................

The Exclusive OR Operation

Arithmetic Unit

1-18
1-20

...........................................

...........

Alphabetic

1 -15

....................................... .................................

Negative Numbers and Subtraction
Multiplication and Division

1-11

......................................................

Fractions .;

‘

..................................................

1-30
1-31
1-32
1-32

1-3’3

1-33

1-,33
1-34
1-34
1-34

‘

PROGRAMMING FUNDAMENTALS

CHAPTER 2

Program Coding
Binary Coding :.
Octal Coding
Mnemonic Coding

2-2

..................................................................

2-2

................................................................

2-2

....................................................................

2-3

............................................................

‘

PDP-8 Organization and Structure

Input and Output Units
Arithmetic Unit
Control Unit

......

x
'

2-5

........................... ..... ....................

........................................................

2—5

........

2-6
2-7

..................................................................

2-8

..........................................

2-9

................................................................................

TAD

2-9

................................................................................

DCA

2-10

................................................................................

JMP

2-10

................................................................................

2-10

..................................................................................

JMS

2-12

................................................................................

‘

Addressing
PDP-8 Memory Pages
Indirect Addressing

2-13

..........................................................................

2-13

....................................................

................................................

Operate Microinstructions
Group 1 Microinstructions
Group 2 Microinstructions

2-15

2-18

..................................................

Mi'croprogramming

Memory Reference Instructions

182

........................

....................................................................

Memory Unit
AND

2-4

‘

........

2-18

..............................................
‘

......

.......................

2-21

................
,

2-23

..............................................................

Combining “MiCroinstruetions

............. ,.

Illegal Combinations
Combining Skip Microinstructions

“’7—23

.......................

...................................

...'

‘

72—23

...................

2-25
‘

Order of Execution of Combined MicroinstructiOns‘
Exercises

.......... ...................
,

CHAPTER 3

2-26

........
_

2-28

t ..............................................

ELEMENTARY PROGRAMMING
TECHNIQUES
'

‘

Programming Phases

.................................................... ......

‘

3-2
‘

Flowcharting

...........................................................

gm.

.......

3-3

Example 1—Straight—Line Programming
Example 2—eProgram—Branching

3-4

........................
p

3-4

.............................

Coding a Program
Location

3-6

‘

............

................................................

Assignment

................ ...................
.

Symbolic Addresses
Symbolic Programming Conventions

‘....'.

3—6

..............

........................................................

Programming Arithmetic Operations
Arithmetic Overflow

Subtraction

3-10

..................................

3-11.

.........

3-13

....................................... ...........................
.

PoWers of Two

Writing Subroutines

...........

Address Modification

3-13

...........................

3-14

......................................

..........................

3-7

3-8

..............................

...........................................

Multiplication and Division
Double-Precision Arithmetic

“

3-16

.....................................

3-16

................................................

3—l9r_

........................................................

Inserting Comments and Headings
'

Looping a Program

Autoindexing

.................................... .......

3-27

3-29

.......................................................

Program Branching

3-30

....................... .......................................

3-34

.........................................................................

CHAPTER 4

3-24

.............................................................

Program Delays
Exercises

...........

3-22

...... ................................

SYSTEM DESCRIPTION AND OPERATION

Programiner’s Console Operation
Manual Program Loading

.........

4-1

................

4-6

........................................ ............

‘

Keyboard/ Printer Console Devices
Teletype Operation
Teletype Control Knob
Teletype Keyboard
Teletype Printer
Teletype Paper Tape Reader
Teletype Paper Tape Punch
Generating a Symbolic Tape

....................... .............

..............................................................

4-9

4-10
4—11

............................................

..............................

............................

..............................................

............

...........

4-12
4-13
4-13
4-14

.......

........

4—14

Paper Tape Formats
Paper Tape Loader Programs

.............................

4-15

..........................................

4-18

Peripheral Equipment and Options
Unit
High- Speed Paper Tape Reader and
Punch
Extended Memory
DECtape System

4- 18

......................................

A-18

............

......... .....................................

4—20

......

‘

4—21

...................................................

DECdisk Systems
Extended Arithmetic
Exercises

4—23

......................

Element

4-24

........................................

4-24

..............................................................................

CHAPTER 5

Introduction

LOADING, EDITING AND DEBUGGING
5-1

........................................................................

Loaders

‘5-1

................................................................ ................

RIM Loader

5-2

Binary Loader
Self-starting Binary Loader
M18—E Hardware Bootstrap

5—5

..................................................................

5-9

.........

5—9

............................................

Symbolic Editor
Introduction

5-11

..................................................................

5-11

....................................................................

Modes of Operation

Command Structure

5 -12

...............

.........................................

....................................

5-13

..................................................

5-18

Special Characters and Functions
Switch Register Options
Command Repertoire

5-19,“

.........
,

......................... .............................

5-28

................................................................

5—35

Operating Procedures
Error Messages

5—12

........................................................

Example of Use
Summary of Editor Operations

5—35

..............................................................

............

5-38

...........................

............................................................

5-42

..................................

5-42

Debugging Programs
Debugging Without DDT or ODT
Debugging With DDT

5-43

.......................... , .............................
p

Loading DDT
Symbol TableeTapes
Storage Requirements

.................................

;

..................................

................................................

.......

5-43
5-44
5-48

xii

5-48

Definitions
Mode Control .:

Program Examination andModification

Example Program Debugged......-.
Command Summary

5-49

.......

.............................................

5-50

........................

...................................

563»

5-65

.......................................................

Internal Symbol Table

5-67

............................. .............._. ........
‘

‘

Debugging With ODT
Features

5-68

........................................................

5—68

.........................................................................

Using ODT .f
Operation and Storage

5 -69

.................. ..................................... .............

Commands

Additional Techniques

............ ..........

..................

541

CHAPTER 6

5-80A

......................................... .........
.

Programming Notes Summary

Introduction

5—69

..........

......................................................................

Command Summary

5—82

..........................................

5-83

......................

..........................

INPUT / OUTPUT PROGRAMNIING

.................................

6-1

......................................

Programmed Data Transfers

6-2

......................

‘

IOT Instruction Format

Checking Ready Status

6-2

..................................................

6-3

....................................................

Instruction Uses

6-3

................................................ ..............
'

ASCII Code

6-4

...................................

Programming the Teletype Unit

Keyboard/ Reader Instructions.
Printer/Punch Instructions
Format Routines

‘.

................

6-4

.......................

,6‘-4

........................................

6-6

..............

6-8

................................................

Text Routines

6-8

........................

Numeric Translation Routines

6-11

.....................................

Program Interrupt Facility
Programming an Interrupt
Multiple Device Interrupt Programming
A Software Priority Interrupt System
Multiple Interrupt Demonstration. Program

..................................................

6-22

6-24

..............................................

........................

Data Break

.....

..........................................................................

Accessing Data

................................................................

xiii

6-28
6-30

6-31

6-407
6—40

‘

Data Break

Single-Cycle
Cycle Stealing
3—Cycle Data Break

................................................

..................................................................

Exercises

............................................

..............................................................................

CHAPTER 7
Introduction

DECTAPE

6-40
6—42
642

6-43

PROGRAMMlNG

........................................................................

Data Blocks

..........

7—1
7-1

Data Channels
Standard

.............................. ......................................

7-2
7-2

DECtapes

DECtape Control Unit

........................................................

DECtape Status Registers
Status Register B
Status Register A

‘

..............

......................................

............................................................

DECtape IO’I‘ Instructions

..................................................

Programmed DECtape Operation
Use of the DECtape Flag

........

......

....................................................

7-4
7—4
7-6
7-6

7—9
7-11

7-13

Selecting Direction
A

........................................... ...................

7 —14

..................................................

7 ~14

Reversing Direction

........

Accessing Data Blocks

................. .........

Allocating Storage Areas

....................................................

Programming for Error‘Conditions
Programming for Interrupts
IDTAPE Subroutine

....................................

7—15
7-15
7 -16

.......................................... .. .....

7~16

............................................................

7 -17

, .................................................

7-19

.....................

7—20

........................................................

7—21

DECtape System Software
DECtape Subroutines
‘

.....

DWAIT Subroutine

...........

SEARCH Subroutine‘

.......................................... ............

xiv

7-21

READ and WRITE-Subroutines

....................................

DECtape copy Program
DECtape Formatting Program

..............................

........................................

DECtape Library System
The Directory
Using the Library System
TCOI Bootstrap Loader

..................

....................................................

.............

‘.
‘

.................................. ..............

.........

......................

...................

7-25'
7—26
7-27
7 ~28

.................................................

7-327-33’
7-34

FLOATING-POINT PACKAGES

CHAPTERS

8-1

Introduction

Assembly Instructions

................................. . ......................

Floating-Point Notation
Normalization

......................................................

..................................................................

Number Representation

..................................................

Using the Floating-Point Package
Floating Input and Output

User Subroutines

Floating Switch
Floating Halt

..............

‘

........... ...................................

Use of F182 and Auto-Indexing

....................................

............................................................

................................................................

8-2‘
8-4

8-5
8-6
8-7
8 -17

8-20
8—22

8-26

.......................................................... ........

7-23

7—31

........... .......

TDS-E DECtape Subroutine

Assembly Parameters
Calling Sequence

7—22

Single Instruction vs Interpretive Mode

................................

8-27

8—27

Error Traps

............................

.........................

...... .............

Extended Function Algorithms

‘

............................................

Execution Time for EAE Operations

..............................

ExecutiOn Time for EAE Extended Functions
Execution Time for Non-EAE Operations

................

...................

Accuracy of Extended Functions
Core Storage Maps

....................................

..........................................................

Summary of Floating-Point Instructions
Memory Reference Instructions
Expanded Instructions

............................

........................................

8-31
8-35
8—36
8-36

8—37

8-38
8-40

8-40

’

......................... ..........

’8-29

8-41

APPENDIX A
Answers to Selected Exercises

...................... ........................

A-l

APPENDIX B

Character Codes

B-l

.......................................................... ........
s

‘

APPENDIX C

Flowchart Guide

C-l

.........

APPENDIX D
,

PDP-S Instruction Set

D-l

........................................... ...............

APPENDIX E

Read-In Mode Loader

Binary Loader

E-l

..............................................

......................................................................

E-3

‘

APPENDIX F

Miscellaneous Tables

........, ........................

F—1

APPENDIX G

Digital Equipment Computer Users Society

xvi

........................

G—l

Fndaa-umentls j
computer
Fundamentals

programming
Fundamentals
—__
'

elementary
programming
techniques
_

—

computerFUndamentals
INTRODUCTION

During the past 20 years, the computer revolution has dramatically
changed our world, and it promises to bring about even greater changes
‘

in the years ahead.
The general purpose, digital computers being built today are much

faster, smaller and more reliable and can be produced at lower cost
than the earlier computers. But even more

significant breakthroughs

have come in the many new ways we have learned to use cemputers.
The first big electronic computers were usually employed as

super

calculators to solve complex mathematical problems that had been im-

possible to attack, before.- In recent years, computer programmers
have begun using computers for non-numerical operations, such as
control systems, communications, and data handling and-processing.
In these Operations, the computer system processes vast quantities of
data at high speed.
The Computer Challenge

that a computer can be programmed to do any
problem that can be defined..The key word here is defined, which
It has been said

problem can be broken down into a
series of steps that can be Written as a sequence of cemputer instrucmeans

that the solution of the

tions. The definition of some problems, such as the translation of natu—
ral languages, has turned out to be very difficult. A few years ago it

thought that computer programs Could be written to translate
French into English, for example. As a matter of fact, it is quite easy
to translate a list of French words into English words with similar
meanings. However, it is very diflicult to precisely translate sentences
because of the many shades of meanings associated with individualwas

words and word combinations. For this reason, it “is not practical to
1—1

try to communicate with a computer using a conventional spoken lan'

guage.
While natural languages are impractical for computeruse, program-

ming languages, such as FOCAL, ALGOL, and FORTRAN with their
precisely defined structure and syntax, greatly simplify communication
with a computer. Programming languages are problem oriented"~and
contain familiar words and expressions; thus, by using a programminglanguage, it is possible to learn to. write programs after a relatively
short training period. Since most computer manufacturers have adopted
standard programming languages and implemented the use of these,languages on their computers, a given program can. be executed on a
large number of computers. PDP—8 programmers use FORTRAN and
ALGOL—8 for scientificand engineering problems and use FOCAL—8
and BASIC—8 for shorter numerical calculations. Computer languages
have been developed for programmed control of machine tools, computer typesetting, music composition, data acquisition, and many other
applications. It is likely that there will be many more new programming
languages in the future. Each new language development will enable
the user to more easily apply the power of the computer to his particular problem or task.
Who can be a programmer? In the early days of computer program—
ming, most programmers were mathematicians. However, as this text
illustrates, most programming requires only an elementary ability to
handle arithmetic and logical operations. Perhaps the most basic re—
quirement for programming is the ability to reason logically.
The rapid expansion of the computer field in the last decade has
1

:made the resources of the computer available to hundreds of thousands
of people and has provided many new career opportunities.

Computer Applications
A computer, like any other machine, is used because it does certain
tasks better and more efficiently than humans. Specifically, it can reinformation and process it faster than a human. Often,
this speed means that weeks or months of pencil and paper work can
ceive

method requiring only minutes of computer time.
Therefore, .computers are used when the time saved by using a combe replaced by

puter offsets its cost. Further, because of its capacity to handle large
volumes of data in a very short time, a computer may be the only
means

of resolving problems where time is of the essence. Because of

the advantages of high speed and high capacity, computers are being.
used more and more in business, industry, and research. Most com-

puter applications can be classified as either business uses, which usually.
1-2

rely upon the computer’ s capacity to store and quickly retrieve large

amounts of information, or scientific uses, which require accuracy
and speed in performing mathematical calculations. Both of these are
performed on general purpose computers. Some examples of computer,

applications are given below.

Solving Design Problems. The computer is a very useful calculating
tool for the design engineer. The wing design of a supersonic aircraft,
for example, depends upon many factors. The designer describes each
of these factors in the form of mathematical equations in a program-

ming language. The computer can then be usedto solve these equations.

Scientific and Laboratory Experiments. In scientific and laboratory
experiments, computers are used to evaluate and store information
from numerous types of electronic sensing devices. Computers are particularly useful in" such systems as telemetry where signals must be
quickly recorded or they are lost. These applications require rapid and
accurate processing for both fixed conditions and dynamic situations.
Automatic Processes. The computer is a useful tool for manufacturing'and inspecting products automatically. A compUter may be programmed to run and control milling machines, turret lathes, and many
other machine tools with more rapid and accurate response than is
humanly possible. It can be programmed toinspect a part as it is being
made and adjust the machine tool as needed. If an'incoming part is defective, the computer may be programmed to reject it and start the
next part.

Training by Simulation. It is often expensive, dangerous and impractical to train a large group of men under actual Conditions to fly a
commercial airplane, control a satellite, or operate a space vehicle. A
computer can simulate all of these conditions for a trainee, respond to
his actions, and report the results of the training. The trainee can therefore receive many hours of on-the-job training without risk to himself,

others, or the expensive equipment involved.

Applications, such as those given above often require the processing
of both analog and digital information. Analog information consists
of continuous physical quantities that can be easily generated and controlled, such as electrical voltages or shaft rotations. Digital information, however, consists of discrete numerical values, which represent
the variables of a problem. Normally, analog values are converted to
equivalent digital values for arithmetic calculations to solve problems.
Some computers, such as the LINC-S, combine the analog digital
characteristics in'one computer system.
1-3

Computer Capabilities and Limitations
A computer is a machine and, as all machines, it must be directed

and controlled in order to perform a useful task. Until a program is

prepared and stored in the computer’s core memory, the computer
“knows” absolutely nothing, not even' how to receive input. Thus, no
matter how good a particular computer may be, it must be “told”
what to do. The usefulness of a computer therefore can not be fully
realized until the capabilities (and the limitations) of the computer
are recognized.
Repetitive operation—A computer can perform similar operations
thousands of times, Without beeOming'bored, tired or careless.

Speed—A computer processes information at enormous speeds,
which are directly related to the ingenuity of the designer and the
programmer. Modem computers can solve problems millions of
times faster than a skilled mathematician.
,

Flexibility—General purpose computers may be programmed to
solve many types of problems.

Accuracy—Computers may be programmed to calculate answers
with a desired level of accuracy as specified by the programmer.
Intuition—A computer has no intuition. It can only proceed as it
is directed. A man may suddenly find the answers to a problem
without working out the details, but a computer must proceed as
'

ordered.
The remainder of this chapter is devoted to the general organization
of the computer and the manner in which it handles data. Included are
the number systems used in programming together with the arithmetic
and logical operations of the computer. This information provides a
of com—
desire a basic
background for

necessary
all who
appreciation
'puters and their uses, and it is a prerequisite to machine-language"

programming, coVeredin chapters 2 through 5.

1—4

‘

NUMBER SYSTEM PRIMER
,

The concept of writing numbers, counting, and performing the basic.

operations of addition, subtraction, multiplication, and division has
been directly developed by man. Every person is introduced to these
concepts during his formal education. One of the most important
‘

development was the invention of the decimal»

factors in scientific

numberng system. The system of counting in units of tens probably
developed because man has ten fingers. The use of the number 10 as
the base of our number system is not of prime importance; any standard unit would do as well. The main use of a number system in early
times was measuring quantities and keeping records, not performing
mathematical calculations. As the sciences developed,- old numbering

systems became more and more outdated. The lack of an adequate
numerical system greatly hampered the scientific development of early
‘

‘

civilizations,

Two basic concepts simplified the operations needed to manipulate

numbers; the concept of position, and the numeral zero. The concept"

of position consists of assigning to a number a value which depends.
both

‘

the

symbol and on its position in the whole number. For

example, the digit 5 has a different value in each of the three numbers
135, 152, and 504. In the first number, the digit 5‘has its‘original
value 5; in the second, it has the value 'of 50; and in the last number,
it has the value of 500, Or 5 tirnes 10 times 10. Sometimes a position
in a number does not have avalue between ‘1 and 9. If this position
were simply left out, there would be no difference in notation between

709 and 79. This is where the numeral zero fills the gap. In the number

709, there are 7 hundreds, 0 tens and 9 units. Thus, by using the i
concept of position and the numeral 0, arithmetic becomes quite easy.
A few basic definitions are needed before proceeding to see how .77
these concepts apply to digital computers.
'

Unit—The standard utilized in COunting separate items is the unit.

Quantity—The absolute or physical amount of units.
Number—A number is a symbol used to represent a quantity.

Number System—A number system is

a
means of representing
All
numbers.
modern
number systems use
quantities using a set of

the zero to indicate no units, and other symbols to indicate quan—I

tities. The base or radix Of a numbersystem is the number of symbols it contains, including zero. For example the decimal number.

system is base or radix 10, because it contains 10 different symbols (viz., 0,1,2,3,4,5,6,7,8,‘ and 9).,
I

Binary Number System
The fundamental requirement of a computer is. the ability to physi-

cally represent numbers and to perform operations on the numbers
thus represented. Although computers which are based on other num—

ber systems have been built, modern digital computers are all based
the binary

(base 2) system. To represent ten different numbers
(0,1,2,
9) the computer must possess tenidifierent states with
which to associate a digit value. However, most physical quantities have
only two states: a light bulb is on or off; switches are on or 06; holes
on

‘

in paper tape or cards are punched or not punched; current is positive

‘f or negative; material is magnetized 'or demagnetized; etc. Because it
can be represented by only two such physical states, the binary number

system is used in computers.
To understand the

binary number system upon which the digital
computer operates, an analysis of the concepts underlying the decimal
number system is beneficial.

POSITION COEFFICIENT
In the decimal numbering system (base 10), the value of a numeral

depends noon the numeral’s position in a number, for example:
347

3x-100=300
10:
40
4x
7x
1:
7

327
The value of each position in a number is known as its position coeflicient. It is also called the digit position weighting value, weighting value,
or

weight, for short. A sample decimal weighting table follows:
.

103

102

101

10°

and, as shown above,
347:3 X 102+4 X101+7X100.

Weighting tables appear to serve no useful purpose in our familar deci—
mal numbering system, but their purpose becomes apparent when we
consider the binary or base 2 numbering system. In binary we have
only two digits, 0 and 1. In order to represent the numbersl to 10, we
must utilize a enunt-and—carry principle familar to, us from the decimal
1-6

system (so familiar we are not always aware that we use it).‘To count
from 0 to 10 in decimal, we count as follows:

\OOQONUI-h-WNl—‘O

with a carry to the 101 colum

Continuing the counting, when we reach 0 in the units column again,
carry another 1 to the tens column. This process is continued until
the tens column becomes 0 and a 1 is carried into the hundreds column,

shown below:

12
13

92
93
94
95
96
97

3
4

'15

18
19
20

9
10

one carry

100 two carries

one carry

COUNTING IN BINARY NUMBERS
In the binary number system, the carry principle is used with only
two

digity‘symbols, namely 0 and 1. Thus, the numbers used in the

binary number System. to count up to a decimal value of 10 are the
following.
‘

Binary

Decimal

(0)
(1)
(2)

10
11

(3)

100

(4)
(5)

101

Decimal

Binary
.

110
111.
1000

1001
1010

When using more than one number system, it is

customary to subscript

numbers with the applicable base (e.g., 1012:510)
1-7

(6)
(7)
(8)
(9)
(10)

”

used to convert binary numbers to the

A weighting table is
familiar decimal system
24 2‘3 22 21 20
1

l O l

(Weight Table)
(Binary Number)
Digit

Position
Coefficient

Decimal Number .—: 21

It should be obvious that the binary weighting table can be extended,

like the decimal table, as far as desired. In general, to find the value
of a binary number, multiply each digit by its position coeflicient and
then add all of the products.

ARRANGEMENTS OF VALUES

By convention, weighting values are always arranged in the same
manner; the highest on the extreme left and the lowest on the extreme
right. Therefore, the position coefficient begins at l and increases from
right to left. This convention has two very practical advantages. The
first advantage is that it allows the elimination of the weighting table,
as such. It is not necessary to label each
binary number with weighting
values, as the digit on the extreme right is always multiplied by l, the
digit to its left is always multiplied by 2, the next by 4, etc. The second
advantage is the elimination of some of the Os. Whether a 0 is to the
right or left, it will never add to the value of the binary number. Some
Os are required, however, as any US to the right of the highest valued
l are utilized as spaces or place keepers, to keep the ls in their correct
positions. The 0s to the left, how ever, provide no information about
0001010111:
be discarded, thus
the number and
the
may
number
1010111
The PDP-8 family computers operate upon 12-bit (binary digit)
numbers. This

means

that the

numbers from 0 to 1111111111112

(409510) can be directly represented.
SIGNIFICANT DIGITS
The “leftmost” 1 in a binary number is called the most Significant
digit. This is abbreviated MSD. It is called the “most significant” in
that it is multiplied by the highest position coefficient. The least sig-

nificant digit; or_ LSD, is the extreme right digit. It may be a l or 0,
and has the lowest weighting value, namely 1. The terms LSD and
1—8

MSD have the same meaning in the decimal system as in the binary sys-

tem, as shown below.
1

MSD

1 0

Ulb—dl-d OBQLSD

0 0

0 1

9 7,1

CONVERSION OF DECIMAL TO BINARY
There are two commonly used methods for

converting decimal

num-.

bers to binary equivalents. The reader may choose whichever method
he finds easier to use.

1. Subtraction of Powers Method—To convert any decimal number
to its binary equivalent by the subtraction of powers method, proceed
'

as follows.

Subtract the highest possible power of two from the decimal number,
and place a “l” in the appropriate weighting position of the partially

completed binary number. Continue this procedure until the decimal
number is reduced to 0. If, after the first subtraction, the next lower
power of 2 cannot be subtracted, place a 0 in the appropriate weighting position. Example:
'42,,

——32

*—

binary

—2

Power

2O
_

32
1

Value

Binary

Therefore, 4210

2. Division

1010102.

Method—To convert a decimal number to binary by

the division method, proceed as follows.
Divide the decimal number by 2. If there is a remainder, put a 1 in the
LSD of the partially formed binary number; if there is no remainder,

put a 0 in the LSD of the binary number. Divide the quotient from the
first division by 2, and repeat the process. If there is a remainder,

1—9

record

1; if there is no remainder, record a 0. Continue until the

quotient has been reduced to 0. Example:
4710 = ?Binary

Quotient

,
.

Remainder

EIJZJE’IEQ

Therefore, 4710

.23

1011112.

EXERCISES

Decimal—to-Binary Conversion
numbers to their binary equivalents.
a.

l 510

11.
12.

1 8m

p—a

P.“>‘.°‘E-":‘>.WE\’T‘

4210
1 00m
2 3 5m

14.

15.

1 10

16.

29410
1 1 7m
8 610
409010

17.
18.

'19.
20.

1967”

Binary to Decimal Conversion
numbers to their decimal equivalents.
b.

9°9SAPE’N‘N“

1102
1012
11101102
10111102
01101102
111112
10102
1101112

9.
10.
11.
V

12.
13.
14.
15.
16.

Convert the following decimal

4095m
1502m
377m
501m
828m
907m
4000m
3456.0
2273”

13.
'

——

—-

Convert the following binary

110110111012
1110001110012
1110101101002
1111111101112
1010110101012
1111112
0001010012

1111111111112
1-10

‘

Octal Number

System

probably quite evident at this time that the binary number
system, although quite nice for computers, is a little cumbersome for
human usage. It is very easy for humans to make errors in reading and
writing quantities of large binary numbers. The octal or base 8 numbering system helps to alleviate this problem. The base 8 or octal number system utilizes the digits 0 through 7. in forming numbers. The
count—and-carry method mentioned earlier applies here also. Table 1-1
shows the octal “numbers with their decimal and binary equivalents.
It is

Table 1—1

Decimal-Octal-Binary Equivalents.
L

Decimal

Octal

Binary

Decimal

111

1000

1001

.11

1010

100

1011

101

_14

1100

110

1101

Octal

Binary

The octal number system eliminates manyof the problems involved

in handling the binary number system used by a computer. To make the

12-bit numbers of the PDP—8 computers easier to handle, they are
often separated into four 3- bit groups. These 3-bit groups can be represented by one octal digit using the previous
-

seen below.

Abinary number
is

table of equivalents as

11010111101

separated into 3-bit groups by starting with the LSD end of the

number and supplying leading zeros if necessary:
.

011010111101

The binary groups are then replaced by their. octal equivalents:

011.; 3.
0102: 25
11 12: 78
1012: 53
and the binary number is converted to its octal equivalent:
3

Conversely, an octal number can be expanded to a binary number using the same table of equivalents.
'

53078: 101 011
1—11

000 111.

OCTAL-TO-DECIMAL CONVERSION
Octal numbers may be converted to decimal by multiplying each
digit by its weight or position coefficient and then adding the resulting

products. The position co‘eflficients in this case are powers of 8, which
is the base of the octal number system. Example:
2167i: ‘.7 decimal
7X8027X
1:
21678::
8:
+6><81=6><
64:
+l><82:1><
+2 X 83 2: 2 X 512 =

7
48
64

+1024
1143

Therefore,

2167g :'

114310.

DECIMAL-TO-OCTAL CONVERSION
There are two commonly used methods for converting decimal num-

bers to their octal

equivalents. The reader may choose the method

which he prefers.

SUBTRACT ION OF POWERS METHOD. The following procedure
is followed to convert a decimal number to its octal equivalent. Subtract from the decimal number the highest possible value of the form

a8”, where a is a number between 1 and 7, and n is an integer. Record
the value of a. Continue'to subtract decreasing powers of 8 (recording
the value of a each time) until the decimal number is reduced to zero.

Record a value of a=0 for all powers of 8 which could not be subtrac-

ted. Table l—2tmay’ be used to convert any number which can be represented by 12—bits (409510 or less). Appendix F contains a similar
'

table forceneerting larger numbers. Example:

259110 2 ‘2 octal

2591

—2560:5><83:5><512
31

31
74
A—

0:0xs2zox

—

3X81: 3

7X80:7><1

Therefore, 259110

l5037

50378.

1-12

Octal-Decimal Conversion

Table 1-2

Position Coefficients

3:3:

(Multipliers)

Position/
8n

7
y

lst

(230)11

_‘4

256

320

384

448.

2,048

2,560

3,072

3,584

2nd

(81)

3rd

(83)

128

192

4th

(83)}

512

1,024

1,536

DIVISION METHOD. A second method for converting a decimal
number to its octal equivalent is by successive division by 8. Divide the
decimal number by 8 and record the remainder as the least significant
digit of the octal equivalent. Continue dividing by 8, recording the re—
mainders as the successively higher significant digits until the quotient
is reduced to zero. Example:

13761,

‘2 octal

Quotient

8)1376

Remainder
'0

172

‘

87—172

8)21

8T2

‘

2—1

Therefore, 137610

2540,.

1—13

EXERCISES
3.

Convert the following binary numbers to their octal equivalents.
1.

1110

2..

0110

111

1011111011

5.
6.
7.

100000

011000

9.
'

110111110
.

11000111

10111111

10.

111111111111

11.

010110101011

12.
13.

111110110100
010100001011

14.

000010101101

15.
16.

110100100100
010011111010

b. Convert the following octal
354
2.
736
3.
15
4.
10
5.
7
6.
5424
7.307
8.
1101

'10.
11.

12.

14.

15
16.

70
64
7777
7765
3214
4532
fln3
1243

Convert the following decimal numbers to their octal equivalents.
1.

13.

numbers to their binary equivalents.

796

1080

1344

4037

580

10.

1000

11.

12.

1512
3077
4056
4095

Convert the following octal numbers to their decimal equivalents.
7;

7773

7777

3257

1000

10.

4577

1200

11.

0012

742

12.

0256

2.
'3.
4.
5.

734
.

1—14

Fractions
The. binary and octal number systems represent fraCtional parts Of
numbers in a similar manner to the decimal system. Furthermore, frac—
tions may be converted from one number system to another by the

same techniques developed for converting whole numbersBefore investigating the mechanics of fraction conversion, consider
what a fraction is. A fraction is a number between 0 and 1, or a number less than a unit. Until now only whole numbers in the following

Octal.

In
three systems have been considered: decimal, binary, and
each of these systems, the position of the symbol in the number denotes

its power, and the symbol is the coefficient of that power. These are
positive powers. For example, in the decimal system the number 598,
5 isthe coefficient of 102, 9 is the coefficient of 101, and 8 is the coeffi-

cient of 10°. In binary and octal the same rule applies to using the
powers of the base of the system.

When working with fractions,

important point to keep in mind
is that fractions contain coefficients of negative powers, with the radix
point being the dividing line between the non—negative and negative
powers of the number system being used. Any number to, the immediate right of the radix point has a power of negative (minus) 1.
The first digit of the fractional number is the MSD. For example, in
an

the decimal fraction .637; 6 is the coefficient of 101, 3 is the coeflicient of 102, and 7 is the coefficient Of 10's. The coefficient of a nega—

tive pOwer of the base is actually the numerator of a proper fraction
.

‘

whose denominator is the positive power of that base. For example,

.610 (6 X 101) is equivalent to 6 divided by 101. or 6/10, and also
.38 (3' X 8—1) is equivalent to 3 divided by 81 or 3/8. It should be ap—
parent that this general rule applies to any base that may be considered.
Table 1-3 contains proper fractions which have been changed to deci-

mal, binary, and octal for comparison purposes.
CONVERTING DECIMAL FRACTIONS
-

BINARY AND OCTAL FRACTIONS
SUBTRACTION OF POWERS METHOD. One method of converting
a

decimal fraction to a different number system is the subtraction of

powers method. In this method, subtractions of the highest possible
negative power of a number in another system that is contained in the
decimal

fraction, are performed. In each subtraction, recording the
power and itS coefficient gives the equivalent number in the other sys—
“tem. When no subtraction is possible, a 0 is recorded. To convert a
decimal fraction to a binary fraction, the powers of 2 are associated
1-15

Table 1-3

Fraction Equivalents

Proper

Decimal

Octal

Fraction

Equivalent

Binary
Equivalent
’

1/ 2

1/ 4

.25

.20

.01

.125
.0625

.10

.001

.04

.0001

1/ 8
1/ 16

‘

1/32
1/64
1/ 128
1/ 256

.03125

.02

.00001

.015625
.0078125
.003 90625

.01

.000001

.004

.0000001

1/512

.001953125

1/ 1024

.0009765625

.002
.001
.0004

.00000001
.000000001
.0000000001

with coefficients of 0 or 1, since they are the only coefficients used in
this system. In the octal system, the coefficients 0 through 7 are used.

The following example and explanation will show the conversion of
the decimal fraction .5625 to binary.
.0625
——.0625=2‘4

.5625
——.5000‘:2‘1

.0000

.0625
,

Negative Powers of 2
Decimal

Equivalents

2-1

2-2

2‘3

2'4

.5000

.2500

.1250

.0625

Bit Values of Answer

.1001

The largest negative power of 2 contained in the decimal fraction .5625
is 2'1, which is equivalent to decimal .5000; subtract .500010 from

.562510 and record a l in the 2‘1 column. It is not possible to subtract
2’2 from the remainder, so record a 0 in the 2‘2 column, 2‘3 cannot be
subtracted from the remainder, so record a 0 in the 2‘3 column; 2‘4
can be subtracted from the

remainder, so record a 1 in the 2‘4 column.

Thus, the binary equivalent of a decimal .5625 is .10012.
Conversion to octal fractions follows the same procedure, but more
than one subtraction of a given poWer of the base is possible; The
number of times this subtraction is possible yields the coefficient of that
particular power of the base. This method will not bejdemonstrated

here, since it is very cumbersome, and easier methods are available.
1-16

VMULTIPLICATION METHOD. This method of conversion is fre—'
quently used to change from a decimal fraction to another base. To
convert, the decimal fraction is multiplied through by the base of the
system being converted to. For example, convert decimal fraction .5625
to binary. Multiply the decimal fraction by 2. Since a whole number is
obtained, recorda 1 in the 2‘1 column, discard the whoie number portion of the number, and multiply the remainder by 2 again. No whole\
number is obtained, so record a 0 in the 2‘3 column, and multiply the
result by 2. No whole number is obtained, so record a 0 in the 2"”
column, and multiply by 2 again. A whole number is obtained, so record
a
1_ in the 2‘4 column. The remainder, nowreduced to 0, completes the
conversion, and .5625“, is .10012. The following examples show the conversion just described, and the same decimal fraction converted to octal.
Decimal to Binary

Decimal to Octal
.5625

.5625
'

4.5000

1.1250
.

.44<-——"- 4.0000

0.2500
,

‘

0.5000
,

.1001<—— 1.0000

CONVERTING BINARY AND OCTAL TO DECIMAL
FRACTIONS
EXPANSION METHOD. This method can be used in converting frac-

tions from any base to a decimal fraction. Remember that the MSD is
the first digit to the right of the radix point in a fractional number, and
that it is multiplied by the base to the r—l power. The second digit is
that digit multiplied by the base to the -2 power, etc. For example, to
convert the binary fraction .10001 to decimal, proceed, as follows. The

(2-1) or 1/2, the second digit is 0 X (2‘2) 0r 0, the third
digit is 0 X (23) or 0, the fourth digit is 0 X, (24) or 0, and the
fifth digit is 1 X (2‘5) or 1/ 32. The binary numbers are multiplied by
the respective powers and added together to get the answer. Thus
1/2 + 1/32 which is 16/32 +1/32 equals 17/32 or ,5312510.
MSD is 1 X

1-17

The octal fraction .42 can be converted in the same manner, asfol-

lows. The MSD is 4 X (8‘1) or 4/8-and 2 X (8‘2) or 2/64. The fractions are now added together to get the result; 4/8 + 2/64 or

16/32 + 1/32 :2 17/32 or .5312510. If you look carefully at the
binary fraction .100012 and divide it into groups of 3 to convert to
octal, you can see that .100012 does equal .428. Zeros may be added
to the right of a fraction without changing the vaIUe.

“SHORT CUT”¢METHOD. This is another method of converting frac—
tions from another base to decimal. In this method, start at the LSD of
the fraction and

proceed to the MSD of the fraction, counting the
powers of the base, the next higher power of the base will be utilized
as a common denominator. The number is assumed to be a whole num-

ber for counting purposes. The number .100012 would be converted
as follows:
.1

2.3

O
22

The MSD is 24 or 16, so the common denominator is the next higher
power of 2, or 32. The numerator is converted as if it were a whole
number. The result is then 17/32 which is .5312510.The same method

with the octal fraction .42 should yield the same result.
.4

8°

The MSD is 81, or 8, so the common denominator is the next higher
power of 8, or 64. Multiplying the digit values by the powers of the
base and adding the products gives us the value of the numerator; thus,
4 X

(81) —f— 2 X (8")

34, and the fraction 34/64 equai‘s 531251...

Arithmetic Operations with Binary and Octal Numbers
Now that the reader understands the conversion techniques between
the familiar decimal number system and the binary and octal number

systems, arithmetic operations with binary and octal numbers will be
described. The reader should remember that the binary numbers are
used in the computer and that the octal numbers are usedas a means
of representing the binary numbers conveniently.

BINARY ADDITION

Addition of binary numbers follows the same rules as decimal or
other bases. In adding decimal 1 + 8 we have a sum of 9. This is the
highest value digit. Adding one more requires the least significant digit
1-18

to become a 0 with a carry of 1 to the next place in the number. Simil-_

arly’, adding binary 0 + 1 we reach the highest value a} single digit can
have in the. binary system, 51nd adding one more (1 + 1) requires a;
carry to the next higher power (1 + 1__ 10) Take the binary numbers
—

101 + 10 (5 + 2).
101

+010

111

510
210
710
_

1,1 + 0 :1, and 0 +1: 1 with noicarries required. The
answer is 111, which is 7. Suppose we add 111 to 101.

O + 1

1 1

<—-carries

111

+101

1100

710
510
1210

Now 1 + 1 :: 0 plus a carry of 1. In the second column, 1 plus the
carry 1 : 0, plus another carry. The third column is 1 + 1 : 0 with
a carry,

plus the previous carry, or 1 + 1 + 1

is equal to 1

X 2"" + 1 X 22 or 8 + 4

11, Our answer 1100

12, which is the correct

solution for 7_ + 5.
OCTAL ADDITION
Addition for octal numbers should be no problem if we keep in mind

the following basic rules for addition.
If the sum of any column is equal to or greater than the base

of the system being used, the base must be subtracted from the
sum to obtain the final result of the column.

If the sum of any column is equal to or greater than the base,

there Will be a carry to the next column equal to the number
of times the base was subtracted.-

If the result of any column is less than the

base, the base is

not subtracted and no carry will be generated. Examples:

+,33

510
310

"—8"
—

To.

58
3s

2910
5110

—8—8
=

‘81,

'1-19

0,,

80,,

Negative Numbers and Subtraction
Up to this point only positive numbers have been considered. Negative numbers and subtraction can be handled in the binary system in
either of two ways: direct binary subtraction or by the two’s comple_

ment method.

BINARY SUBTRACTION (DIRECT)

Binary numbers may be directly subtracted in a manner similar to
decimal subtraction. The essential differenee is that if a borrow is re-

quired, it isequal to the base of the system or 2.
110

_‘101

001

610
510
110

To subtract 1 from 0 in the first column, a borrow of 1 was made from

the second column which eflectively added 2 to the first column. After
the borrow, 2
1 : 1 in the first column; in the second column
0
0 : 0; and in the third column 1
1 : 0. The same‘numbers
——~

——

which

——

using the twos complement method are subtracted directly in the following example.
were

subtracted

011

001

100 010
’

010 010 010 111

000 111 001 011

B-A

TWO’S COMPLEMENT ARITHMETIC
To see how negative numbers are handled in‘the computer, consider

mechanicai register, such as a car miieage indicator, being rotated
backwards. A Sedigit register approaching and passing through zero
a

would read the following.
00005
00004
00003

00002
00001

00000
99999
99998
etc

1-20

It should be clear that the number 99998 corresponds to —2. Fur-

ther, if we add
00005
99998
1

00003

andignore the carry tothe left, we have effectively

performed the

operation of subtracting
5=—2=3

The number 99998 in this example is described as the ten’s complement

of 2. Thus in the decimal number system, subtraction may be performed by adding the ten’s complement of the number to be subtracted.
If a system of complements were to be used for representing negative

—

numbers, the minus sign could be omitted in negative numbers. Thus
all numbers could be represented with five digits; 2 represented as
00002, and —2 represented as 99998. Using sucha system requires
that a convention be established as to what is and is not a negative
number. For example, if the mileage. indicator is turned back to 48732,"
is it a negative 51268, or a positive 48732? With an ability to represent
a total of 100,000 different numbers (0 to 99999), it would seem
reasonable touse half for positive numbers and half for negative num—
bers Thus, in this situation, 0 to 49999 would be
regarded as positive,
and 50000 to 99999 would be regarded as negative.
In this same manner, the two’s complement of binary numbers are.»
used to represent negative numbers, and to carry out binary subtraction,
in the PDP- 8 computer. In octal notation, numbers from 0000 to 3777
are regarded as positive and the
numbers from 4000 to 7777 are re—
garded as negative.
The two’s complement of a numberIs defined as that number Which
'

when added to the original number will result in a sum of zero. The

binary number

110110110110 has

001001001010 as

two’s

complement equal

shown1n the following addition.
110110110110
001

001

001 010

l 000 000 000 000

The easiest method of finding a tWo s complement is to first obtain the
one ’s

which formed by setting each bit to the opposite
complement,
value.
IS

101

000 110 111

010 111 001 000

Number
.

Onescomplement of the number
1-21

The two’s complement of the
_

number is then obtained by adding 1 to

the one’s complement.
110001
001

001

11.0 010

110 001

Number
One’s complement of the number
Add 1

110

Two’s complement of the number

101

Subtraction in the PDP-8 is performed using the two’s complement

method. That is, to subtract A from B, A must be expressed as its two’s

complement and then the value of B is added to it. Example:
010 010 010 111
101 001

Two’s complement ofA

011 001

100 010

1 000 111

001 011

B—A

101
(carry is
ignored)

101

OCTAL SUBTRACT ION

Subtraction is performed in the octal number system in two ways
‘

which are directly related to the subtractions in the binary system. Sub-

traction may be performed directly or by the radix (base) complement
method.
OCTAL SUBTRACTION

(DIRECT). Octal subtraction can be per-

formed directly as illustrated in the following examples.
3567—2533: ?

2022—1234: ?

3567
—2533

—1234

1034

0566

2022

Whenever a borrow is needed in oetal subtraction, an 8 is borrowed as
in the second example above. In the first column,

8 is borrowed

which is added to the 2 already in the first column and the 4 is sub>~~traeted from the resulting .10. lathe secondtcolumn, an .8. is borrowed,
and added to the 1 which is already in the column (after the previous
borrow) and the 3 is subtracted from the resulting _9. In the third
column the 2 is subtracted from a borrowed I (originally a borrowed
8), and in the last column 1——1:0.
EIGHT’S COMPLEMENT ARITHMETIC. Octal subtraction may be
performed by adding the eight’s complement of the subtrahend to the
minuend. The eight’s complement is obtained in the following manner.
3042
4735

+1
473 6

Number
Seven’s complement of the number

Add 1 to seven’s complement to obtain
Eight’s' complement
1-22,

The seven’s complement of the number is obtained by setting each digit

of the complement to the value of 7 minus the digit of the number, as
seen

above. The eight’s complement of the number is then obtained by

adding 1 to the seven’s complement. To prove that the complement is
intact a complement, the number is added to the complement. and a re—
sult of Zero and an overflow of 1 is obtained.
'

3042

+4736

1 0000

The

following example uses the eight’s complement to subtract a

number.
3567—2533: ?
2533
5244

Number

Seven’s complement

+1
5245

Eight’s complement

3567

Minuend

.Eight 3 complement of subtrahend

+5245

(carry is
ignored)-—>l 1034

Difierence

Multiplication and Division1n Binary and Octal Numbers
Though multiplication in computers is usually aChieved by means

other than formal multiplication, a formal method will be demonstrated

as a teaching vehicle.

BINARY MULTIPLICATION
In binary multiplication, the partial product is moved one position to

the left as each successive multiplier is used. This is done in the same

‘

manner as
.

in decimal multiplication. If the multiplier is a 0, the partial

product can be a series of 0s as in example 2, or the next partial product
can be moved two places to the left as in
example 3, or three places as

in example 4.

Example

46210
127m

Multiplicand
Multiplier

3234

First partial product

924

Second partial product

462

Third partial product

58674

Product

1-23

Example 2.

1110110.;
1011;;~
1110110
1110110

0000000
1110110

101000100102

Example 3.

11101102
10112

1110110
1110110

1110110

101000100102

Example 4.

11001110:
110012
11001110
11001110
11001110

10100000111102
Because of the difficult binary additions resulting from multiplications such

the previous examples, octal multiplication of the octal

equivalents of binary numbers is often substituted.
OCTAL MULTIPLICATION

Multiplication

of octal

numbers is the same as multiplication of

decimal numbers as long as the result is less than 108. Obviously this
could be a problem if it weren’t for the fact that an octal multiplication
table can be set up, similar to the decimal multiplicatiOn table, to make
"

muitiplieatien of ectal numbers quite simple Table l- 4
is a partially completed octal multiplication table that
will be quite use-

the‘ job of

ful once you have filledin the blank squares.

Using the completed octal

multiplication table, the following prob-

lems may be solved.

22613 X 128 = ?
2265;
X 128

454

226

27348

1-24

"nbg<wi:?

’

12478x3058

6503
0000

3765
405 203i
'

Table 1-4‘
0

Octal Multiplication Table
3

5
6
0

BINARY DIVISION
Once the reader has mastered binary. subtraction and multiplication,

binary division is easily‘learned. The following problem solutions illustrate binary division.
'

‘"

Divide 100102

10.;
1001
‘

510010
10

100102

18m
:

10010
_

102
-

00
00

01
00
10

10
0

1-25

210

910

’

Divide 1 1 102

14m

—_

1 00;.

_—

410

3.510

11.1

100)11100

1111: 15m

100

'iib
100

”166
100
0

OCTAL DIVISION
Octal division uses the same principles as decimal division. All mul-

tiplication and subtraction must however be done in octal. (Refer to the
octal multiplication table.) The following problem solutions illustrate
octal division.
621

5010

1714s

_
“

2.21

211

“

318

2510

)1714

562

154

154
154

""6

EXERCISES
a.

Perform the following binary additions.
1.

10110

111001

-+101

+101101

+110

100111

10.

101

‘—

100

+10

11011001

11.

1110

10010011

100

+11100011

+11

11011

~F0010

10110111

101

+1000

+
5.

1111

1101

110111

100100

101
'

+11

+110001

1—26

11011011’

12.
_

10111011
00101011
01010111

+01111101

b. Find the one’ s complement and the two’s

complement of the fol-

lowing numbers.
'

100 110 010

000 000 000 111

111

100 000 000 000

100 000 010 010

10.

100 001

100 110

‘11.

111

I, 12.

111

111 111 111

011

010 111 011

011

000 000 000 000

000 000 000 001

-000 100~100 100

1101000 000

'_
.

Subtract the following binary numbers directly.
1.

101000001

101011010111
011111111101

101111100111
010101110010

010111101

1010111010
0101110101

(1.

Perform— the following subtractions by' the two’s complement
method. Check your
work by direct“ subtraction. Show all work.
f‘

_011

011

9’!”
P‘:h

000 111

111

011

111

101

001

101

111

110

011 111

111

——

111.010 110

001

000 001 001 101

——

100 011

——

010 101

——

001

100 101 011

-—

010

101

Multiply the following binary numbers.

11011

X110

110'

1011101

101011101011

X101

X10000

Divide the following binary numbers.
1.

100

To‘

10000
100

1-27

3.
.

1100100

10100

'
‘

g. Add the following octal numbers.
1.

127

-+53

777

256

543

+724

+612

_#§§

‘“

77‘

437

426

+1]

772

+76

3357

747

+562

+575

h. Subtract the following octal numbers directly.
42

'1.

2543

~23

-—.2174

,—-44
‘

—-34
3.

—6373

7000

—6573

~11

7500

7474
—4777

following octal subtraetions by the eight’s complement method. Check your work by subtracting directly. Show all
Perform the

,work.
1.

0377

2345

3.1144
4.

3000

—-

0233

2311

1456

0044

1046

3234

0011

llll

—

——

—

——

—

2277

0017
2777
0777

j. Multiply the following octal numbers.
1.

716

14
x13

X472

——

425

x377

X65

x4
2.

571

x246

k. Prove the answers to the problems in (j) by division, as follows:

Multiplicand
>< Multiplier
Product

Multiplicand
-

Multiplier) Product
1-28

LOGIC OPERATION PRIMER

Computers use logic operations in addition to arithmetic operations
to solve problems. The logic operations/have a direct relationship with
the algebraic system to represent logic statements known as Boolean
algebra. In logic, there are two basic. connectives that are used to express the relationship between two statements.
the OR.

The

AND Operation

The following simple circuit

These are the AND and

with two switches illustrates the AND

operation. If current is allowed to flow through a switch, the switchIS
said to have a value of 1. If the switch is open and current cannot flow,
the switch has a value of 0. If the whole circuit is considered, it .will
have a value of 1

(i. e., current may flow through

and B are 1. Thisis the AND operation

it) whenever

both A

c,/..,.____.,/o___._<g

The AND operation is often stated A 0 B : F. The multiplication symbol (0) is used to represent the AND connective. The relationship be—
tween the variables and the, resulting value of F is summarized in the

following table.
B
0
l

O
1

i-a'o o

When the AND operation is applied to binary numbers, a binary 1 will
appear in the result if a binary 1 appeared1n the corresponding position
of the two numbers.
The AND operation can be used to mask out a portion of a 12—bit

number.
To Be

To Be Retained

Masked

for Subsequent

Out

Operation

f—“'~—flr—‘—*—'_1

010 101 010 101
000

000_

111

(12—bit number)
(mask)
(result)

111

000 000 010 101

1-29

The OR Operation

A second logic operation is the OR

(sometimes called the inclusive

OR). Statements which are combined using the OR connective are
illustrated by the following circuit diagram.

~—:/;>°"1—-—o
Current in the above diagram may flow whenever either A or B (or
both) is closed (F21 if A: 1, or B21, or A21 and B: 1 ). This operation is expressed by the plus

(+) sign; ,thus A+B-_:F. The following
table shows the resulting value of F for changing values of A and B.
V‘"

0
1

O
1

Thus, if A and B are the 12-bit numbers shown below,
'

uated follows.
as

011010 011111

100110 010 011

A+B

111110011111

1s eval-

A+B

Remember that the “+” in the above example means “inclusive OR”,
'

not “add

”

The Exclusive OR Operation

exclusive

The third and last logic operation is the exclusive OR. The
OR is similar to the inclusive OR with the exception that one set of

conditions for A and B are excluded. This exclusion can be symbolized
in‘the circuit diagram by connecting the two switches mechanically to-

gether. This connection makes it impossible for the switches to be closed
1-30

simultaneously, although they may be open simultaneously or individu'

‘

ally.

Thus, the Circuit is completed when A21 and B: 0, and when A40
and B:1 The results of the exclusive
operation are sUmmarized
”

in the

table below.
A
O
O
1

The exclusive OR of two 12-bit numbers is evaluated and labeled F
'

in the following operation.
A: 011010 011111
B

100110 010 011

111

100

001

100

GENERAL ORGANIZATION OF THE PDP—8

Almost every general purpose digital computer has the basic units
shown in Figure 1-1, on the following page.
If a machine is to be called a computer, it must'have the capability of
r

‘

performing some types of arithmetic operations. The element of a digital
computer that meets this requirement is called the arithmetic unit. In
order for the arithmeticunit to be able to do its required'task, it- must
be told what to do. Therefore; a control unit is necessary.
Since mathematical operations are performed by‘the arithmetic unit,
it may be necessary to storeya partial answer while the unit'is computing

another part of the problem. This stored'partial answer can then be used
to solve other parts of the problem. It is'also helpful for the control unit

and arithmetic unit to have information immediately available for their
use, and for the use of other units within the computer. This
ment is met by the portion of the computer

unit, or core storage unit.

1-31,

designated

require-

the memory

r'“—‘
F‘“
I
I
I

i
INPUT
UNIT

——a

I
'
1

CONTROL
____

UNIT
"

INTERNAL

o UTPUT

STORAGE

UNIT

i
r

i
L,

Figure 1-1

ARITHMETIC
UNIT

PDP—8 General Organization

The prime purpose of a digital computer is to serve humans in some
manner.

In order to do this there must be a method of transmitting our

wants to the computer, and a means of receiving the results of the com-

puter’s calculations. The portions of the computer that carry out these
functions are the input and output units.
Arithmetic Unit
The arithmetic unit of a digital computer performs the actual work

of computation and calculation. It carries out its job by counting series
of pulses or by the use of logic circuits. Modern computers use com-

ponents such as transistors and integrated circuits. Switches and relays
were used

previously, and were acceptable as far as theirability to'per—
form computations was concerned. Modern computers, however, because of the speed desired, make use of smaller electronic components
Whenever possible.
.

The arithmetic unit of the PDP—8 has, as its major component, a
12—bit accumulator, which is simply a register capable of storing a number of 12 binary digits. It is called the accumulator because it accumu—

during the operation of the PDP—S. All arithmetic
operations are performed in the accumulator of the PDP—S.
lates partial

sums

Control Unit

The control unit of a digital computer is an administrative or switch—
ing section. It receives information entering the machine and decides
how and when to perform operations. It tells the arithmetic unit what

get the necessary information. It knows when the
arithmetic unit has completed a calculation and it tells the arithmetic

to do and where to

unit what to do with the results, and what to do next.
1-32

The control unit itself knows whatto tell the arithmetic unit to do by

interpretng a set of instructions. This set of instructions for the control
'

unit is called

a program and is stored in the computer memory.

Memory Unit
The memory unit, sometimes called

the

core

storage unit contains

information for the control unit (instructions) and for the arithmetic
unit (data). The terms core storage and memory may be used inter‘

changeably. Some computer texts refer to external units as storage, such'
as magnetic tapes\and disks, and to internal units as memory, such as
magnetic cores The requirements of the internal storage units
vary
may
to
from
greatly
computer
computer
The PDP-8 memory unit is composed of magnetic cores which are
often compared to tiny doughnuts. These magnetic cores record binary
information by the direction in which they are magnetized (clockwise
or counterclockwise). The
memory unit is arranged in such a way that
it can store 4096 “words” of binary information. These words are each
12,—bits in length. Each core storage location has an address, which is a
unique number used by the control unit to specify that location. Storage
of this type in which each location can be specified and reached as easily
as any other is referred to as random-access storage. The other type of
storage is sequential storage such as magnetic tape, in Which case some
locations (those at the beginning of the tape) are easier to reach than
others (those at the end of the tape)
‘

Input Unit

Input devices are used to supply the values needed by the computer
and the instructions to tell the computer how to operate on the values.
Input unit requirements vary greatly from machine to machine. A
manually operated keyboard may be sufficient for a small computer.
Other computers requiring faster input use punched cards for data in-.
puts. Some systems utilize removable plugboards that can be pre-wired
to perform certain instructions. Input.may also be Via punched paper
tape or magnetic tape, two forms of input common in PDP—8 systems.
Output Unit

Output devices record the results of the computer operations. These
results may be recorded in a permanent form (e. g., as- aprintout on the
teleprinter) or they may be used to initiate a physical action (e.g., to
adjust a pressure valve setting). Many of the media used for input, such
as paper tape, punched cards,-and magnetic tape, can also be used for
Output.

—

‘

1-33

‘

COMPUTER DATA FORMATS

The PDP-8 uses 12—bit words to represent data. Some of the formats
in which this data is represented are described in the following para—

graphs.
Alphabetic Characters
Computers are designed to operate upon the binary numbers which
it conveniently represents with electronic components. There are occasions however when it is

desirable to have the computer represent.

alphabet and punctuation marks. Binary codes are
used to represent such characters. For example, the reader is familiar
with punched cards, which use a system of punched holes to represent
characters of the

information. Each of these codes associates some character with a particular binary number. The computer can store the binary number (not
the character) in its memory. When so directed, the computer will out-

put the binary code to a device which will interpret the code and print
the character. Some specific binary codes used to represent alpha—
numeric information (letters, numbers, and punctuation symbols) are
I

presented in Appendix B.
Number Representations

The PDP- 8 operates upon 12—bit words

(namely 0 to 111 111 111

1113, or 0 to 77778). By convention, one half of the numbers are con—
sidered positive (0 to 011 111 111 1113, or 0 to 37778), and one
half (100 000 000 000.2 to 111 111 111 1112 or 40005: to 7777s) are

negative. Therefore the PDP-8 can directly represent the
portion of the number line shown in Figure 1—2.
considered

40008

4001a
(-3777‘3)

soooa
(-20003)

/ 0\

20003

3777,,

777.,

(— n

Figure 1-2

PDP-8 Octal Number Line

Notice that the first digit of the 12-bit binary numbers is in effect a

“sign bit.” That is, bit 0 (the first bit) specifies the Sign of the number
by the following rule. If bit 0 is a O, the number is positive; if bit 0 is
a 1, the number is negative. This is the means by which the computer
1-34

and, negative numbers. Thus, the zero is considered

tests for positive

positive. In figure 1-2 it should be noted that the number 4000 is
peculiar in that it has no positive counterpart. (Expressed in octal, the
“two’s complement” of 3776 is 4002; of 3777 is 4001; of 4000 is
4000.)

When the octal to decimal conversions are performed, the number

line of Figure 1-2 is converted to the number line of Figure 1—3. Thus

‘

the PDP-S can represent directly the numbers between -—204819Hand

+204710. This would seem to be a serious restriction. Throught‘two'
techniques however this limitation is overcome.

""1
-

1""

1‘

+2047

2048

Figure 1-3

PDP-S Decimal Number Line

DOUBLE PRECISION NUMBERS
The PDP—8 memory is made up of lZ-bit storage locations. Suppose
however that ‘a number larger than 12-bits were to be stored. By using

12-bit storage locations, numbers between —-8,3 88,60810 and;
8,388,60710 may be represented directly. This method of representation-

two

is appropriately called double precision. The method could be extended

precision and further if necessary.
It should be noted that to add double precision numbers, tWo addi—

to triple

tions are needed. Double precision arithmetic is described in Chapter 3._

FLOATING POINT NUMBERS
'

Another method of representing numbers in the PDP-8 with more

than

12'-bit word is

floating point notation. In this notation, a
number is divided into tWo parts, namely a mantissa (number part)
.

one

and an exponent (to some base). In the decimal number system for
example, the number 12 can be written in the following ways.
MANTISSA
.12
-

1.2

EXPONENT
102

101

x
'

12.

120.

100
10—1

1200.

1&2

PDP-S floating point notation makes use of a representation similar to the

‘

’

above with the exception that the exponent and the mantissa are binary

1-35

numbers. The binary mantissa (number part) is stored in two locations
and a third location stores the exponent. The exponent is selected such

that the mantissa has no leading zeros, thereby retaining the
number of

significant

digits.

1-36

maxinnini

programming

Fundamentals

This chapter describes the three general types of computer instruc«

tions and the way in which they are used in computer programs. The
first type of instruction is distinguished by the fact that it operates upon
memory location and must tell the computer where the data is located in core so that the computer can
find it. This type of instruction is said to reference a location in core
data that is stored in

some

memory; therefore, these instructions are often called

instructions (MRI).

memory

reference

When speaking of memory locations, it is very important that a clear

distinction is made between theaddress of- a location and the contents
of that location; A memory reference instruction refers to a location by
a

12-bit address; hoWever, the instruction causes the computer to take

specified action with the content of the location. Thus, although
the address of a specific location in memory remains .the same, the content of the location is subject to change. In summary, a memory refer-

some

ence

instruction

uses

12—bit address

value to refer to' a memory

location, and it operates on the 12—bit binary number stOred in the
referenced memory location.
The second type of instructions are the operate microinstructions,
which perform a variety ofprogram operations without any need for
reference to a memory location.” Instructions of this type are used to

perform the following operations: clear the accumulator, test for neg—
ative accumulator, halt program execution, etc. Many of these Operate
microinstructions can be combined

(microprogrammed) to increase the

operating efficiency of the computer.

The third general type of instructions are the input/output transfer

(IOT) instructions. These instructionsperform the "transfer of infor—
mation between a peripheral device and the computer memory. IOT.

instructions are discussed in Chapter 5.
2-1

PROGRAM CODING

Binary numbers are the only language which the computer is able
to underStand. It stores numbers in binary and does all its arithmetic
operations in binary. What is more important to the programmer, how—
eyer, is that in order for the computer to understand an instruction it
must be represented in binary. The computer can not understand in‘

structions which use English language words. All instructions

the form of binary numbers (binary code).

must beIn

Binary Coding
The computer has a set of instructions in binary code which it “un

derstands”. In other words, the circuitry of the machine is wired to react
to these binary numbers in

the

certain manner. These instructions have

appearance as any other binary number; the computer can
interpret the same binary configuration of 0’s and 1’s as data or as an
same

instruction. The programmer tells the computer whether to

interpret

the binary configuration as an instruction or as data by the way in which

the configuration is encountered in the program.

Suppose the computer has the following binary instruction‘set.
Instruction A

This binary number instructs the
computer to add the contents of
location 000 000 010 010 to the

001 000 010 010

accumulator.
Instruction B

001

This binary number instructs the
computer to add the contents of

000 010 111

location 000 000 010 111 to the
accumulator.

memory location with an
address of 000 000 010 010 and the binary number 000 111 111 111
If instruction B is contained in

is stored1n a location with an

"ing program could be Written.

core

address of 000 000 010 11.1, the followContent

Location
000 000 010 010
'000 000 010'111

001 000 010 111

‘

000111111

111

If this program were to be executed, the number 000 111 111 111
would be added to the accumulator.
Octal Coding

binary configurations appear cumbersome and confusing, the
reader will now understand why most programmers seldo'm use the
binary number system in actual practice. Instead, they substitute the
If

2—2

octal number system which was discussed in Chapter 1. The reader
should not proceed until‘he understands these two number systems
and the conversions between them.
'

Henceforth, octal numbers will be used to represent the binary numbers which-the computer uses. Although the programmer may use octal
numbers to describe the binary numbers within the computer, it should.
be remembered that the octal representation itself does not exist within

the computer.

~
-

‘

When the conversion to octal is performed, Instruction B becomes

10278 and the previous program becomes

Content

’

Location

‘

1027.

0022.

0027,,

0777..

To demonstrate that a computer cannot distinguish between'a num—7

her and an instruction, consider the following program.
'

LOcation
0021

Content
1022

(Instruction A)
(Instruction B)

0022

1027

0027

0777
.,

(The number 777,.)
\

Instruction A, which adds the contents of location 0022 to the accu-

mulator, has been combined with the previous program. Upon execution of the program (assuming the initial accumulator value:0), the
computer will executexinstruction A and add 10278 as a number to the
accumulator obtaining a result of 1027... The computer will then execute
the next instruction, which is 1027, causing the computer to add the
contents of 0027 to the accumulator. After the execution of the two

instructions the number 20268 is in the accumulator. Thus, the above

program caused the number 10278 to be used as an instruction and as
a number by the computer.
'

Mnemonic Coding

Coding
binary coding, is nevertheless very inconvenient. The programmer must
learn a complete set of octal numbers which have no logical con-_
nection with the operations they represent. The coding is diflicult for
the programmer when he is writing the program, and this difiiculty is
compounded when he is trying to debug or correct a program. There is
no easy way to
remember the correspondence between an octal number
and a computer operation.
a program

in octal numbers, although an improvement upon

‘

2-3

simplify the process of writing or reading a program, each instruction is often represented by a simple 3- or 4-letter mnemonic
symbol. These mnemonic symbols are considerably easier to relate to a
computer operation because the letters often suggest the definition of
the instruction. The programmer is now able to write a prOgram in a

language of letters and numbers which suggests the meaning of each
instruction.
_

The cOmputer still does not understand any language except binary
numbers. Now, however, a program can be written in a symbolic lan-

guage and translated into the binary code of the computer because of
the one-to-one correspondence between the binary instructions and the

mnemonics. This translation could be done by hand, defeating the purpose of mnemonic instructions, or the computer could be used to do the
translating for the programmer. Using a binary code to represent alpha-

betic characters as described in Chapter 1, the programmer is able to
store alphabetic information in the computer memory. By instructing

the computer to perform a translation, substituting binary numbers for
the alphabetic characters, a program is generated in the binary code
of the computer. This process of translation is called “assembling” a
program. The program that performs the translation is called an
assembler.

Although the assembler is described in detail in Chapter '6, it is well
to make some observations about the assembler at this point.
1. The assembler itself must be written in binary code, not
I

mnemonics.
2.

It performs a one-to—one translation of mnemonic codes into

binary numbers.
3.

It allows programs to be written in a symbolic language which

is easier for the prOgrammer to understand and remember.
.

A specific mnemonic language for the PDP-8, called PAL

(Program
The
neitt secis
introduced
this
i’é‘téi’in“
Assembly LangUage),
chapter.

tion describes the general PDP-8 characteristics and components. This
information is necessary to an understanding of the PDP-8 instructions
and their uses within a program.

PDP-S ORGANIZATION AND STRUCTURE

The PDP-S is a high—speed, general purpose digital computer which

12-bit

binary numbers. It is a single—address parallel
machine using two’s complement arithmetic. It is composed of the five
basic computer units which were discussed in Chapter 1. The com—

operates

2—4!

‘

ponents Of the five Units and their interrelationships are shown in
Figure 2-1. For simplicity, the input and output units have been
combined.
I

INPUT/

OUTPUT

UNITS

ARITHMETIC

UNIT

CONSOLE

ACCUM‘

MEMORY

ADDRESS
‘

MEMORY
‘

BUFFER
REGISTER

INSTRUCTION

I
I

-l

.
_

I
I

|
I
_

COUNT ER

”LATOR

|
I

ETC.

UNIT

TYPE,
DISK,
DECTAPE,

MEMORY

I
|
I

PROGRAM

‘

TELE—

|
I

LINK

OUTPUT
DEVICES

INPUT/

UNIT

I
|

CONTROL

|
I
I

Elli);
GENERATOR

Figure 2-1

|
|
|

CORE
4096

MEMORY

I2—BIT WORDS

Block Diagram 'of the PDP-S

Input and Output Units
The input and output units are combined in Figure 2—1 because in
many cases the same device acts as both an input and an output unit:
The Teletype-console, for example, can. be.uSed to input information
which'will be accepted by the computer, or it can accept processed in-”
formation and print it as output. Thus, the two units Of input and output
are very Often joined and referred to as input/output or simply I/O.
Chapter 5 describes the methods of transmitting data as either input or
output; but for the present, the reader can assume that the computer is
able to accept information from devices such as those listed in the block

diagram and to return Toutput information to the devices. The PDP-8
console allows the programmer direct access to core memory and the
program counter by setting a series of switches, as described in detail
in Chapter 4.

Arithmetic Unit

The second unit contained in the PDP-8 block diagram is the arith—
metic unit. This unit, as shown in-the diagram, accepts data from input
-

devices and transmits-processed data tO the output devices. as well. Pri-

marily, however, the unit performs calculations under the direction of
the control unit. The Arithmetic Unit in the PDP-8 consists Of
accumulator and a link bit.

‘

2—5

ACCUMULATOR (AC)
The prime component of the arithmetic unit is a 12—bit register called
the accumulator. It is surrounded by the electronic circuits which per-

form the binary operations under the direction of the control unit. Its
name comes

from thefact that it accumulates partial sums during the

execution of a program. Because the accumulator is only twelve bits in

length, whenever a binary addition causes a carry out of the most sig—
nificant bit, the carry is lost from the accumulator. This carry is re;
'

‘

corded by the link bit.
LINK (L)

Attached logically to the accumulator is a 1-bit register, called the

link, which is complemented by any carry out of the accumulator. In
other words, if a carry results from an addition of the most significant
bit in the accumulator, this carry results in a link value change from 0
to l, or 1 to 0, depending upon the original state of the link.
Below is a diagram of the accumulator and link. The twelve bits of

the accumulator are numbered 0 to 11, with bit 0 being the most sig—
nificant bit. The bits of the AC and L can be either binary 0’s or l’s as

shown below.

ACCUMULATOR

UNK
o

o
I

o‘

III'IIIIIIIII

'\
MOST

SIGNIFICANT BIT

/
LEAST SIGNIFICANT BIT

Control Unit

The instruction register, major state generator, and program counter
can be identified as part

of the control unit. These registers keep track

of what the computer is now doing and what it will do next. thus
specifying the flow of the program from beginning to end.
PROGRAM COUNTER (PC)

’

The program counter is used by the PDP—S control unit to record
the locations in memory (addresses) of the- instructions to be executed.
The PC always contains the address of the next instruction to be executed.
.

Ordinarily, instructions are stored in numerically consecutive

locations and the program counter is set to the address of the next in-

struction to be executed merely by increasing itself by 1 with each
successive instruction. When an instruction causing transfer of command
to. another portion of the stored program is encountered, the PC is set

2—6

the appropriate address. \The PC must be initially set by input to

specify the starting address of a program, but further actions are controlled by program instructions.
INSTRUCTION REGISTER (IR)

The 3—bit instruction register is used by the control unit to specify

the main characteristics of the instruction being executed. The three

significant bits of the current instruction are loaded into the IR
each time an instruction is loaded into the memory buffer register fromcore memory. These three hits contain the operation code which
specifies the main characteristics of an instruction. The other details
are specified by the remaining nine bits (called the operand) of the

most

instruction.
MAJOR STATE GENERATOR
The major state generator establishes the proper states in sequence

for the instruction being executed. One or more of the following three

major states are entered serially to execute eaCh programmed instruc—
tion. During a Fetch state, an instruction is loaded from core memory,
at the address specified by the program counter, into the memory
buffer register. The Defer state is used in conjunction with indirect ad,dressing-to obtain the effective address, as discussed under “Indirect
Addressing” later in this chapter. During the Execute state, the instruction in the
memory buffer register is performed.
Memory Unit
The PDP-8 basic memory unit consists of 4 ,096 12-bit words of
magnetic core memory, a 12-bit memory address register, and a 12—bit
memory bufier register. The memory unit may be expanded in units of

4,096 words up to a maximum of 32,768 words.
CORE MEMORY
The

memory provides storage for the instructions to be performed and informationto be processed. It is a form of random access
core

storage, meaning that any specific location can be reached in memory

readily as any other. The basic PDP—S memory contains 4,096 12-bit
magnetic core words. These 4,096 words require that'12-bit addresses
be used to specify the address for each location uniquely.
MEMORY BUFFER REGISTER (MB)
All transfers of instructions or informatiOn between Core memory and
the processor registers (AC, PC, and IR) are temporarily held in the
memory bufier register. Thus, the MB holds all Words that go into and
out of memory, updates the program counter, sets the instruction
register, sets the memory address register, and accepts
as

from or provides information to the accumulator.
2—7

information

MEMORY ADDRESS REGISTER (MA)

The address specified by a memory reference instruction is held in
the memory address register. It is also used to specify the address of the
next instruction to be brought out of memory and performed. It can be

used to directly address all of core memory. The MA can be set by
the memory buffer register.

by input through the program counter

MEMORY REFERENCE INSTRUCTIONS

The standard set of instructions for the PDP-8 includes eight basic

instructions. The first six of these instructions are introduced in the
following paragraphs and are presented in both octal and mnemonic
form with a description of the action of each instruction.
The memory reference instructions (MRI) require an operand to
specify the address of the location to which the instruction refers. The
manner

in which locations are specified for the PDP—S is discussed in

detail under “Page Addressing” later in this chapter. In the following

discussion, the first three bits (the first octal digit) of an MRI are used
to specify the instruction to be performed. (The last nine bits, three
'octal digits, of the 12—bit word are used to specify the address of the

referenced location——that is, the operand.)
The six memory reference instructions are listed below with their
mnemonic and octal equivalents as well as their memory cycle times.

Octal
Mnemonicg' Value

Instruction

Logical AND
Two’s Complement Add
Deposit and Clear the Accumulator
Jump
Increment and Skip if Zero
Jump to Subroutine

AND

Memory
Cycles‘

Onnn

lnnn

DCA

3nnn

J MP

Snnn

TAD
'

182

JMS

2mm

4min

PDPMemory cycle time for the PDP~8 and -8/l is 1.5 microseconds; for the
8/L, it is 1.6; for the PDP-S/S, it is 8 microseconds. (Indirect addressmg re-

quires an additional memory cycle.)
2The mnemonic code is
binary code.

meaningful

2-8

and translated

assembler into

AND (0nnn8)
_

The AND instruction causes

bit——-by bit

Boolean AND operation

between the contents of the accumulator and the data word specified

by the instruction. The result is left in the accumulator as illustrated
below.

LINK

The

12)

I25

£2!

(25

125

52>

DATAWORD

(25

(25-

{26

Acmssum

following points should be noted with respect to the AND

instruction.

1. A 1 appears in the AC only when a 1 is present in both the AC

(The data word is often referred to as a

and the data word

‘

mask);
2. The state of the link bit is not affected by the AND instruction;
‘

and

3. The data word in the referenced location is not altered.

TAD (lnnng)

The TAD instruction performs a binary addition between the speci—
.

fied data word and the contents of the accumulator, leaving the result
of the addition in the accumulator. If a carry out of the most significant
bit of the accumulator should occur, the state of the link bit is comple—

mented. The add instruction is called a Two’s Complement Add to re—

mind the programmer that negative numbers must be expressed as the
two’s complement of the positive value. The following figure illustrates
'-

the operation of the TAD instruction.

LINKI

LINK

¢¢¢¢¢¢¢¢¢l¢lACIf5
II

¢I

DATAWORDI—3

I25

(25

AC(RESULT)Z+2

2-9

The following points should be remembered when using the

instruction:

TAD

of
Negative numbers must be expressed as a two’s
complement
the positive value of the number;
2. A carry out of the accumulator will complement the link; and
1.

3. The data word1n the referenced locationis not affected.

DCA (3nnn5)
The DCA instruction stores the contents of the AC1n the referenced

location, destroying the original contents of the location. The AC is
then set to all zeroes. The following example shows the contents of the

accumulator, link, and location 225 before and after executing the in—
struction DCA 225.
DCA 225
AC

Loc. 225

Link
'

Before Execution
After Execution

1234

0000

7654
1234

The following facts should be kept in mind when using the DCA in-

struction:
1. The state of the link bit is not altered;
2. The AC is cleared; and

3. The original contents of the addressed location are replaced by
the value of the AC.

JMP (Sunns)

The JMP instruction loads the effective address of the instruction
into the program counter, thereby changing the program sequence since
the PC specifies the next instruction to be performed. In the following

example, execution of the instruction in location 250 (J MP 300) causes
the program to jump over the instructions in locations 251 through 277
and immediately transfer control to
LOcatiOn

Content

250

JMP 300

the instruction in location

300.

(This instruction transfers program
control to location 300.)

DCA 330

300

NOTE: The JMP instruction does not affect the contents of
the AC or link.

ISZ (2nnn8)

The ISZ instruction adds a 1 to the referenced data word and then
examines the result of the addition. If a zero result occurs, the instruction following the ISZ is skipped. If the result is not zero, the instruction

2-10

following the ISZ is performed. In either case, the result of the addition

replaces the original data wOrd in memory. The example in Figure 2—2
illustrates one method of adding the contents of a given location to the
AC a specified number of times (multiplying) by using an ISZ instruc—
tion to increment a tally. The elfect of this example is to multiply the
contents of location 275 by 2. (To add the contents of a given location
to the AC twice, using the ISZ loop, as shown in Figure 2—2, requires
more instructions than merely repeating the TAD instruction. However,
when adding the contents four or more times, use of the ISZ loop requires fewer instructions.) In the first pass of the example, execution of
ISZ 250 increments the contents of location 250 from 7776 to 7777
and then transfers control to the following instruction (JMP 200). In
the second pass, execution of ISZ 250 increments the contents of loca—
tion 250 from 7777 to 0000 and transfers control to the instruction in
location 203, skipping over location 202.
CODING FOR ISZ LOOP
Location

Content

200

TAD 275

201

ISZ 250

202

JMP 200
'

203

DCA 276

250

7776'
0100'

275
276

0000

SEQUENCE OF EXECUTION FOR ISZ LOOP
Content After Instruction EXecution

Location

Content

‘

250

275

276

FIRST PASS
200
TAD 275

0100

7776

0100

0000

201

0100

7777

0100

0000

0100

7777

0100

0000

0200

7777
0000

0100

0000

ISZ 250
JMP 200

202

SECOND PASS
TAD 275
200
ISZ 250
201
JMP
200
202
DCA
276
203
I

0200

0100

0000
‘

(Skipped during second pass)
0000
0000
0100

Figure 2-2.

ISZ Instruction Incrementing a Tally

2—11

0200

The following

points should be kept in mind when using the ISZ

instruction:
1. The contents of the AC and link are not disturbed;
2. The

original word is replaced in main memory by the incre-

mented value;

3. When using the ISZ for looping a specified number of times,

the tally must be set to the negative of the desired number; and
4. The ISZ performs the incrementation first and then checks for
a zero result.

JMS (4nnn8)
A program written to perform a specific operation often includes sets
of instructions which perform intermediate tasks. These intermediate
tasks may be finding a square root, or typing a character on a keyboard.
Such operations are often performed many times in the running of one

program and may be coded as subroutines. To eliminate the need of
writing the complete set of instructions each time the operation must be

performed, the J MS (jump to subroutine) instruction is used. The J MS
instruction stores a pointer address in the first location of the subroutine
and transfers control to the second location of the subroutine. After the
subroutine is executed, the pointer address identifies the next instruc—
tion to be executed. Thus, the programmer has at his disposal a ‘simple
means

of exiting from the normal flow of his program to perform an

intermediate task and a means of return to the correct location upon
completion of the task. (This return is accomplished using indirect ad—

dressing, which is discussed later in t .i-s chapter.) The following exam—
ple illustrates the action of the J MS instruction.
Location

Content

PROGRAM
200

JMS 350
.

(This instruction stores 0201 in location 350 and'transfers program control
to location 351.)

201

DCA 270

(This instruction stores the contents of
the AC in location 270 upon return
from the subroutine.)

2-12

SUBROUTINE
350‘

0000

351

iii

375

J MP I 350

(This location is assumedgto have an
initialgvalue of 0000; after JMS 350 is
executed, it is 0201.)
(First instruction of subroutine)

(Last instruction of subroutine)

The following should be kept in mind when using the J MS:
1. The value of the PC (the address of the JMS instruction +1)
is always stored in the first location of the subroutine, replacing
'

the original contents;

Program control is always transferred to the location designated
by the operand +1 (second location of the subroutine);
3. [The normal return from a subroutine is made by using an in—'

direct J MP to the first location of the subroutine .(JMP I 350
in the above example); (Indirect addressing, as discussed later
_

in this chapter, effectively transfers control to location 201.);

4. When the results of the subroutine processing are contained in
the AC and are to be used in the main program, they must be

stored upon return from the subroutine before further calculations are performed. (In the above example, the results of the
"

subroutine processing are stored in location 270.)

ADDRESSING‘
When the memory reference instructions were introduced, it was
stated that nine bits are allocated to specify the operand (the address
referenced by the instruction). The method used to reference a memory
location using these nine bits will now be discussed.
PDP-8

Memory Pages
As previously described, the format of an MRI is three bits (0, l,
and 2) fer the operation code and the remaining nine bits the operand.
However, a full twelve bits are needed to uniquely address the 4,096
(10,000 Octal) locations that are contained in the PDP-S memory unit.
To make the best use of the available nine bits, the PDP-8 utilizes a
logical division of memory into blocks (pages) of 2008 locations each,
as shown in the following table.
2-13

Memory

Page

Locations

0-1 77

4000-4177

200-377

4200-4377

4400—45 77
4600—4777

Page

400-5 7 7

600-77 7

1000-1177
1200-1377
14004577
1600-1777

5
6
7
10

Locations

5000-5177
5200-5 377
5400-5577
5600-5777

30
31
32

6200-63 77
6400-6577

25
26

2000-2177
2200—2377
2400-257 7

2600-2777

6600-6777

15
16

3000-3177
3 200-3377
3400-3577

3600-3777

7000-7177
7200-7377
7400-7577
7600-7777

.1 1

6000-6177

Since there are 2008 locations on a page and seven bits can represent

2008 different numbers, seven bits (5 through 11 of the MRI) are used

discussing the use of the page ad—
dressing convention by an MRI, it should be emphasized that memory
does not contain any physical page separations. The computer recog—
nizes only absolute addresses and does not know what page it is on, or
when it enters a different page. But, as will be seen, page addressing
to specify the page address. Before

allows the programmer to reference all of the 4,096“; locations of
memory using only the nine available bits of an MRI. The format of an
MRI is shown in Figure 2-3.

OPERAND
'

BIT

POSITION

o
o

EITHERO
'

ORI

o
I

EACHBmS

o
I

I.__r__1

o
l

o
I

I
,

PAGE ADDRESS ans

OPERATION
cone

(0 TO I773)
CURRENT PAGE 0R PAGE 0 an

ADDRESS MODE BIT
OZ DIRECT ADDRESSING

02 PAGE 0

I I INDIRECT ADDRESSING

Figure 2-3.

o
I

II CURRENT PAGE

Format of a Memory Reference Instruction

2-14

As previously stated, bits 0 through 2 are the operation code for the

MRI; Bits 5 through 11 identify a specific location on a given page, but
they do not identify the page itself. The page is specified by bit 4, often
called the current page or page 0 bit. If bit 4 is a O, the page address is

interpreted as a location on page 0. If bit 4 is a 1, the page address
specified is interpreted to be on the current page (the page on which
the MRI itself is stored). For example, if bits 5 through 11 represent
1238 and bit 4 is a 0, the location referenced is absolute address 1238.
However, if bit 4 is a l and the current instruction isin a core memory

‘

location whose absolute address is between 4,6008 and 4,7778, the page
address 1238 designates the absolute address 4,7238. Note that, as.
shown in the following example, this characteristic of page addressing
results in the octal

coding for two

instructions on different
memory pages being identical when their operands reference the same
relative location (page address) on their respective pages.
TAD

’

Content.
Location

Mnemonic

Octal

Explanation

200

TAD 250

1250

TAD 250 and TAD 450 both
mean-add the contents of location 50 on the current page (bit

400

TAD 450

1250

4 =

1) to the accumulator.

Except when it is on page 0, a memory reference instruction can reference 4008 locations directly,
namely those 2008 locations on the page
containing the instruction itself and the 2008 locations on page 0, which
can be addressed from any memory location.

NOTE: If an MRI is stored in one of the first 2008 memory locations (0 to
1778), current page is page 0; therefore, only locations 0 to 1778 are
directly addressable.
Indirect Addressing

preceding section, the method of directly addressing 4003
memory locations by an MRI was described—namely those on page 0
In the

and those on the current page. This section" describes the method for
'

addressing the other 7400S memory locations. Bit 3 of an MRI, shown
in Figure 2—3 but nOt discussed in the preceding section, designates the
address mode. When bit 3 is a 0, the operand is a direct address. When
bit 3 is a 1, the operand is an indirect address. An indirect address
(pointer address) identifies the location that contains the desired address
(effective address). To address a location that is not directly address—
able, the abSolute address of the desired location is stored in one of
the 4008 directly addressable locations (pointer address); the pointer
"address is written as the operand of the MRI; and the letter I is written
2-15

between the mnemonic and the operand.

‘

(During assembly, the pres—
ence of the I results in bit 3 of the MRI being set to 1.) Upon execution,
the MRI will operate on the contents of the location identified by the
address contained in the pointer location.
The two examples in Figure 2-4 illustrate the difference between
direct addressing and indirect addressing. The first example shows a
TAD instruction that uses direct addressing to get data stored on page 0
in location 50; the second is a TAD instruction that uses indirect ad-

dressing, with a pointer on page 0 in location 50, to obtain data stored
in location 1275. (When references are made to them from various
pages, constants, and pointer addresses can be stored on page 0 to 'avoid
the necessity of storing them on each applicable page.) The octal value

0); the
1050,. in the first example, represents direct addressing (bit 3
octal value 1450, in the second example, represents indirect addressing
(bit 3: 1). Both examples assume that the accumulator has previously
:

been cleared.
Location

Content
TAD

200

(TAD 50

\Address

501275

QDam
,

‘

1275

10503)

Instruction

(Number) To Be Acted Upon By

Instruction Address

(Content of location 1275 is not used in

the execution of the instruction in location 200.)
1275 after executing the instruction in loca-

NOTE: AC :
tion 200.

LoCation

Content

200

TAD I 50

50
.

\Pointer
Designates Indirect Addressing
Instruction

(TAD I 50
14505)
Address

1275

Q
.

‘1275

Effective Address

Pointer Address

\Data (Number) To Be Acted Upon By
Instruction
’

NOTE: AC

Effective Address
20 after executing the instruction in location-

200-

Figure 2-4.

Comparison of Direct and Indirect Addressing

_2-16

following three examples illustrate some additional ways in
which indirect'addressing can be used. As shown in example 1-,- indirect
addressing makes it possible to transfer program control ofi'page 0 (to
any desired memory location). (Similarly, indirect addressing makes-it
possible for other memory reference instructions to address any of the
4,09610 memory locations.) Example 2 shows a DCA instruction that
uses indirect addressing with a pointer 'on the current page. The pointer
in this case designates a location off the current page (location 227) in
which the data is to be stored. (A pointer address is normally stored on
the current page when all references to the designated location are from
thecurrent page.) Indirect addressing provides the means for returning
to a main program from a subroutine, as shown in example 3. Indirect
addressing is also effectively used in manipulating tables ‘of data as described and illustrated in conjunction with autoindexing in Chapter 3.
The

EXAMPLE 1
Content

Location
75

JMP

100

6000
~

/
.

6000

DCA

(JMPI 1002550011)

Pointer Address
Designates Indirect Addressing

Instruction

Address

Effective
Pointer Address

100

‘

K Next Instruction To Be Executed
‘

Effective Address

NOTE: Execution of the instruction in location 75 causes pro-.
‘gram control to be transferred to location 6000, and
the next instruction to be executed is the DCA 6100
instruction.

EXAMPLE 2

[N
’

Location

Content

450

j DCA 1' 577

577

;

'(DCA1577: 3777,.)
Pointer Address

Designates Indirect Addressing
Instruction

N \1

\ Effective Address
.

227
‘

Pointer Address

nnnn\
\Data
'

(Number) Stored By
Effective Address

Instruction

NOTE: Execution of the instruction in location 450 causes the
contents of :the accumulator to be-stored- 'in-Iocation
‘

227.

2-17_

EXAMPLE 3
Location

Content

207
210

TAD 250

(JMS I 70 ‘: 4470“)
(The next instruction to be executed
upon return from the subroutine.)

2000

“(Starting address of the subroutine

JMS

stored here.)
2000
200i

(Return address stored here by JMS
instruction.)
(First instruction of subroutine.)

aaaa

‘

2077
NOTES:

iii

J MP I 2000

(Last instruction of subroutine.)

Execution of the instruction in location 207 causes
the address 210 to be stored in location 2000 and
the instru‘ction in location 2001 to be executed
next. Execution of the subroutine proceeds until
the last instruction (JMP I 2000) causes control
to be transferred back to the main program, continuing with the execution of the instruction stored
in location 210.

A JMS instruction that uses indirect addressing is
useful when the subroutine is too large to store on
the current page.

Storing the pointer address on page 0 enables instructions on various pages to have access to the
subroutine.

OPERATE MICROINSTRUCTIONS

The operate instructions (octal operation code 2: 7) allow the pro—

manipulate and/ or test the data that is located in the
accumulator and link bit. A large number of different instructions are
possible with one operation code because the operand bits are not
needed to specify an address as they are in an MRI and can be used to
specify different instructions. The operate instructions are separated
into two groups: Group 1, which contains manipulation instructions,
and Group 2, which is primarily concerned with testing operations.
Group 1 instructions are discussed first.
grammer to

Group 1 Microinstrucfions
The Group 1 microinstructions manipulate ”the contents of the accu—
mulator and link. These instructions are microprogrammable; that is,
they can be combined to perform specialized operations with other
Group 1 instructions. Microprogramming is discussed later in this
chapter.
‘

2-18

I2345678'9I0II
O

CLA CLL CMA CML RAR RAL

OPERATION
CODE

Q'I ROTATE ONE PLACE
I I ROTATE TWO PLACES

ZERO SPECIFIES

‘% IAC

GROUP |

The preceding diagram illustrates the manner in which a PDP-8 in-

struction word is interpreted when it is used to represent'a Group 1

operate microinstruction. As previously mentioned, 78 is the operation
code for operate microinstructions; therefore, bits 0 through 2 are all
1’s. Since a reference to core memory is not necessary for the operation
of microinstructions, bits 3

through 11 are not used to reference an
address. Bit 3 contains a 0 to signify that this is a Group 1 instruction,
and the remaining. bits are used to specify the operations to be performed by the instruction. The operation of each individual instruction
specified by these bits is described below.
-

Clear the accumulator. If bit 4.~is a 1, the

CLA

instruction sets

the accumulator to all zeroes.
CLL

Clear the link. If bit 5 is-a 1, the link bit is set to 0.
‘

Complement the accumulator. If bit 6 is a 1, the accumulator is set to the 1’s complement of its original value; that
is, all 1’s become 0’s, and all 0’s become 1’s.
Complement the link. If bit 7 is a 1, the state of the link bit

CMA

CML

is reversed.

RAR

‘

Rotate the-accumulator and link right. If bit 8 is a 1 and

bit 10 is a O, the instruction treats the AC and L as a closed
.

loop and shifts all bits in the loop one position to the right.
This operation is illustrated by the following diagram.
Ac

L
'

RTR

-l

BEFORE RAR

AFTER RAR

Rotate the accumulator and link twice right. If 'bit 8 is a 1’

and'b'it 10 is also a 1, asth of two places to the right is
executed. Both the ‘RAR and RTR instructions use what is

commonly called a circular shift, meaning that any bit
rotated offbne end of the accumulator will reappear at the
other end. This operation is illustrated below.
2—19

BEFORE RTR

l
I

AFTER RTR

RAL

Rotate the accumulator and link left. If bit 9 is a 1 and bit
10 is a 0, this instruction treats the AC and L as a closed

loop and shifts all bits in the loop one position to the‘left,
performing a circular shift to the left.
Rotate the accumulator and link twice left. If bit 9 is a 1
and bit 10 is a 1 also, the instruction rotates each bit two
positions to the left. (The RAL and RTL microinstructions
shift the bits in the reverse direction of that directed by the

RTL

RAR and RTR microinstructions.)
IAC

Increment the accumulator. When bit _11 is a

I, the con—
I

tents of the AC is increased by 1.

No operation. If bits 0 through 2 contain operation code

NOP.

78, and the remaining bits contain zeros, no operation is

performed and program control is transferred to the next
instruction in sequence.
A summary of Group 1

instructions, including their octal forms, is

given below.
Mnemonic1

Octal2

NOP

.7000

CLA
CLL
CMA
CML
RAR
RAL

7200

RTR
RTL
IAC
1 Mnemonic

code.

7100

7040
7020
7010
7004 A
7012
7006
7001

Sequence?-

Operation
No operation

Clear AC
Clear link bit
Complement AC

Complement link bit
Rotate AC and L right one position

Rotate AC and L left one ”position

Rotate AC and L right two positions

Rotate AC and L left two positions
Increment AC

code is meaningful to and translated

by an assembler into binary

2. Octal numbers conveniently represent binary instructions.
3
Sequence numbers indicate the order in which the operations

are

performed-

by the PDP~8/ I and PDP-S/ L (sequence. 1 operations arcrperformed first,
sequence 2 operations are performed next, etc..).
‘

2-20

Groupz Microinstructions
Group 2 operate microinstructions are often referred to as the “skip
microinstructions” because they enable the programmer to perform
tests on the accumulator and link and to skip the next instruction de—
pending upon the results of the test. They are usually followed in a program by a JMP (or possibly a JMS) instruction. A skip instruction
causes the computer to check for a specific condition, and, if it is pres—
ent, to skip the next instruction. If the condition were not present, the
-

‘

next instruction

would

executed.

CLA

OPERATION
CODE

SZA

SNL

O/I

SPA

SNA

SZL

SKP

VALUE OF BIT 8
DETERMINES THE
ACTION SPECIFIED
BY BITS 5 6,8 7

CONTAINS A I
To SPECIFY

/
/

OS R HLT

CONTAINS A 0
'

To SPECIFY

GROUP 2

REVERSE SENSING BIT
O. SMA SZA BISNL ARE ENABLED

:,SPA SNA aSZL ARE ENABLEO
(UNCONDITIONAL SKIP WHEN
BITS 5,6,8 7 ARE O’S)

The available instructions are selected by bit assignment as shown in
the above diagram. The operation of each individual instruction

fied by these bitsis deScribed below.

CLA

Clear the accumulator If bit 4 is a 1, the instruction sets
the accumulator to all zeros

SMA

speci-

‘

Skip on minus accumulator. If bit 51s a l and bit 8IS a O,
the next instruction is skipped if the accumulator is less
than zero.

SPA

Skip on positive accumulator. If bit 5 is a 1 and bit 8 is a
l, the next instruction is skipped if the accumulator is
greater than or equal to zero.

SZA

Skip on zero accumulator. It hit 6 is a 1 and bit 8 is a 0,
the next instruction is skipped if the accumulator is zero.

SNA

Skip on nonzero accumulator. 1f bit 6 is a 1 and bit 8 is a
1 also, the next instruction is skipped if the accumulator is
not zero.

2-2 1

SNL
SZL

‘

SKP

;

OSR

Skip on nonzero link. If bit 7 is a 1 and bit 8 is a 0, the
next instructidn is skipped when the link bit is a 1.
Skip on zero link If bit 7 is a 1 and bit 8 is a 1, the next
instruction is skipped when the link bit'is a O.
Unconditional skip. If bit 8 is a 1 and bit 5, 6 and 7 are
all zeros, the next instruction is skipped. (Bit 8 is a reverse
sensing bit when bits 5, 6 or 7 are used—see SMA, SPA,
SZA, SNA, SNL, and SZL above.)
Inclusive OR of switch register with AC. If bit 9 is a 1, an

inclusive OR operation is performed between the content
of the accumulator and the console switch register. The re—

sult is left in the accumulator and the original content of
the accumulator is destroyed. In short, the inclusive OR

operation consists of the comparison of the corresponding
bit positions of the two numbers and the insertion of a 1 in
the result if a 1 appears in the corresponding bit position
in either number. See Chapter 1 for further discussion. The
action of the instruction’ is illustrated below.

LINKD [ilololololnloli[OIIIOIIJACCUMULATOR
[1 [ololllolllolojolill [OJSWITCHREGISTER

LINKE
HLT

[ilololiloliloTqulxlil'aJResuumAc

Halt. If bit 10 is a 1, the computer will stop at the conclu—
sion of the current machine cycle.

A summary of Group 2 instructions, including their octal representa-

tion, is given in the following table.
"
'

"Mnemonic

"Octal

CLA
SMA
SPA»

7600
7500

Clear the accumulator
Skip on minus accumulator

7510

Skip

Sequence

‘

Operation

positive accumulator

(or AC : 0)
SZA

7440

SNA

7450
7420

SNL

SZL

SKP
OSR

7430
7410
7404

Skip on zero accumulator
Skipon nonzero accumulator
Skip on nonzero link
Skip on zero link
Skip unconditionally
Inclusive OR, switch register
with AC

HLT

7402

Halts the program

2-22

1
.

1
1

MICROPROGRAMMING
BecausePDP-S instructions of Group 1 and Group 2 'are determined
by bit assignment, these instructions maybe combined, or microprogrammed, to form new instructions enabling the computer to do more
operations in less time.
l"

—

Combining Microinstructions
The programmer should make certain that the program clears. the
accumulator and link before'any arithmetic operations are performed.
To perform this task, the program might include the followinginstructions (given in both octal and mnemonic form).
7200 (octal)
7100 (octal)

CLA
CLL
_

However, when the Group 1 instruction format is analyzed, the follow—
'

ing is observed.

CLA C'LL

OPERATION
CODE

LMUST

BE A I TO

SPECIFY CLL
'

'
-

MUST BE A ITO SPECIFY CLA
MUST BE A 0 TO SF‘ECIEY GROUP |

Since the CLA and the CLL instructions occupy separate bit posi-

tions, they may be expressed in the same instruction, thus combining
the two operations into one instruction. This instruction would be written as follows.

CLA CLL

7300 (octal)

In this manner, many operate microinstructions can be combined mak-

ing the execution of the program much more efficient. The assembler
for the PDP-8 will combine the instructions properly when they are
written as above, that is, on the same coding line, and separated by a
space.

Illegal Combinations

Microprogramming, although very efficient, can also be troublesome
for the new programmer. There are many violations of coding which

the assembler will not accept.
2-23

One rule to remember is: “If you can’t code it, the computer can’t do

it.” In other words, the programmer could write a string of mnemonic

microinstructions, but unless these microinstructions can be coded correctly in octal representation, they cannot be performed. To illustrate
this fact, suppose the programmer would like to complement the accu:
mulator (CMA), complement the link (CML), and then skip on a
nonzero link (SNL). He could write the following.
CMA CML SNL

These instructions require the following bit assignments.
O

CMA

CML

SNL

The three microinstructions cannot be combined in one instruction because bit

3 is required to be a 0 and a 1 simultaneously. Therefore, no

instructions may be used which combine Group 1 and Group 2 microinstructions because bit 3 usage is not'compatible. The CMA and CML“
can,

however, be combined because their bit assignments are com-

patible. The combination would be as follows.
CMA CML

7060 (octal)

To perform the original set of three

operations, two instructions are

‘

needed.
CMA CML

SNL

7060 (octal)
7420 (octal)

Because

Group 1 and Group 2 microinstructions cannot be combined, the commonly used microinstruction CLA is a member of both
groups. Clearing the AC is often required in a program and it is very
convenient to be able to microprogram the CLA with the members of
i

‘

both groups.

The problem of bit assignment also arises when some instructions
within a group are combined. For example, in Group 1 the rotate instructions specify the number of places to be rotated by the state of bit

10. If bit 10 is a 0, rotate one place; if bit 10 is a 1, rotate two places.

Thus, the instruction ,,RAL can not be combined with RTL because bit
10 would be required to have two difierent,_values at once. If the pro2-24

grammer wishes to rotate right three places,'he must use two separate
instructions.
RAR
RTR

,
.

7010 (octal)
7012

(octal)

Although he can write the instruction “RAR RTR”, it cannot be correctly converted to octal by the assembler because of the conflict1n bit
10; therefore, it is illegal.

Combining Skip Microinstructions
Group 2 operate microinstructions use bit 8 to determine the instruc-_
tion specified by bits 5, 6, and 7 as previously described. If bit 8 is a 0,
the instructions SMA, SZA, and SNL are specified. If bit 8 is a l, the
instructions SPA, SNA, and SZL are specified. Thus, SMA cannot be
combined with SZL because of the opposite values of bit 8. The skip
condition for combined microinstructions is established‘by the skip conditions of the individual mcroinstructions in accordance with the rules
for logic operations (see “Logic Primer” in Chapter 1).
0R GROUP—SMA OR SZA 0R SNL
If bit 8 is a 0, the instruction skips on the logical OR of the condi—
tions specified

by the separate microinstructions. The next instruction

is skipped if any of the stated conditions exist. For example, the com—
bined microinstruction SMA SNL will skip under the following conditions:
1. The accumulator is

negative, the link is

zero.

2. The link is nonzero, the accumulator is not negative.

3. The accumulator is negative and the link is nonzero.

(It will not skip if all conditions fail.) This manner of combining the
test conditions is described as the logical OR of the conditions.
AND GROUP—SPA AND SNA AND SZL

specifies the group of microinstructions SPA,
'SNA, and SZL which combine to form instructions which act according
to the logical AND of the conditions. In other words, the next instruction is skipped only if all conditions are satisfied. For example, the instruction SPA SZL will cause a skip of the nextinstruction only if the
accumulator is positive and the link is zero. (It will not ski-p if either
A value of bit 8: 1

of the conditions fail )
programmer is not able to specify the
NOTES 1. The
of combination. The SMA, SZA, SNL conditions

manner

are always combined by the logical OR, and the
SPA, SNA, SZL conditions are always joined by a

logical AND.
2.

Since the SPA microinStruction will skip on either

positive or a zero accumulator, to skip on a
strictly positive (positive, nonzero) accumulator
a

the combined microinstruction SPA SNA is used.

2-25

Order of Execution of Combined Microinstructions
'

The combined microinstructions are performed by the computer in a

very definite sequence. When written separately, the order of execution
of the instructions is the order in which they are encountered in the pro:
'

gram. In writing a combined instruction of Group 1 or Group 2 micro—

instructions, the order written

has

bearing upon the

order of

execution. This should be clear, because the combined instruction is a
12—bit binary number with certain bits set to a value of .1. The order in
which the bits are set to i has no bearing on the final execution of the
I

whole binary word.
The definite

sequence, however, varies between members of the

PDP-S computer family. The sequence given here applies to the PDP-

8/1 and PDP-S/L. The applicable information for other members of
p,

the PDP-8 family is given in Appendix E. The order of execution for
PDP—8 / I and PDP-8/ L microinstructions is as follows.

GROUP 1

CLA, CLL—Clear the accumulator and/or" clear the
link are the first actions performed. They are effectively

Event 1
.

performed simultaneously and yet independently.
CMA, CML—Complement the accumulator and/ or complement the link. These operations are also effectively
performed simultaneously and independently.
IAC—Increment the accumulator. This operation is performed third allowing a number in the AC to be complemented and then incremented by 1, thereby forming the
two’s complement, or negative, of the number.

Event 2

Event 3

RAR, RAL, RTR, RTL—The rotate instructions are per-

Event 4

formed last in sequence. Because of the bit assignment

previously discussed,‘only one of the four operations may
be performed in each combined instruction.
I

GROUP 2
Either SMA or SZA or SNL when bit 8 is a 0. Both SPA

Event 1

and SNA and SZL when hit 8 is a 1. Combined micro-

specifying a skip are performed first. The
microinstructions are combined to form one specific test,
therefore, skip instructions are efiectively performed
simultaneously.
Because of bit 8, only members of one skip group may be

instructions

‘

combined in an instruction.
-

2—26

Event 2
'

CLA—Clear the accunfulator. This instruction 'is performed second in sequence thus allowing different arith-

meticoperations to be performed after testing (see Event
1) without the necessity of clearing the accumulator with
a separate instruction before some subsequent arithmetic
operation.
OSR—Inclusive OR between the switch register and the
AC. This instruction is performed third in sequence,
..

‘

Event 3
,

allowing the AC to be cleared first, and then loaded from
,

:Event 4

the switch register.
HLT—The HLT

is performed last to allow any other

operations to be concluded before the program stops.

This is the order in which all combined instructions are performed.

operations in a different order, the instructions
must be written separately as shown in the following example. One
might think that the following combined microinstruction would clear
the accumulator, perform an inclusive OR between the SR and the AC,
and then skip on a nonzero accumulator.
In order to perform

CLA OSR SNA

However, the instruction would not perform in that proper manner,
because the SNA wOuld be executed first. In order to perform the skip
last, the instructions must be separated as follows.
CLA OSR
SNA

Microprogramming requires that the programmer carefully code
mnemonics legally so that the instruction does in fact do what he desires
it to do. The sequence in which the operations are performed and the
legality of combinations is crucial to PDP—S programming.
The following is a list of commonly used combined microinstructions,
some of whichhave been assigned a separate mnemonic;
'

Instruction

Explanation

CIA

CLA CLL
CMA IAC

Clear the accumulator and link.
Complement and increment the accumulator.
(Sets the accumulator equal to its own ‘nega—

LAS

CLA OSR

Load accumulator from switches.
(Loads the accumulator with the value of the

CLL CML
CLA IAC
CLA CMA

Set the link (to a 1).

"—

tive.)
~

switch register.)

STL
*'

——-

Sets the- accumulator to a 1.
Sets the accumulator to a —1.

2-27

In summary, the basic rules for combining operate microinstructions

are given below.
1.

Group 1 and Group 2 microinstructions cannot be combined.
(Group 1) cannot be Combined with

2. Rotate microinstructions

each other.
3. OR Group (SMA, SZA, or SNL) microinstructions cannot be

combined with AND GroupJSPA, SNA,
structions.

4. OR Group microinstructions

are

SZL) microin—

combined as the logical OR

of their respective skip conditions. AND Group microinstructions are combined as the logical AND of their respective skip

conditions.
5. Order of execution for combined instructions (PDP-8/ I and
PDP-S / L only) is listed below.
'

Group 2

Group 1
l.

CLA,’ CLL

CmdA,CmdL

IA“:

SMA/ SZA/ SNL or
SPA/ SNA/ SZL
(flak
()SR.

ILAR,RIU;I§TR,RJT,

IiLT

EXERCISES
.1.

The following is

list of current addresses and locations to be

addressed. Determine whether the second location should be di-

rectly or indirectly addressed from the first.
Current Address
a.

2456

Location to be Addressed

2577

1500

1600

1230
0050

0030

6555
6555

6400
6600

d.
e.

01 20

4343

h.
i.

2742

4100
2450

2507

5507

3200

3377

2-28'

2. ”What.

type of instruction is each of the following (MRI, operate

Group 1 or operate Group 2 microinstruction)?
7430

19‘s»
rm
3.

7240
7000
4706
7700

Why are each of the following not legal instruCtions for the PDP-S?
a.

4._

0024

15007

6509

1581

635

7778

What is the effect of each of the following octal instructions?

Octal

M nemonic

Operation

0000

9‘.” 4010'
2300
9.0 1777
3500
rm 5400
1030

r'r‘P‘w

2577,

5273
3150

Separate the following octal instructions into

microinstruction

mnemonics.

7260

9“.» 7112
7440
11.0 7632
7550
rm 7007
<20

7770

Write the octal

representation‘for each-location in the following

program. What are the contents of the accumulator and locations
205, 206, and 207 after execution of the program?
Location.

Mnemonic

0200

CLA
TAD 0205
TAD 0206

0201

0202

0203
0204
0205
0206

DCA 0207
HLT

0207

0000

1537
2241

2-29

Octal

Write the octal form of the following microinstructions. Identify

any illegal combinations.
CLA CLL CMA CML
CLL RTL HLT

9‘?
9-9
5‘?

SPA CLA
CLA IAC RTL
CLA IAC RAL RTL
SMA SZA CLA
SMA SZL

*‘F'PWP
.8.

CLA OSR HLT
CLA OSR IAC
CLA SMA SZA

What instructions could be used to

perform a skip only if the

accumulator is zero and the link is nonzero?
9.

10.

Why is it not possible to write one combined microinstruction that
will load the accumulator from the console switch register, and
then test that number, skipping on a positive value?
Write the following programs.
a. Program starts in location 0200 and adds 2 and 8. Give both
mnemonic and octal representations.
b. Program beginning in location 400 which interchanges the con-

tents of locations 550 and 551. Give both mnemonic and octal

representations.
ll.

Write programs to add three numbers A, B, and C in the specified
locations below and put the result in the

given address for the

SUM. All programs start in location 200. Give octal and mnemonic

coding.
.A

SUN!

0030

0031

0032

0033

0300

0301

0302

0303

3000

3001

3002

3003

2—30

elementary
programming
techniques
‘

Mastery of the instruction set is the first step in learning to‘program
the PDP—8 family“ computers. The next step is to learn to use the in?
struction set to obtain correct results and to obtain them efficiently. This
is best done by studying the following programming techniques. Exam—
ples, which should further familiarize the reader with the instructions
and their uses, are given to illustrate each technique.
The modern digital computer is capable of storing information, per-

forming calculations, making decisions based on the results and arriving
at a final solution to a given problem. The computer cannot, however,
'

perform these tasks without direction. Each step which the computer is
to perform must first be worked out by the programmer.

The programmer must write a program, ~which is a list of instructions

for the computer to follow to arrive at a solution for a given problem.
This list of instructions is based on a computational methOd, sometimes

called an
-

algorithm, to solve the problem. The list of instructions is
placed in the computer memory to activate the applicable circuitry so
that the computer can process the problem: This chapter describes the
procedure to be followed when writing a program to be used on the
PDP-8 family of computers
3-1

PROGRAMMING PHASES
In order to successfully solve a problem with a computer, the programmer proceeds through the five programming phases listed below:
1. Definition of the problem to be solved,

S‘P‘H’P

Determination of the most feasible solution method,
Design and analysis of the solutiOn—flowcharting,
Coding the solution in the programming language, and
Program checkout.

The definition of the problem is not always obvious. A great amount
of time and energy can be wasted if the problem is not adequately de-

fined. When the problem is to sum four numbers,'the defining phase is
obvious. However, when the problem is to monitor and control a per—

formance test for semiconductors, a precise definition of the problem
is necessary. The question that must be answered in this
“What precisely is the program to accomplish?”

phase is:

Determining the method to be followed is the second important
phase in solving a problem with a computer. There are perhaps an infinite number of methods to solve a problem, and the selection of one
method over another is often influenced by the computer system to be
used.

Having decided upon a method based on the definition of the
problem and the capabilities of the computer system, the programmer
must develop the method into a workable solution.
The programmer must design and analyze the solution by identifying
the necessary steps to solve the problems and arranging them in a
logical order, thus implementing the method. Flchharting is a graphical

representing the logical steps of the Solution. The flowcharting
technique is effective in providing an overview of the logical flow of a
solution, thereby enabling further analysis and evaluation of alternative
approaches.
means of

”

Having designed the problem solution, the programmer begins coding
the solution in the programming language. This phase is commonly
called programming but is actually coding and is only one part of the
programming process. When the program has been coded and the pro—
gram instructions have been stored in the computer memory, the 'problem can be solved. At this point, however, the programming process
is rarely complete. There are very few programs written which initially
function as expected. Whenever the program does nOt work properly,
the programmer is forced to begin the fifth step of programming, that

of checking out or “debugging” the program.

3—2

The program checkout phase requires the programmer to methodi—

cally retrace the flow of the instructions step—by-step to find any. program errors that may exist. The programmer cannot 'tell a computer:
“You know what I mean!”, as he might say in .daily life. The computer.

does not know what is meant until it is told, and once given a set of

instructions, the computer follows them precisely. If needed instructions.
are left out orcoding is done inCorrectly, the results may be surprising.
These flaws, or “bugs” as they are often called, must be found and
corrected. There are many different approaches to finding bugs in a—
program; however, the chosen approach must be organized and pains—
takingly methodical if it is to be successful. Several techniques for de—
bugging programs for the PDP-8 family of computers are described
in Chapter 6.
7

FLOWCHARTING
'

A simple problem to add three numbers together is solved in a few,

easily determined steps. A programmer could sit at his desk and Write
out three or four instructions for the computer to solve the problem.
However, he probably could have added the same three numbers with,
paper and pencil in much less time than-it took him to write the program. Thus, the problems which the programmer is usually asked to
solve are much more complex than the addition of three numbers,'b.ecause the value of the computer is in the solution of problems which are
inconvenient or time consuming by human standards.

When a more complex problem is to be solved by a computer, the
program involves many steps, and writing it often becomes long and".
confusing. A method for solving a problem which is written in words
and mathematical

equations-is extremely hard to follow, and coding
computer instructions from 'such a document would be equally difficult.
A technique called flowcharting is used to simplify the writing of pro—
grams; A flowchart is a graphical representation of a given problem,
indicating the logical sequence of operations that the computer is to»
perform. Having a diagram of the logical flow of a program is a tree,
mendous advantage tothe programmer when he is determining the
method toflbe used for solving a problem, as well as when he writes
the coded program instructions. In addition, the flowchart is often a
valuable aid when the programmer checks-the written program for
errors.

3—3

The flowchart is basically a collection of boxes and lines. The bOXCS
indicate what is to be done and the lines indicate the sequence of the
boxes. The boxes are of various shapes which represent the action to
be

performed in the program. Appendix C is a guide to the flowchart

symbols and procedures which are used in this text.
The following are examples of flowcharts for specific problems, illus—H'

trating methods of attacking problems with a computer program as well
as illustrating floweharting techniques. Example 1 adds three numbers
together. Example 2 puts three numbers in increasing order.

Example 1
Example

—

Straight-Line Programming

1 is

illustration of straight-line programming. As the

flowchart shows, there is a straight—line progression through the process-

ing steps with no change in course. The value of X, which is equal to
A+B+C is in the accumulator when the program stops.

CLEAR
ACCUMULATOR

l
GET A mro

ACCUMULATOR

l
ADD 8

l
ADD C

Example I

—-

Add Three Numbers

Program Branching
Example 2
Example 2 is designed to arrange three numbers in increasing order.
——-

The program must branch to interchange numbers that are out of
order. (Branching, a common feature of programming, is-dcscribed in

detail later in this chapter.) Note that the arithmeticoperations-of sub—
traction are done in the accumulator, which must'be cleared initially.

3-4

‘

START

CLEAR THE
ACCUMULATOR

GET FIRST
NUMBER INTO
ACCUMULATOR

I
SUBTRACT
SECOND NUMBER

IS AC
~

POSITIVE
?

INTERCHANGE
ISTANDZND
NUMBERS
COMPARE
2ND AND 3RD
NUMBERS
As ABOVE

I
_

IS Ac
POSITIVE
?

INTERCHANGE

2ND 'AND 3RD
NUMBERS

COMPARE
IST AND 2ND
NUMBERS
AS ABOVE

'
-

‘

IS AC

POSITIVE
:2

INTERCHANGE
1SrAN02ND
NUMBERS

Example 2 ‘———- Arrange Three Numbers in Increasing Order
3-5

CODING A PROGRAM
The introduction of

Chapter 2' enabled the programmer to write a symbolic program using meaningful mnemonic codes
rather than the octal representation of the instructions. The programmer
could now write mnemonic programs such as the following example,
which multiplies 1810 by 3610 using successive addition.
,

200/
201/

TAD 210

202/

CIA

203/
204/
205/
206/
207/
210/

DCA 212

assembler in

CLA CLL

(Initialize)
(Set up a Tally
equal to —1 810 to
count the additions of 36)

TAD 211

182 212
J MP 204
HLT
0022

21 1/

0044

212/

0000

(Add 36)
(Skip if Tally is 0)
(Add another 36 if not done)
(Stop after 18 times)
(Equal to 1810)
(Equal to 36m)
(Holds the tally)

Writing the above program was greatly simplified because mnemonic
codes were used for the octal instructions. However, writing down the
absolute address of each instruction is clearly an inconvenience. If the
programmer later adds or deletes instructions, thus altering the location
assignments of his program, he has to rewrite those instructions whose
operands refer to the altered assignments. If the programmer wishes to
move the program to a-different section of memory, he must rewrite the

program. Since such changes must be made often, especially in large
programs, a better means of assigning locations isneeded. The assem—

bler provides this better means.

Location Assignment
As in the previous program example, most programs are written in

successive memory locations. If the programmer assigned an absolute
location to the first instruction, the assembler could be told to assign
the next instructions to the following locations in order. In programming
the PDP-8, the initial location is denoted by a precedent asterisk (*).
The assembler maintains a current location counter By which it assigns
successive locatiOns to instructions. The asterisk

causes

the current

location counter to be set to the value following the asterisk. With this
as shown
improvement incorporated, the previous example
appears
in the following example.
’

13—6

*200

‘CLA_CLL,
134r) 210
CLA
I)Cv\ 212
THAI) 211
ISZ 212.
114? 204

IILT‘
‘

0022

0044

0000

NOTE: In this example, CLA CLL is stored in location
successive instructions are stored1n 201, 202, etc.

Symbolic Addresses
The programmer does not at the outset

200 and the

know Which locations he will

use to store constants or the tally. Therefore he must leave blanks after
each MRI and come back to fill these in after he has assigned locations

tolth'ese numbers. In the previous program, he must count the number'
of locations after the

assigned initial address in order to assign the
correct values to the MRI operands. Actually this is not necessary, because he may assign symbolic names (a symbol followed by a comma
is a symbolic address) to the locations to which he must refer, and the

assembler will assign address values for him. The assembler maintains

symbol table in which it records the octal values of all symbolic
addresses. With symbolic address name tags, the program is as shown
a

below.
*200

START,

CLA CLL
TAD A
CIA
DCA TALLY
TAD B

MULT,

ISZ TALLY
J MP MU'LT
HLT

A,
B,
TALLY,
$

0022
0044
0000

NOTES: 1. The dollar sign is the terminal character for the assembler.
1

2. The

comma after a symbol (e. g.,
START,) indicates to the
assembler that the symbol is a symbolic address.

3-7

‘

Symbolic Programming Conventions
Any sequence of letters (A, B, C

Z) and digits (0, 1,,

9)
beginning with a letter and terminated by a delimiting character (see
Table 3-1) is a symbol. For example, the mnemonic codes for the
.

PDP—8 instructions are symbols for which the assembler retains octal

equivalents in a permanent symbol table.
User—defined symbols (stored in the external symbol table) may be
of any length; however, only the first six characters are considered, and
any additional characters are ignored. (Symbols which are identical in
their first six characters are considered identical.)

Any sequence of digits followed by a delimiting character forms a
number. The assembler will accept numbers which are octal or decimal.
The radix is

initially set to octal and remains octal unless otherwise
specified. The pseudo-instruction DECIMAL may be inserted in the»
coding to instruct the assembler to interpret all numbers as decimal until
the next occurrence of the pseudo-instruction OCTAL in the coding.
These pseudo-instructions affect all numbers included in the symbolic
*
to denote change of origin.
program including those preceeded by an
Each symbol or number written in a PDP-8 program must represent
a

12—bit binary value in order to be interpreted by the assembler.
The special characters in Table 3-1 are used to specify operations to

be performed by the assembler upon symbols or numbers in PDP-8

symbolic programs.
The comma after a symbol in a line of coding (e. g., MULT, TAD B)

indicates to the assembler that the value of. MULT is the address of
the location in which the instruction is stored. When an instruction that
references MULT (now a symbolic address) is encountered, the assembler supplies the correct address value for MULT. (Care must be taken
that a symbolic address is never used twice in the same program and

that all locations referenced by an MRl are identified somewhere in the

program.)
The space and tab are used to delimit a symbol or number. In a com-

bined microinstruction such as CLA CLL, the space delimits the first
mnemonic from the second, and the assembler combines the two mne—
monic into one instruction. The space and tab similarly delimit the mne'

monic from the symbolic address.
TAD A
'

SPACE

3—8

TAD

"‘— CTRL/TAB.

Table 3-1

Special Characters for the PDP-8 Symbolic Language

Character

Keyboard
'

SPACE

U se

Name

combine symbols or numbers

space

(delimiting)

(nonprinting)

CTRL/ TAB

combine symbols or numbers or format the symbolic tape (delimiting)

carriage return
(nonprinting)
plus

terminate line (delimiting)

minus

—

comma

tab (nonprinting)

RETURN

combine symbols or numbers
combine symbols or numbers
assign symbolic address

equals

define parameters

asterisk
semicolon
dollar sign

set current location counter
terminate coding line (delimiting)
terminate pass (delimiting)
has value equal to current location

‘;

point

counter

slash

indicates start of a comment

The carriage return is 'used to terminate a line of coding: The assembler will also recognize a semicolon as a line terminating character.
‘

$23 3

is the same as

One of these two characters
‘

must be

TAD A; TADB

(i.e., semicolon or carriage return)

used to separate each line of coding.

The assembler will recognize the arithmetic

in
symbols + and
conjunction with numbers or symbols, thereby enabling “address arith—
metic”. For example, the instruction JMP START—H will cause the
—

computer to execute the instruction in the next location after START.
The numbers specified in such instructions are subject to the pseudo—

f instructions DECIMAL and OCTAL, therefore the number is inter—-'
l

,preted as an octal number unless the pseudo-instruction DECIMAL is
in effect.
The decimal point, or period, is a character which is interpreted by,

the assembler as the value of the current location counter. This special

symbol can be used as the operand of an instruction; for example, the
instruction J MP .—1 causes the computer to execute the preceding in-

struction.

The equal sign is used to define symbols. This character is used to

replace an undefined symbol with the value of a known quantity. For
example, the programmer could define a “new instruction” NEGATE
3-9

by writing that NEGATE— CIA The programmer could then write
_.

the following instructions to subtract B from A.
TAD B

START,

NEGATE
TAD A
NEGATE

HLT
CIA

The above coding would be assembled as if the instruction CIA had
been included in the‘ actual coding.
The slash is used to insert comments and headings as described later
in this chapter.
The dollar sign as previously noted, is a terminal character for the
assembler itself. When this character is

encountered, the assembler
stops accepting input and terminates the assembly pass, as described in
Chapter 6.
These characters and conventions will be used throughout the

re-

mainder of this text to code programs in PAL III, the symbolic lan—
guage of the PDP-8 family of computers. Thus, all examples given may
be directly punched on paper tape as described in Chapter 4 and

assembled by the procedure described in Chapter 6.

PROGRAMMING ARITHMETIC OPERATIONS
The instructions for the PDP—8 may be used to perform the basic
arithmetic operations within the limits of the machine to represent the
necessary numbers. That is, numbers may be added unless the sum ex:
ceeds 409510 or 77778. When a sum exceeds the size of the accumulator,

overflow occurs and incorrect answers result, This
usually be detected by checking the value of the link bit;

condition

can

The following instructions will add numbers and check for overflow,

halting the program if the link is 1.
ADD,

CLA CLL
TAD A
TAD B
SZL
HLT
DCA SUM

3-10

Since the link is initially cleared in the above example, a link value

equal to 1 is an indication that the sum of the contents of locations A
and B is too large to be represented by the 12-bit accumulator alone.
The computer will halt if the overflow is detected with the actual sUm
in the combined 13 bits of the

accumulator and link.

Arithmetic Overflow
Since the PDP— 8 regards the numbers 0

through 37778

positive

numbers and the numbers 40008 through 77778 as negative numbers,

the addition of two positive numbers could result in either a positive

negative number depending upon the size of the numbers added.
Arithmetic overflow is said to occur whenever two positive numbers
add to form a negative 'number, as shown in the following example.

or a

(a positive number)
(a positive number)

24338
+22118

46448

(considered
PDP- 8)

negative number by the

Likewise, two negative numbers could be added to
ber as in the following example.

yield positive
a

num—

5275.

Disregarded—»1

3256

(—25038)

(considered

number

positive

PDP-8)

by' the
_

Because of situations like those illustrated in the, two preceding ex-

amples, the programmer must consider the size of the numbers used
inprogrammed arithmetic operations.- If the programmer suSpects that
overflow may occur in the result of an arithmetic operation, he should
follow such an operation by a set of instructions to correct the error or
at least to indicate that such an overflow occurred.

The conditions outlined below may be used to test for arithmetic

overflow.

Signs of Numbers Added

Overflow and Link Value

Positive + Negative
Positive +‘ Positive

No overflow possible; link value ignored.

Negative + Negative
'

‘

May result in negative sum; no change in
link value.
May result in positive sum; link is always

complemented regardless of the sign of the
result.

The program coding on the next page uses the following facts, as-

suming an initially cleared link, to quickly determine the sign of the
sum of two unknown quantities, A and B.
3-11

Result of Adding only Bit 0 of A mail of B

Sign

of A

of B

Positive

Negative

Positive

Positive
Positive

Negative

Link Value

Bit 0 of AC

O
_

/ CODING TO ADD TWO NUMBERS

/ TESTING FOR ARITHMETIC OVERF LOW.

START,

CLA CLL
TAD A
AND MASK

/MASK OUT ALL BUT BIT 0.
/ADD B TO BIT 0 OF A.
/LINK :2 I IMPLIES BOTH
/ARE NEGATIVE.
/ROTATE BIT O INTO LINK.
/BIT 0 = 1 IMPLIES
/OPPOSITE SIGNS.
/BIT 0 = 0, BOTH POSITIVE.

TAD B
SZL

OPPSGN,

BTHNEG,

JMP BTHNEG
RAL
SZL CLA
JMP OPPSGN
JMP BTHPOS
TAD A
TAD B
DCA SUM
HLT

/SIGNS, THE ADDITION
/ CANNOT RESULT IN
/OVERFLOW.

CLA CLL

TAD.A
TAD B

SMA
JMP NEGERR

BTHPOS,

/ IF A AND B ARE OF OPPOSITE

DCA SUM
HLT
TAD A
TAD B
SPA

/IF TWO NEGATIVE NUMBERS
/ADD TO FORM A
/POSITIVE NUMBER,
/JMP TO ERROR ROUTINE.
/OTHERWISE, STORE SUM.
‘

/IF TWO POSITIVE

/NUMBERS ADD TO FORM

JMP POSERR

DCA SUM
HLT

SUM,
MASK,

A,
B,
POSERR,

/A"NEGATIVE NUMBER, m1:
/TO ERROR ROUTINE.
/OTHERWISE, STORE SUM.

4000
/ANY NUMBERS A AND B

nnnn

/ROUTINE TO SIGNAL
/ARITHMETIC OVERFLOW
/OF POSITIVE NUMBERS.
/ROUTINE TO SIGNAL
/ARITHMETIC OVERFLOW
/OF NEGATIVE NUMBERS.

NEGERR,

3-12

Subtraction

Subtraction in the PDP-8 family of computers is accomplished by
_

negating the subtrahend (replacing it by its two’s complement) and
then adding it' to the minuend, ignoring the overflow if any. The following example shows the contents of the accumulator for each step of the
subtraction process.
Subtraction Program

Resulting Contents
Accumulator

Link

000 000 000

CLA CLL

000

TAD B

000 000 011

CMA

111

IAC

TAD A

1‘

100 001.
000 000 000 111

111

111
111

111

100 000

(0000)
(0037)
(7740)
(7741)
(0007)

A,
B,

0046

0037

(000 000 100 110)
(000 000 011 111)

(subtrahend) is brought into
the accumulator, complemented (1’s complement) and incremented by
1 (to form the 2’s complement). (The 2’s complement could be obtained directly through the .one microinstruction CIA.) The number
from which A is to be subtracted (minuend) is then added to the ac—
Note that the number to be subtracted

cumulator and the difference is obtained.
If A were already in the accumulator from‘a previous calculation, an

alternate procedure could be followed. The number A could be negated
first, then B added to it to get B—A. Negating this result yields the same
answer because
(B——A) is equal to A—B.
—

Multiplication and Division
A previous example illustrated the method of performing multiplica—
tion with the basic PDP- 8 instructions, namely by repeated addition.
Obviously, multiplication by this method is also subject to the limitation of overflow. The largest positive number which can be directly
represented is 20471,J or 37778.
Multiplication by repeated addition will properly handle positive and
negative numbers within the limits of positive or negative arithmetic
overflow. For example 77778 is the PDP—8 representation for —1. If it
is multiplied by itself the answer should be +1. In other words, adding
77778 to itself 77778 times should leave (after carries from the most

significant bit) the accumulator equal to 1.
3-13

‘

7777

1st

+7777
1

2nd

7776

+7777

Disregarded

3rd

7775

Carries

0003
‘

+7777

7776th

0002

7777th

+7777
1

0001

Thus, successive addition will work properly as a method of multiplying negative as well as positive numbers in the PDP-S family of com—
puters.

Similarly, division could be performed by repeated subtraction. This
method of division could be used to obtain a quotient and remainder,
because only whole numbers are directly represented in the PDP-S.
There are, however, much more efficient means of multiplying and dividing numbers in the PDP-S. One means is through the extended arithmetic- element (EAE) option, which is described in Chapter 4. Multi—
plication and division can also be performed through use of the floating
point-packages, mathematical routines, and interpretive languages of
the system software for the PDP-S. These “software”

approaches to

multiplication and division are described in Chapter 6 of this book.
Double Precision Arithmetic
Two memory location (24 bits) are used to express double precision.

numbers. Using these 24 bits allows the representation of numbers in
the range —8X 106 to 8X106. The following program adds two double

precision numbers, obtaining a double precision result.
Note that if the addition of AL and BL

produces a carry, it will

appear in the link. The accumulator is cleared by the DCA CL instruc—
tion, and the RAL instruction moves the value of the link into the least

significant bit position. The values of AH and BH are then added to
the carry (if any) and the higher part of the answer is deposited in CH.
3-14

This technique may be extended to any‘order of multiple precision.
'

*200

CLA CLL
TAD AL
TAD BL

DUBADD,
‘

‘

DCA CL
RAL
TAD AH
TAD BH

DCA CH

AH,
AL,
BH,_
BL,
CH,
CL,
$

HLT
1345
2167
0312

0110
0

0
,

procedure is followed to subtract'two double precision
numbers. The following program illustrates the technique.
A similar

*200

DUBSUB,

CLA CLL
TAD BL
CIA
TAD AL.
DCA CL
‘RAL
DCA KEEP
TAD BH
CMA
TAD AH
TAD KEEP

DCA CH

CLL
HLT
1345

AH,
AL,
BH,
BL,
CH,
CL,
KEEP,
$

2167
0312
0110
0
0
0

The location KEEP is Used to save the contents of the link while the

value of BH was complemented in the accumulator. To__ form a double

precision two’scomplement number, a double precision one’s comple3-15

ment is formed and the 1 is added to it once.

Thus, the value of BL is

complemented using the CIA instruction, while the value of BH is
complemented with the CMA instruction. The CLL instruction is used
to clear the link and disregard the
carry resulting from using two’s complement numbers to perform subtraCtion.
_

Powers of Two

In the decimal number system, moving the decimal'point right (or
left) multiplies (or divides) a number by powers of ten. In a similar way,

rotatingfia binary. number multiplies (or divides) by powers of two. However, because of the logical connection between the accumulator and
the link bit, care must be taken that unwanted digits do not reappear in
the accumulator after the passage through the link. Multiplication by
powers of two is performed by rotating the accumulator left;'division is
performed by rotating the accumulator-right. Multiplicationand division
by this method are subject to the limitation of 12-bit numbers (unless
double precision is used). That is,. significant bits rotated out of the
accumulator by multiplication or division are lost and incorrect results
are therefore obtained. For example, the following program multiplies a
number by 8 (23).
*200

MULTS,

NUMBER,
$

CLA CLL

TAD NUMBER
CLL RAL
CLL RAL
CLL RAL
DCA NUMBER
HLT
0231

The program will replace the number 0231;; by 23mg. Notice that
multiplying any number with four significant octal digits (such as

12348) using this program will yield incorrect results.

WRITING SUBROUTINES

Included in the memory reference instructions, given in Chapter 2,
was the instruction JMS (jump to subroutine). This instruction is a
modified J MP command which makes return to the point of departure
from the main program possible. The J MS instruction automatically
stores the location of the next instruction after the J MS in the location
to which the program is instructed to jump, thereby enabling a return.

3-16

The programmer need only terminate the subroutine with ,an indirect
IMF to' the first location of the subroutine in order to return to the

following the J MS instruction. The following simple

next instruction

program illustrates the use of a subroutine to double
tained in the accumulator.

number con—

(Main Program)
'

CLA‘ CLL
TAD N
J MS DOUBLE
DCA TWON

START,

(Get the number in the AC)
(Jump to subroutine to double N)
(First instruction after the subroutine)

(Any number, N)

‘

N,~
TWON,

nnnn

DOUBLE,

0000

(2N will be sto'red here)

nnnn
_

(Subroutine)
A

DCA STORE

(Save value of N)

TAD- STORE

(Get N back in the AC)

CLL RAL
SNL
J MP I DOUBLE
CLA CLL
TAD STORE
HLT
0000
,

STORE,

(Rotate left, multiplying by 2)
(Did overflow occur?)
_

(If

overflow

occurs,

display

the

number to be doubled in the AC
and then stop the computer.)
.

Notice that the first instruction of the subroutine'is loCated in the second

location of the subroutine. Any instruction stored in location DOUBLE
would be lost when the return addressis stored. Also note that the sub—
'

routine as it is written must be located on page 0 or current page, because it is

directly addressed. (A subroutine is often located on another
page and addressed indirectly as the next example demonstrates.)
The following program multiplies a number in the accumulator by a
number stored in the location immediately following the JMS instruc'

tion.
3-17

(Main Program)
*200

START,

CLA CLL
TAD A

DCA .+3
TAD B
1 MS I 30

(2000

DCA PRDUCT

PRDUCT,
A,
B,

‘

0000

‘

0051
0027
'

*30

MULT

(Subroutine)
*6000

MULT,

0000

MTALLYy

CIA
DCA MTALLY
TAD I MULT
ISZ MTALLY
IMP .—2
ISZ MULT
J MP I MULT
0000

The preceding example illustrates the following important points.
1. The JMS I 30 instruction could be used

anywhere in core
memory to jump to this subroutine because the pointer word
(stored in location 30) is located on page 0 and all pages of

memory can reference page 0.
2. The period was used to

denote the currentlocation in the in-

structions DCA .+3 and 1MP .-——2.
3. Since the result of the subroutine is left in the AC when jump—

ing back to the main program, the next instruction should store
the result for future use.

4. The first instruction of the subroutine is in location MULT +1
since the next address in the main program is stored in MULT

by the JMS instruction.
3—18

5. The first two instructions of the subroutine set the tally with
the negative of the number in the AC.
6. The second number to be multiplied is brought into the sub- -,
routine by the TAD I MULT instruction since it is stored in

the location specified by the address that the J MS instruction

automatically stores in the first location of the subroutine. This
is a common technique for transferring information into a sub'

routine.
7. The ISZ MTALLY

instruction is» used in the subroutine to

count the number of additions. The ISZ MULT instruction‘is

used to increment the contents of MULT by one, thereby mak—

ing the return jump (JMP I MULT) proceed to the next instruction after the location which held the number to be multi—

plied.
8. An interesting modification of the previous program is achieved
.

by defining a “new operation” MLTPLY by including in the
coding the statement MLTPLYzJMS I 30. The assembler
would make a replacement such that any time the programmer
writes MLTPLY, the computer would perform a jump to the
subroutine and return to the program with the product in the
'

AC.

ADDRESS

MODIFICATION

A very powerful tool often used‘ by the programmer is address‘modi-

fication, meaning the inclusion of instructions in a program to modify
the operand portion of ‘a memory
reference instruCtion. It is a particularly useful technique when working with large blocks of stored data
as illustrated by the
two programs that follow.
The first program sums 1008 numbers in locations 3008 to 3778. The

program begins in location 2003- The block of 1008 numbers is sum—
med using only one TAD instruction merely by repeatedly increment—

ing and performing 'the instruction.
The second eXample program moves‘ data between memory pages as
well as performing an operation upon the data. The program computes
the square. of the 2008 numbers in locations 40008 to 41773. The pro—
"

gram starts in location 2008. All numbers to be squared must not exceed 4510 or the square is too large to be represented in the normal
'

format.
3-19

START

CLEAR AC

ADD

FIRST NUMBER

‘

;

N0
'

L ADD .1 To me ADD
INSTRUCHON‘

Program
*200

START,

‘CLA CLL
TAD K100
CIA

ADD,

DCA TALLY
TAD 300
182 ADD
ISZ TALLY
JMP ADD

DCA SUM
'

K100,
TALLY,
SUM,
55

HLT
0100
0000
0000

3—20

The second example illustrates the method of using indirect address-

ing.in an address modification situation, It should be noted that in the
first example the actual instruction

perform the
modification. In the second example, the modification was done' by
: incrementing the contents of a
location which was used for indirect
addressing. The second example could be simplified further through
use of autoindexing, a feature that‘will be discussed later.
was

incremented to

*200
START,

CLA CLL
.TAD K200

CIA

DCA TALLY
TAD

K4000

DCA NUM
'

_TAD K4200

DCA RESULT
TAD I NUM

AGAIN,

JMS‘ SQUARE
DCA

IRESULT

ISZ RESULT
ISZ NUM
ISZ TALLY
JMP AGAIN
HLT
'

K200,
TALLY,

0200
0000

K4000,

40000000
4200
0000

NUM,
K4200,
RESULT,
,

*300

0000

SQUARE,

DCA STORE

TAD STORE
CIA
_

DCA‘COUNT‘
TAD STORE
ISZ. COUNT

STORE,

JMP .--2
JMP I SQUARE
0000

COUNT,

0000?

3—21

The reader should note that the first eight instructions of the second

example are concerned with intializing the program. This intializing
enables the stored program to be restarted several times and still operate

the correct locations. If the program had merely incremented

locations K4000 and K4200 and utilized those locations for indirect

addressing the program would only operate on the correct locations of
the first running. On successive runnings the program would be opera—
ting on successively higher locations in memory. With the program
written as shown however the pointer words are automatically reset.
This procedure is often referred to as “housekeeping.”

INSERTING COMMENTS AND HEADINGS
Because programs very seldom are written, used, and then forgotten,
the programmer should strive to document his procedure and coding

reasonably possible. There are many instances where
changes or corrections must be made by people unfamiliar with a pro—
gram, or more commonly the original programmer is asked to modify
a program months after his original effort. In both cases, the success of
the attempt to change the program depends largely upon the documen—
tation provided by the original programmer. A complete and accurate
flowchart is the first form of documentation. It is extremely important
to document modifications made in the program by incorporating these
changes in the flowchart as well.
as

much

Many times it is desired to include headings and dates to identify a
program within the actual coding of the symbolic program. It is often
helpful to add comments to simplify the reading of a symbolic program
and to indicate the purpose of any less than obvious instruction. PDP- 8

programming allows comments and headings to be inserted simply by
preceding any comments with a slash ( / ).
following example illustrates the method used to insert com—
ments and headings in a PDP-8 program. It also illustrates the use of
a rotate instruction. The program takes a binary word stored in memory
and counts the number of non-zero bits. Although the program may
The

have no useful application, it does serve to familiarize the reader with
the structure of the accumulator and link bit and the action of a rotate

instruction. The flowchart and comments will ”aid the reader to finder—
0

stand the program.
3-22 .:

‘

(

)

START

GET THE NUMBER

ROTATE LE F T

CLEAR [INK

INCREASE COUNT

‘

/ COUNT THE BINARY ONES PROGRAM
/ 20 SEPTEMBER 1968
*200

CLA CLL
DCA COUNT
TAD I WORD
SNA
HLT

START,

ROTATE,

RAL
SNL
J MP ——2
CLL
ISZ COUNT

/ SET COUNT TO 0.
/GET THE WORD.
I

/STOP IF THE WORD IS 0.
/ROTATE ONE BIT INTO LINK.
/WAS THE BIT : 0?
/YES: ROTATE AGAIN.
/NO: CLEAR LINK.
/COUNT THE NUMBER OF l’S.
'

SNA
HLT
J MP ROTATE

COUNT,
WORD,

/ STOP IF THE WORD IS NOW 0.
,

3000

/ANY 12-BIT NUMBER.

3—23

The

following points should be observed in the preceding example.
1. The word was'checked to see that it was non-zero to begin
with. If this check were not made, a zero word would be rotated

endlesslyby the remaining instructions in the program.
2. Because a rotate right instruction

(RAR) would transfer the
bits into the link just as the RAL instruction does, either could
be used1n the above program. Both instructions use a circular
shift of the accumulator and link bits.-

L»)

ecause

rotate

thel ink bit i rotated int

the accumulator by the

instructions, the link must be cleared each time a 1 is

rotated into it.

LOOPING A PROGRAM
As many of the examples

given have already shown, the use of 'a
program loop, in which a set of instructions is performed repeatedly,
is common programming practice. Looping a program is one of the
.most powerful tools at the programmer’s disposal. It enables him to perform similar operations many times using. the same instructions, thus
saving memory locations because heneed not store the same instructions
many times. Looping also makesa program more flexible because it is
relatively easy to change the number of loops required for differring
conditions by resetting a counter. It is good to remember that looping
is little more than a jump to an earlier part of the program; however,
the jump is usually conditioned upon changing program conditions.
There are basically two methods of creating a program loop. The

first method is using an ISZ (2nnn8) instruction to count the number
of passes made through the loop. The ISZ is usually followed by a JMP
instruction to the beginning of the loop. This technique is very efficient
when the required number of passes

determined.

through the loop

can

be readily

The second technique is .to use the Group 2 Operate Microinstructions to test conditions other than the number of passes which have

Using this second technique, the program is required to
loop until a specific condition is present in the accumulator or link bit,

been made.

rather than until a predetermined number of passes are made.
To illustrate the use of an ISZ instruction in a program loop situa-

tion, consider the following program which simply sets the contents of
all addresses from 2000 to 2777 to zero.

3-24

#200 3»
CLA
TAD CONST
DCA COUNTTAD TTABLE.
DCA STABLE
DCA I.STABLE
ISZ STABLE
ISZ COUNT,

CLEAR,
,

/SET COUNT TO —1000.

-‘

’

JMP .-—-3
HLT

CONST,
COUNT,
TTABLE,
STABLE,

7000

/~SET STABLE TO 2000..
/CLEAR ONE LOCATION.
/SELECT NEXT LOCATION.

/IS OPERATION COMPLETE?-~
-/NO: REPEAT.
/YES: HALT.
/2.’S COMP OF 1000;

TABLE
‘

/POINTERTO TABLE.

*2000

TABLE,
$

Several points should _be=:car.efully noted;
1. The-first five instructions initialize the loop, but are not in it,

The location COUNT is set to -1000 at the beginning,and 1
is added to it during each passage. of the loop. After the 1000th

(octal) passage, COUNT goes to. zero, and the program skips
the J MP instruction, .and executes the HLT instruction. On
each previous occasion, it executed the J MP instruction.
.

In the list of constants follOwing the HLT instruction, TTABLE

contains TABLE, which is in turn defined below as having the‘
value 2000,.- and

containing 0. Therefore, STABLE contains
'

2000 initially. In order to understand this point it must be ref

membered that an asterisk character causes the first locationafter the asterisk to be set to the value after the asterisk. There-

fore, in the previous example CLEAR equals 200 and TABLE
equals2000.
‘

ISZ STABLE adds 1 to the contents of location

STABLE,

forming 2001 on the first pass, 2002-on the second pass, and
SO on.

Since it never reaches Zero, it will never skip. This is a

very common use. It is said to be indexing the addressesfrom
2000 to 2777. (When using an ISZ instruction in this way, the

programmer must be certain that it does not reach 0. Follow
the ISZ instruction with a NOP if necessary.)

3-25

4. For every 182 instruction used in

program, thereimust be
two initializing instructions before the loop, and there must be
a constant

and a counting location in a table of constants. This

procedure allows the program to be rerun with the counting
locations reset to the correct values.

The following program utilizes a Group 2 skip instruction to create
a

loop. lhe program will search all of core memory to find the first

occurrence of the octal number

1234.

*0
‘
'

NUMBER,
*200

1234

’

BEGIN,

CLA CLL
TAD NUMBER
CIA
DCA COMPARE
DCA ENTRY

REPEAT,

ISZ ENTRY
CLA

/STORES MINUS NUMBER.
/SETs ENTRY To 0.
/INCREASES ENTRY.
,

TAD I ENTRY

/COMPARISON Is

TAD COMPARE

/DONE HERE.
'

SZA CLA
IMP REPEAT
TAD ENTRY
HLT

COMPARE,
ENTRY,

/ENTRY IS‘IN AC.

The previous example is not very useful perhaps but it is interesting
'to note that the program will search itself

well

all other

core

memory locations.

Also notice the following points with regard to the example.
1. The 182 ENTRY instruction is used to index the locations to
be tested. The next instruction

(CLA) is unnecessary, thus if

ENTRY becomes zero during the course of the program, the
program will not be affected. It is very important to protect
against an 182 instruction going to zero and skipping a neces—
sary part of
index.

program, if the 182 is

3-26

being used to simply

2. The number to be searched for. was ,stored in location 0, and

the search starts in location 1. TherefOre, the program will find
at least one occurrence of the number” and will .halt after one

complete pass through memory if not before.
3. The program could be modified to bound the area of the search.
By setting the contents of ENTRY equal to one less than the

desired start location and putting the number being searched for
in the location following the last location to be searched, the
program will search only the designated area of memory.

4. The program could be restarted at location REPEAT in order
to find a second occurrence of 1234 after the program had
_

halted with the first occurrence.

AUTOINDEXING

The PDP-8 family computers have eight special registers in page 0,

namely locations 0010 through 0017. Whenever these locations are
addressed indirectly by a memory reference instruction, the content of
the register is incremented before it is used as the operand of the instruction. These locations can therefore be used in place of an ISZ instruction in an indexing application. Because of this unique action these
eight locations are called autoindex registers. It is important to realize
that autoindex

registers act as any other location when addressed

directly. The autoindexing feature is performed only when the location
is addressed indirectly.
The following example is a modification of the first program example

in the preceding section with an autoindex register used in place of the
182 instruction. (The purpose of the program is to clear memory locations 2000 through 2777.)

Carefully notice the difference between the two examples, especially

that TABLE now has to be set to TABLE—1 since this is incremented

by the autoindexing register before being used for the first time. This
3-27

point must be remembered when using an autoindex register. The
register increments before the operation takes place, therefore it must
always be set to one less than the first value of the addresses to be
indexed.

10
0

INDEX,
*200

CLA
TAD CONST

CLEAR,

DCA COUNT
TAD TTABLE
'

DCA INDEX

DCA I INDEX
152 COUNT
JMP —2
BLT

CONST,
COUNT,
TTABLE,
*

7000
0

TABLE—1

2000

TABLE,
15

The memory search example of the" preceding sectiOn could also be

simplified using an autoindex register as shown below.
*0

NUMBER,

1234

*10
.

ENTRY,

Notice that in this case ENTRY

originally equals 0 because its

*200

BEGIN,

content is incremented before

CLA CLL
TAD NUMBER

being used to obtain data for

CIA

the

DCA COMPARE
DCA ENTRY

REPEAT,

TAD I ENTRY

TAD COMPARE
SZA CLA
JMP REPEAT
TAD ENTRY
HLT

COMPARE,
$

3-28

comparison.

PROGRAM DELAYS

Because the development of a computenwas primarily sparked by a
desire for speed in performing calculations, it seems inconsistent and

‘

self-defeating to slow the computer down with program delays. How—.
ever, there are many occasions when a computer must be told to slow

down or to wait for further information. This is because most peripheral

equipment, and certainly the human operator, is very much slower than
the computer program. A temporary delay may be introduced into the
execution of a program when needed by causing the computer to enter
one or more futile loops which it must traverse a fixed number of times
before jumping out. It is often necessary to have a computer perform a
temporary delay while a peripheral device is processing data to be submitted to the computer. The delays can be accurately timed so as not
to waste any more computer'time than necessary.
The following is a simple delay routine using the 182 instruction for
an inner loop and an outer loop. The reader should rememberfwhen
analyzing the example that the PDP—8 represents only positive numbers

up to 3777., or 2047,... Therefore, the computer counts up to 20471..
and then continues to count starting at the next octal number 40003,

which the computerinterprets

——2048w. Successive increments of

this number will finally bring the count to zero; Thus, a location could
be used to cOunt from 1 up to O-by using an 152 instruction.
3

(main program)
0

TAD CONST

DCACOUNT
ISZ COUNTl
JMP .-1
ISZ COUNT

/ START OF DELAY ROUTINE
,

/INNER
/LOOP

JMP .—3

CONST,

6030

COUNT,

COUNTL

/SETS DELAY
.

The inner loop consists of an 182 instruction with an execution time
on

the PDP-8/I of 3.0 microseconds (a microsecond is 10’6 seconds)

JMP instructiOn with an execution time of 1.5 microseconds.
Therefore, the inner loop takes 4.5 microseconds for one pass, and each
and

time it is entered the program will traverse it 4096“, times before leav—

ing. This means that a delay of 18.432 milliseconds (a millisecond is
3-29'

10‘3 seconds) has occurred. if, as in the example above, the value of

CONST is 60308, this loop will be entered 100010 times giving a total

delay of 18.432 seconds. For any given purpose, a desired delay
from milliseconds to seconds can be obtained precisely by varying the
values of CONST and the initial value of COUNT‘l. Similar reasoning
can be used to design delays
for other'members of the PDP-8 family.

‘

A second type of delay, which waits for a device response, is "dis-

cussed in Chapter 5. This type is not a timed delay but causes the com-

puter to wait until it receives a response from an external device.

PROGRAM BRANCHING

Very few meaningful programs are written which do not take ad—
vantage of the computer’s ability to determine the future course the
program should follow based upon intermediate results. The procedure
of testing a condition and providing alternative paths for the program
to travel for each of the different results possible is called branching a
program. The Group 2 microinstructions presented in Chapter 2 are
most often used for this purpose. The 182 instruction also provides a
branch in a program. These instructions are often referred to as conditional skip instructions. The 152 instruction operates upon the contents of a memory location, while the

Group 2 microinstructions test
V

the contents of the AC and L.
A typical example of a conditional skip would be a program to com—

pare A and B and to reverse their order if B is larger than A.

{

START

t
FORM

A-B

.3
”0

RESULT

YES

SAVE A
DUMMY

NEG
7

STORE B W

A'S LOCATION

STOP

STOREDUMMYIN
33 LOCATION

3-30

”*200
TEST,

CLA CLL
TAD B

/SUBTRACT B

/FROM A

CIA
TAD A
SMA CLA

/HERE.

HLT

/STOP HERE IF A IS GREATER
OR EQUAL

‘

TAD A
DCA DUMMY
'

‘

TAD B
DCA A

/THE REMAINDER OF
/THE PROGRAM
/DOES THE SWITCH.

TAD DUMMY
DCA B
'

A,
B,
DUMMY,
$

HLT
1234
2460

/SUBSTITUTE ANY POSITIVE
/VALUES FOR A AND B.

If A is less than B, their diflerence will be negative and the HALT will
be skipped. The program will proceed to reverse the order of A and B.

If A is greater than or equal to B, the program will halt.
The concept illustrated by the above example can be included in a

larger program that will take a set of elements and arrange them in
increasing order. The following important concepts should be learned
from the example.
l. The program contains two loops to perform the sort. The inner»
loop starts at TEST and is traversed 208 times‘to switch ad—
jacent elements of the set. The outer loop begins at START
and is re-entered until the elements are in the correct order.
A “software flag” was created to signal the program that a
switch has been performed on the last pass. The flag is checked
upon every exit from the inner loop. If the flag is non—zero
(equal. to —1), a reverse was performed on the last pass and
the next
pass is started. If the flag is zero, the set is now in
'

.order and the program halts.
.

The flag is set to zero on each pass through the outer loop by

depositing AC:O in it. It can only be set to a non—zero value
by a pass through the REVERSE subroutine.
The TALLY had to be set to ~(AMOUNT) +1 or in this
case to —20,s because
if the set contains n elements there are

comparisons between an element and the immediately
succeeding element, thus, in this case, TALLYz—ZOS.
n——1

3—31

5. The

following sort of five elements illustrates the technique

used in the program.

INHWAL

‘

PASS!

@126)
n9

PASSZ

(9 C3)
0 0

-@
,

@ ®©

53%;

Go
t,

In performing the above sort, the program makes three passes.

On the third pass thrOugh the table of data, thetflag is not
raised; therefore, the program stops.

(

START

)

FORM

am?”
2

Yes

1NCREMENT

SET FLAG

Xg,_X2

[

‘

RESULT o.
OR NEG

‘

xl'xz

X112

STOP
CLEAR FLAG

3-32

*ZDD

START,

CLA

CLL

TAD

AMOUNT

TALLY

CIA
IAC

/UP

DCA

IPASSES

THRU

TEST

/CLEARS

FLAG

BEFORE

DCA

TALLY
FLAG
‘

DCA
.TAD

N'-

BEGIN

/xr AND
IPROPER

TAD

EACH

PASS

THE

VALUES

/INITIALLY.

/SUBTRACTION

CIA
I

LOOP.

POINTERS

THE

ISET

IAC

THE

COUNT

-T

/THESE INSTRUCTIONS

DCA

TAD

/AMOUNT

.JAD BEGIN“

TEST:

INSTRUCTIONS SET
EOUAL To

/THESE

THE

FOR
~

/TEST

/DONE

HERE.

SPA' SNA

CLA

SKP

SWITCH

JMS
152

REVERSE

/DO

ISET

152

/THE

NEXT PASS.

152

TALLY

v/HAVE

ALL

JMP
TAD

TEST

IND:

KEEP

FLAG

/YES:

HERE'ANY

xOONE

THE

/YES;

THRU

-SZA

JMP
HLT
AMOUNT,

TALLY,

0660

BEGIN,

ZZGD

X1,

6000

X2,

@366

FLAG,

@800

HOLD,

0600

START

/N0:

X'S

THE

X‘S

TESTED?

BEEN

TESTING.

STOP.

POSITIVE.

FOR

SWITCHES

LAST

PASS?

PROGRAM

TABLE

AGAIN.
ORDER.

REVERSE,DDD@
TAD I

/SUBROUTINE

SWITCH

X'S

DCA

HOLD

TAD

DCA

TAD

HOLD

DCA

CLA

CLL

DCA

FLAG

JMP

CMA

/SETs
/SET

AC EOUAL T0
ELAG=~1

-1.

SWITCH.

REVERSE

6. This program can perform a sorting for any specified block of
data merely by specifying the octal number of entries to be
.

sorted in the location AMOUNT and by specifying the begin-

ning address of the block in BEGIN. The data to be sorted
must be placed in consecutive memory locations.
3—33

Exercises
1. Write a subroutine SUB to subtract the number in the AC from

the number in the location after the JMS instruction that calls the
subroutine. Return to the main program with the difference in the

AC. Use a flowchart and comments to document the procedure.

2. Write two programs to put 0 into memory location 2000, 1 into

2001, 2 into 2002, etc., up to 7778 into 2777 using (a) an ISZ
instruction for indexing and (b) autoindexing. Use fiowcharts
and comments to document the procedure.
3. The following program was previously given to multiply two num—

bers together.
I

*200

CLA CLL
TAD A
CIA
DCA TALLY

START,

:
.

MULT,

TAD B
ISZ TALLY
J MP MULT

HLT

g’

}substitutc
0000

TALLY,
$

any numbers for A and B

largest product that the PDP-8 can compute using

What is the

this program?

Using the following value for A and B, verify that the program will
.

obtain the correct answers. Remember that any carry from the most
significant bit is lost from the accumulator.

b.
c.

7756(—1810)

0027(2310)

0000

0005

7700( —6410)

0000

A X B

program TRIADD which will add two triple precision
numbers A+B:C. There are three parts to each number, namely

4. Write

AH (A high) AM (A medium), and AL (A low); BH, BM, and
BL; CH, CM, and CL. Use a flowchart and comments to document the procedure.

3-34

write a program to perform a multiplication between two single-

precision numbers to yield a double-precision product. Use comments and a flowchart to document the procedure.
Write a program to multiply any number n by a power of 2 (the

exponent is stored in location EXP), the product being expressed
in double precision. Use comments and a flowchart to document

the procedure.
Write a program to find how many of the numbers stored in a
table from address 3000 to address 3777 are negative. Use a flowchart and comments to document the procedure.
Write a program that will run for exactly 20 seconds on
or

PDP-8/ I before it halts. Use

document the procedure.

the PDP-8

flowchart and comments to
,.

Modify the program written for exercise 8 such. that if bit 11 of
the console switch register is a 1, the program runs for 20 seconds,
and if it is a 0, the program runs for 40 seconds
Hint: The OSR instruction must be used to check
.

the switch

10. The program on the next page rotates a bit left or right depend-

ing on the value of bit 0 and faster or slower depending on the
value of the remaining bits. Analyze the program and comment
each instruction to indicate its use in the program.

3-35

'
-

*200

CLA CLL CML

ROTATE,
BEGIN,

HLT
DCA SAVEAC
RAL
DCA SAVEL
TAD MASK

OSR

DCA COUNT

GO,

INSTR,

OSR
RAL
SZL CLA
JMS LEFT
J MS RIGHT
CLL
TAD SAVEL
RAR
TAD SAVEAC
RAR
ISZ COUNTR
JMP .—l
182 COUNT
J MP .—3
J MP BEGIN

SAVEAC,
SAVEL,
MASK,

COUNTR,

0
0

COUNT,
LEFT,

RIGHT,

KRAR,
KRAL,

7000

182 LEFT
TAD KRAL
DCA INSTR
JMP I LEFT
0
TAD KRAR
DCA INSTR
JMP I RIGHT
7010

7004

3-36

onahn

system
description
-

and

-_
'

operation

loading editing
’

and debugging

ehopt4er
system
description
.

and

operation

A PDP-S / E system is composed of the computer (programmer’s

console), a keyboard/ printer terminal and possibly other peripheral

equipment. While normal operation of a computer system is

by programmed control, manual operation is necessary for many
tasks. This chapter describes the manual control and operationof
the PDP-8/E and provides an introduction to the more common
‘

peripheral devices which may be included in a PDP-8/E system.
Chapter 6 describes the programmed control of peripheral devices
and the means for transferring information between peripheral
equipment and the central processor.
’

‘

PROGRAMME-R’s CONSOLE OPERATION
The programmer’s console allows manual control cf the cem"

puter and provides the most elementary means of storing a program in memory. It consists of switches and indicator lamps which
enable the programmer to examine or alter the contents of memory

locations and determine the current status of a running program.
The PDP-S/E programmer’s console is shown in Figure 4-1. For

reference purposes, the switches and indicators
Table 4—1.

are

identified in

’

4—1

O O O O

a: 23$

now:

in)

or:

my:

O
sq

Eu]

0 O O O O

Inn-1'3: m

O
5"“

:"us
In

no
nos

Anon Elm

Law AM

Figure 4-1

57m

W412“- “GET“

PDP-S/ E Programmér’s Console

4—2

Table 4-1
‘

Control

Programmer’s Console Control and Indicator Functions
or

Indicator

Function

OFF/ POWER/ PANEL LOCK

In the

counter-clockwise, or OFF
position, this key operated switch
disconnects all primary power to
the computer;‘In--the POWER, or
vertical position, it applies power

the computer and all manual
controls. In the PANEL LOCK, or
to

clockwise position, it applies power.
to the computer, the

and the RUN

switch register

light only. In this
position, a running program is protected

from

inadyertent

switch

operation.
.When this switch is up, the Omnibus SW line is high.(logical 1).
When it is down, the SW line is

SW
'

low. This switch is used by special
peripheral routines.
'

(SR)

SWITCH REGISTER
may be loaded with a 12—bit binary
number by setting the
twelve
switches either up, for' a 1, or
down, for a 0.
The

SWITCH REGISTER

ADDR

Pressing

LOAD

switch

the ADDRess
LOAD
loads the contents of the
SR into the central processor MA
register and forces the processor to

enter a fetch state. This causes the
contents of the core memory loca-'
tion designated by the SR to be

loaded into the MB register.

EXTD

Pressing the EXTendeD ADDRess'

ADDR
LOAD

LOAD switch loads the contents of
SR bits 6-8 into the instruction
field register and the contents of
SR. bits 9-11 into the data field

CLEAR

Pressing the CLEAR switch loads
binary 0 into bits 0-11 of the accumulator, the link, all I/O device
flag registers, and the interrupt
a

4-3

request flag register. This is equivalent
All

executing a CAF (Clear
Flags) instruction.
to

Pressing the CONTinue switch sets

.CONT

the run filp-fiop and issues a memory start to begin program execution
at the address specified by the current contents of the central processor MA

Pressing the EXAMine switch loads
the contents of core memory at the
address specified by the MA.register into the MB register and then
increments the MA register and
the PC. Repeated operation of this
switch permits the contents of sequential core memory locations to
be examined.

EXAM

Pressing HALT clears the run flipflop and causes the computer to
stop at the beginning of the next
fetch state. Operating the computer
with HALT depressed causes one
machine cycle to be executed when-

HALT

the

ever

CONTinue

switch

pressed.

Pressing SINGle STEP clears the
flip-flop and causes the com-

SING”
STEP

run

puter

execute

and halt at the

one

instruction

beginning of the

next fetch state.

the DEPosit switch loads
the contents of the SR into the MB

DEP

Lifting

register and into core memory at
the address specified by the current
of the central processor
register, then increments the

contents

MA
'

PC and the MA registers. This
facilitates manual storage of information in sequential core memory

locations.

EMA

The 3-bit Extended Memory Address register displays the memory
field designation of the memory
field currently being accessed.

4-4

MEMORY ADDRESS

The MEMORY ADDRESS register
displays the contents of the central
processor MA register. It combines
with the EMA register to provide
the 15-bit address of the next core
location to be. accessed.

The RUN indicator is lit whenever
all machine timing circuits are ac-

RUN

tivated and capable of executing
instructions.

Indicator Selector Switch

6-position rotary knob desigwhich of six possible registers (or combinations of regisThis-

nates

to be loaded
12-bit
adjacent
display.

ters)

into

the

Setting this knob to:
BUS

Displays the logical state of the
data gating lines which connect
the
major registers.

Displays the contents of the multiplier quotient register.
'

Displays the contents of the MB

read

from

written

into core memory.
I

Displays the

contents

of the

cumulator.

ac-

‘

STATUS.

‘

Indicator
Light/ Bit
Position

Turned On to

The link contains a binary 1.
The Greater Than Flag (GTF) is raised.

2
3
4

6-8

9-11

Indicate:

Each display light is turned on to
indicate the designated condition:

The interrupt request line is asserted.
A processor condition which prevents program interrupts has been initiated by software.
The interrupt enable flip- flop is on.
The user mode line is asserted.
register.
Displays the contents of the instruction
Displays the contents of the data field register.

field

4-5

STATE

—

With the Indicator Selector knob in
the STATE position, each display
light is turned on to indicate the
following condition:

Indicator

Light/ Bit

Turned on to indicate:

Position

Currently in fetch state.
Currently in defer state.
Currently in execute state.
Displays the contents of the instruction register.

0
1
2

3-5

The MD DIR line is asserted.
The BREAK DATA CONT line is asserted.”
The SW line is asserted.
The PAUSE I/ 0 line is asserted.
The BREAK IN PROG line is asserted.
The BREAK CYCLE line is asserted.

8
9
10
11

Note: The function of the various transmission lines cited and their associated control logic is documented in the Small Computer Handbook.

MANUAL PROGRAM LOADING
After

writing a program, the programmer must store the instructions in memory before they can be executed. One means of
accomplishing this is to load the octal value of each instruction
directly into core memory from the programmer’s console. The
following procedure will initialize the programmers console for
manual program loading.
1.

Place

OFF/POWER/PANEL LOCK switch in the

the

POWER position.
2.

Load 0000 into the switch register. (all Switches in the down

position)._
3.

Place all

other switches in the up position, except for the

spring—loaded DEPosit switch.
4.

Turn off all peripheral devices.

Once the programmer’s console has been initialized, a program
may be loaded manually by following the instructions in Figure
4-2. Figures 4-3 and 4—4 give: further instructions for checking and
4-6

running the stored program; These: procedures 'will also permit a
single core memory location to be examined or altered. The content of the desired location is considered to be a one-Word “program,” in this case, and the left-hand (clockwise) loops in the flow
charts are never taken.

INITIALlZE

SET INDICATOR
SELECTOR KNOB
TO MD

SET SR To
PROGRAM'S

FIRST ADDRESS

PRESS

ADDR LOAD
V

SET SR T0

PROGRAM'S
FIRST BINARY
INSTRUCTION
7

LIFT DEP

SET SR T0
PROGRAM'S NEXT
INSTRUCTION

‘ROGRAM IS LOADED
_

Figure 4-2

Manually Loading a Program
4—7

INITIALIZE

SET INDICATOR
SELECTOR KNOB
T0 MD

SET SR 1:0
PROGRAM S
FIRST ADDRESS
SET SR TO
VALUE OF
MA—I

PRESS ADDR LOAD

IPRESS

PRESS EXAM

ADDR LOAD

SET SR TO
CORRECT
INSTRUCTION

LIFT DEP

ALL

INSTRUCTIONS

CHE?)KED

Figure 4-3

Checking a Stored Program

INITIALIZE
II

SET SR TO

ADDRESS OF

PROGRAMS FIRST
INSTRUCTION

EwEEAEI
PRESS CLEAR
AND CONT

PROGRAM
IS RUNNING

Figure 4-4

Running a Stored Program
4-8

KEYBOARD/ PRINTER CONSOLE DEVICES
The techniques described in the previous section permit direct

interaction between the operator and the computer. This type of
interaction is very convenient for many special applications such—
as program modification and debugging, data modification and
controlled program execution. In general, however, a keyboard/

printer peripheral device is also necessary so that routine input/
output operations may proceed at the fastest possible rate of speed.
The VTOS Display Terminal (Figure 4-5) and the LA-30 DEC—
writer Data Terminal (Figure 4—6) are console devices which per—
mit fast, convenient interaction between the operator and the com-

puter. They are the recommended medium for most routine input/h
output operations. These devices are described in the Small Computer Handbook. The LT—'33 Teletype may also be Used as a console device. Teletype operation is described in the following sections.

Figure 4-5

VTOS Display Terminal

4-9

Figure 4-6

LA—30 DEeriter Data

Terminal

TELETYPE OPERATION
consists of

printer; keyboard; paper
and
The
reader,
tape
paper tape punch.
Teletype unit can operate
under program control or manual control. Programmed operation
of the Teletype unit is described in detail in Chapter 6. Opera—
tion of the Teletype unit as an independent device
for generating
The LT—33

Teletype

paper tapes is described later in this section.
and. their functions are listed below.
4-10

Major components

‘

rorrr

REL.

8. SP.

START
'STOP
FREE

‘-

i
on:

’

LINE

Figure 4-7

LT-33

0 LOCAL

Teletype Console

Teletype Control Knob

Teletype console (see Figure]
4-7 ) has the following three positions.
The control knob of the LT—33

LINE

Teletype console is energized and connected
the computer as an input/ output device under
computer control.
The

OFF

The Teletype console is de-energized.

LOCAL

The Teletype console is energized for off-line operation under control of the Teletype
and

switches exclusively.

4-11

keyboard

©®®©©H®O©©OO
OWOQW

COCOQOQQCDWQ
.©©©©®®©©©.
L

SPACE

Figure 4-8

Teletype Keyboard

Teletype Keyboard
The Teletype keyboard shown in Figure 4-8 is similar to a typewriter keyboard, except that some nonprinting characters are included asupper case elements. When typing upper case characters
or symbols such as $, % or it, which appear on the upper portion
of numeric keys and certain alphabetic keys, the SHIFT key is held
depressed while the desired character or symbol key is operated.
When typing the nonprinting operational functions which appear
on the upper portion of some alphabetic keys, the control (CTRL)
key is held depressed while the desired key is operated. Table 4-2
lists several commonly used keys that have special functions in the
symbolic language of PDP-S series computers.
Table 4-2

Special Keyboard Functions

‘

Key
SPACE

Function

Space

Use

Used to combine and delimit
symbols or numbers in a

symbolic program.
RETURN

Carriage Return

Used to terminate

input.
HERE 18

Blank Tape
.

4—12

line of

a
-

Used to generate leader/
trailer tape. Effective in L0CAL control mode only.

Special Keyboard Functions.

Table 4-2 (cont’d)

Key

Function

‘

RUBOUT

Use
Used for

Rubout

deleting

erroneous

characters. Punches
channels.

CTRL/ SHIFT/ REPT/ P

Code 200

LINE FEED

Line Feed

all eight

Used for- leader/ trailer on
BIN format tapes. Keys must
be released in reverse order:
P, REPT, SHIFT, CTRL.
Follows carriage return to ad—'
vance

printer one line.

Used to type the characters
and symbols which appear on
the upper portion of certain

SHIFT

keys.

Teletype Printer
The Teletype printer operates at a maximum rate of ten char— _'
acters per second. When the Teletype unit is on line (LINE control
mode), all printed copy is generated under program"'.control by
the computer. When the Teletype unit is off line (LOCAL control
mede), all printed copy is generated automatically whenever a key
is typed.
-

’

Teletype Paper Tape Reader
The Teletype paper tape reader (also called the low-speed
reader) is used to read data'punched on paper tape into core memory. The data is read from an eight-channel, perforated paper tape
at a maximum rate of ten characters per second. Operation is controlled by a three-position switch, shownin Figure 4-7. The control positions are described below.
-

START

Activates the reader; reader
gaged and operative.
’

STOP

sprocket wheel is

Deactivates the reader; reader sprocket wheel is en-

gaged but not operative.
FREE

en-

Deactivates the

’

disengaged.

‘

reader; reader sprocket wheel is
.

4-13

Teletype Paper Tape Punch
The paper tape punch is used to perforate eight—channel, rolled,
oiled paper tape at a maximum rate of ten characters per second.
'

.The punch controls are shown in Figure 4-7 and described below.
REL.

.Disengages
loading.

the

tape

allow

tape removal

B.SP.

Backspaces the tape one space for each firm depression of the B.SP. button.

Activates the paper tape punch.

OFF

Deactivates the paper tape punch.

CHANNELS
3 2

B 7 6 5 4

Data is recorded

.
'
.

'
.

' ‘

. . '

+C°LUMN

(punched) on

paper tape by groups of holes
arranged in a definite format
along the length of the tape. The

tape is divided into channels,
which run the length of the tape,
and into columns, which extend
across the width of the tape, as
shown in the adjacent diagram.
The paper‘ tape readers and
punches used with PDP-8 series

computers accept eight-channel
paper tape-

SPROCKET
HOLE

Generating A Symbolic. Tape

Symbolic tapes to be used as input to the Assembler are most
conveniently prepared on—line, using the Symbolic Editor. This
program, described in Chapter 5, has several formatting and error4-14

correcting features which greatly facilitate the process of writing
and editing symbolic tapes. If it is not possible to generate a ‘sym- 3
bolic tape on—line, the following procedure may be employed to
generate such a tape on the Teletype. This procedure is best employed on an isolated Teletype unit, when the computer is un‘

available.

Teletype Control Knbb to LOCAL and turn the
paper tape punch ON In LOCAL mode the Teletype unit
and functions much like an
is independent of the
computer
electric typewriter.
'2. Press the HERE IS key to
several inches of
leader
1.

Set the

produce

tape.

4.
-

Type out the symbolic program, following the conventions
described in Chapter 3 To correct an error, press B. SP.
until the error is under the print/ punch station, then press
RUBOUT until the error and all subsequent characters have
been deleted. The erroneous character and all subsequent
characters may now be retyped.
Press the HERE IS key to produce several inches of trailer
following the symbolic program, then remove the tape by
tearing it against the plastic cover of the punch.

The following procedure is
ASCII-coded

employed to obtain

listing of an

symbolic tape.

Set the paper tape reader switch to STOP or FREE.

Release the plastic cover of the reader unit and place the

tape over the read station with the small
the sprocket wheel. Close the cover.

sprocket

holes over
,

Set the Teletype Control Knob to LOCAL.

Push the paper. tape reader switch to START and release.
A printed copy of the tape will be produced on the Teletype.

If the paper tape punch is ON, a duplicate of. the tape will
also be generated.

‘

Paper Tape FormatsManual use of the toggle switches on the programmer’s console

is a tedious and inefficient means of loading a program. This procedure is necessary in

some

instances, however, because PDP-8
4-15

series computers must be programmed before any form of input to

the memory unit is possible. For example, before any paper tape
can be used to input information into the computer, the memory
unit must contain a stored program which will interpret the paper

tape code and store the interpreted data in core memory. This
loader program must often be entered into core with the console

switches.
Before a loader program can be written to accept information,
the format in which the data is represented on paper tape must be

established. There are three basic paper tape formats commonly
used by PDP-8 series computers. The
scribe and illustrate these formats.

O O

o o

O o
o

310

311

323

240

311

323

301

240

324
O

o o

. .

g
.

323

o =-

303

3”

’

‘

o o
o o
O O

240

°'

0 0
°

zz:

3?: (F)
322

315

o“

following paragraphs de-

301

324

ASCII FORMAT
The USA Standard Code for
Information Interchange (ASCII)
format uses all eight channels1
of the paper tape to represent a
single character (letter, number,
or symbol) as shown in the diagram at left. The complete code
is given in Appendix Bl.

Channel 8 is normally designated for. parity check. The Teletype units

used with PDP-S series computers do not generate parity, and Channel 8
is always punched.

4-16

{—
.

CHANNEL 7

' °

RIM

(READ IN MODE)

FORMAT
VRIM format tape uses pairs of

columns to represent
: :17 LOCATION adjacent
words directly.
12-bit

binary

3’:
to represent either addresses
':'::°:' 5:;
information to .be stored. A’
channel 7 punch indicates that
: I _. : 1:: CONTENTS the
column and the folbe inter-‘
lowing column
':'::°:: g; LOCATION 'preted
address specify:
ing the location at which the
::
:3
information contained in the
CONTENTS Channels 1 through 6 are used
or

LOCATION

current

are

° '

CONTENTS

........
°

°"

00.

z z z : 3;
° . .

50
'

OOOOOOIOOOO

OO
00
OO
o

77
.0

76
,.

o o

leader and

tape
RIM

format tape
in channel 8

(BINARY) FORMAT
Binary format is similar to RIM
ORIGIN
format except that only the first
address in a series of consecuINSTRUpTION tive
addresses is specified. A
INSTRUCTION Channel 7 punch indicates that
BIN

The
for

FIELD SETTING

stored.
trailer
must

E
o

CONTENTS

following two columns is to be

be punched
2; LOCATION only (octal
200).

o o

LOCATION
.

00
77

WW"

the current column and the fol-

INSTRUCTION lowing column are to be inter-

preted as an address. Successive
'pairs of columns are stored in
INSTRUCTION sequential locations following
this address until another ChanINSTRUCTION
nel 7 punch is encountered. A
INSTRUCTION

INSTRUCTION

Channel

and

channel

punch designate the current" colmemory field

specifi¥

INSTRUCTION

umn

INSTRUCTION

cation. Leader/ trailer tape must
be punched in channel 8 only.

4—17

Paper Tape Loader Programs
Each of the threepaper tape formats has its own
primary ap—
The
ASCII
is
used
format
for
plication.
symbolic programs which
provide input to the AsSembler. As described in the previous
Chapters, the Assembler translates ASCII—coded mnemonic instructions and symbolic addresses into binary instructions and 211380111te
addresses. Once this translation has been performed,
format tape is generated.

binary

Binary format tapes are the usual means of loading assembled
programs into PDP—8 core memory. Binary tapes are loaded under
program control, using the BIN Loader, which is an 83—instruction
program that must be placed in core before any binary format tape
may be loaded.
RIM format is easier to load than BIN
_

format because RIM

format supplies a core address for every instruction. If the BIN
Loader is not in core, the l7-instruction RIM Loader may be tog—

gled into core from the programmer’s console and used to load a
RIM format tape of the BIN Loader. RIM Loader instructions and

corresponding core memory locations are listed in Appendix E
which also includes directions for storing and using the RIM and
BIN Loaders. Chapter 5 contains further discussion of the use of
,

paper tape loaders.

PERIPHERAL EQUIPMENT AND OPTIONS
PDP-S/E computers are used in many different environments
interfaced with many different peripheral devices. The
Teletype unit is the most common peripheral device, but other
and

‘

are

equipment and options often incorporated in a system with the
PDP-S/E include high—speed paper tape reader and punch units,
DECtape, DECdisk, extended memory, and the extended arithmetic element (EAE). These options give the basic PDP-S/E new
capabilities of which the programmer should ,be aware. The purpose and features of each of these
following sections.

options are described in the

High- Speed Paper Tape Reader and Punch Unit
Loading a long paper tape program into core memory with the
low—speed reader of the LT— 33 Teletype unit is very time consuming. Punching a long program on paper tape from an assembly
program is also very slow. If handling of lengthy paper tapes is
commonly required, much computer time is wasted while low4—18

speed input/output devices read or punch data. The high-speed
paper tape reader and punch unit performs paper tape input and
output at a considerably faster rate than the low-speed reader and
punch. It is of great value in any system that relies on paper tape
as a primary medium of data and program storage.

ihH-Specd Papar Tape Unit

The high-speed paper tape reader and punch unit is available in
twoversions: the raCk mounted PCS—EA illustrated in Figure 4—9,.
and the table top PCS—EB. Both units consist of a PRS-E highspeed paper tape reader and a PC8—E high-speed paper tape
punch, mounted on a single chassis. The reader and punch are
also available separately
The high—speed reader accepts input data from eight—channel,

fan-folded, non-oiled paper tape at a maximum rate of 300 characters per second, or thirty times the LT-33 maximum input rate.
The high-speed punch records output data at a maximum rate of
,

50 characters per second. All reader and punch operations are
executed on-line. They may be controlled directly by the Computer
or from the

keyboard through the computer.
The reader and punch are each supplied with an ON/OFF
rocker switch Which applies power to the respective units in the
I

4-19

‘

position and disconnects power in the OFF position. Each
device is also provided with a FEED switch which advances the
tape without reading, in the case of the reader, or advances tape
with only the feed holes punched, in the case of the‘punch unit.
The reader is supplied with a control knob‘which may be turned
counter- clockwise to raise the

tape retaining lever and free the

tape, or clockwise to lower this lever and engage the sprocket
wheel.

following procedure is employed to position tapes in the
high-speed reader:
The

Turn the control knob to raise the tape retaining lever.

'2.

Place a fan-folded tape in the right-hand bin.

Place several folds of leader in the left—hand bin and posi—
tion the tape
holes.

that the sprocket wheel engages the feed

Turn the control knob to lower the tape retaining lever.

Press the FEED switch

properly positioned.
Tape is advanced and read by programmed computer in-

briefly to ensure that the tape is.

struction's.
Extended Memory
PDP—8 series computers have a basic core memory composed of

4096 twelve-bit words. Core memory may be expanded by the
addition of up to seven 4096-word memory modules, providing a

maximum storage capacity of up to 32,768 words. Each module
is called a memory field; Memory fields are numbered from 0 to 7,
with memory field 0 designating the original 4096 words of core.
Core locations within each memory fieldare numbered from 0 to

77778 (409510).
‘

One twelve— bit data word is capable of addressing a maximum

4096, unique locations. However, PDP 8 series com—
puters use two special 3—bit registers which permit 215, or 32,768,
of 212,

locations to be addressed.

During program execution the content of the instruction field
register determines the memory field from which the operand of a

directly addressed instruction should be taken. Any directly addressed TAD, AND, 182 or DCA will obtain its operand from the
designated memory field, and any indirectly addressed TAD,
4—20

AND, ISZ or DCA will obtain its pointer from the designated
'

memory field.

After an indirectly addressed TAD, AND, ISZ or DCA instruc-

tion obtains its pointer from the memory field designated by the

instruction field register, it then obtains its operand in a similar

designated by the data field register.
The instruction field and data field registers are originally set by
loading the desired binary designations into switch register bit
positions 6-8 (instruction field register) and 9-11 (data field reg—
ister), then pressing the EXT ADDR LOAD switch. These regis—

manner from the memory field

ters may also be loaded under program control.

Aside from its more obvious applications, extended memory is

Commonly used to store important system software. For example,
the BIN Loader might be permanently stored in memory field 1.
If the instruction field register is loaded with 001 and the data field
register is loaded with 000, in this case, the BIN Loader will execute in memory field 1 and store a binary program (which is.

sirnply data to the Loader) in memory field 0.

DECtape System

DECtape provides
optional auxiliary magnetic tape storage
for
PDP-8 series computers. A DECtape
updating facility
system consists of at least one DECtape control unit and One to
eight TUS 6 DECtape transport units. In contrast to conventional
magnetic tape devices which store information in sequential, variable-length positions, DECtape permits allocation of fixed, adr
dressable positions for information storage. DECtape is bidirectional, although data is usually read in the same direction that it
was written, and it records information on pairs of identical, noné
adjacent channels, to minimize any chance of data loss.
A standard PDP-8 DECtape contains 2702's blocks of 2018.12—
bit data words each. This provides a total usable storage of
1563 ,3028 (or 190 ,14610) 12- bit words per standard tape.
Nonstandard tapes may be prepared for special applications.
The TUS 6 DECtape Transport Unit shownin Figure 4- 10 reads
and writes the 10- channel magnetic tape. Tape movement may be
controlled by program instructions or by manual operation of
switches located on the front panel of the transport. Data is trans—
ferred only under program control. The manual transport controls
an

"and

are identified in Table

4-3.
4-21

TU56

Figure 4-10

TU56 DECtape Transport Controls

Table 4-3

DECtape Transport Unit

Transport Control Position

REMOTE

Function
This switch position energizes the DECtape transport and places it under program control.

OFF

This switch position disables the DECtape

transport.

This switch position energizes the DECtape transport and places it under operator
control from external transport
switches.

LOCAL

’

WRITE ENABLED

This switch position enables the DECtape
for search, read, and write activities.

WRITE LOCK

This switch position limits the DECtape
transport to search and read activities
only. (This prevents accidental destruction of permanent data.)

Unit Selector
-

The value Specified by this eight-position
rotary dial identifies the transport to the
control unit.
With the transport in LOCAL mode, depressing this switch causes tape to feed
onto the

right hand spool.

With the transport in LOCAL mode, depressing this switch causes tape to feed
onto the left-hand Spool.

4—22

following operations will initialize ‘a DECtape transport
unit for use under program control. It. is assumed that a preformatted DECtape (see Chapter 7) is employed.
The

Set the REMOTE/ OFF / LOCAL switch to OFF.

Place a DECtape on the left spindle with the DECtape label

out.
.

Wind four turns of tape onto the right spindle.

Set the REMOTE/ OFF / LOCAL switch to‘LOCAL.

.Wind a few turns of'tape onto the right spindle with the ->
switch to make sure the tape is properly mounted.
Dial the correct unit number on the unit selector dial.

Set the REMOTE/OFF/LOCAL switch to REMOTE. Se-

lect either WRITE-ENABLE or WRITE LOCK.

DECdisk Systems
The DF32D DECdisk is a

low-cost, random-access, bulk stor—

age device and control mechanism with a storage capacity of
32,768 twelve-bit words. Data is transferred by means of the data

break facility at a maximum rate of 34 microseconds per word.
The DF32D system

can

accommodate up to three DS32D

pander disks, providing an economical
131,072 Words

storage capacity

ex-

of up to

The R508 DECdisk and RF08 controller combination provides

storage for 262, 144 words per disk. Up to four R508 disks may be
driven by one controller, allowing a maximum system storage ca-

pacity of over 1‘ million words. Data is transferred at a maximum
rate of 16. 7 microseconds per word, and average access time is
less than 20 milliseconds.

The RK8 Disk Cartridge System consists of one RK08_- P Disk
Interface Control which will drive up to four RK01 disk files.
System capacity is 831,488 words per disk, or up to 3.3 million

words with the maximum system configuration; The RK8 Disk
System will transfer a full 4096 words in about 80 milliseconds.Extended Arithmetic Element

The KE8-E Extended Arithmetic Element (EAE) is an option

for the PDP-8 / E which provides

circuitry to perform arithmetic
“operations that cannot be performed directly using the basic PDP8/E instruction set. The EAE microinstructions permit multipli4-23

cation and division of unsigned integers to be performed directly.

Other microinstructions

perform arithmetic or logical shifts and
normalization. EAE microinstructions also permit double precision
numbers to be added, complemented, incremented and stored
directly.
The 12-bit Multiplier Quotient (MQ) Register is used in conjunction with the accumulator to perform multiplication, division
and double-precision operations. The content of this register is
displayed on the PDP-8/E programmer’s console when the indi—'

cator selector knob is set to M0.

A one—bit register called the Greater Than Flag (GTF) is used
as an.

extension of the MO Register during shift operations. The

GTF is also used to compare signed numbers while a subtraction
is performed. The state of the GTF is displayed by console indicator bit

position‘l

when the indicator selector knob is set to

STATUS.
The EAE

option is essentially an increase in instruction capability. The additional instructions, which are microprogrammable, are included in Appendix D. These instructions can effect
a significant reduction in core requirements and program execution
time by eliminating iterative coding. In addition, the double precision instructions may be used to good advantage for input buffering operations and whenever full 12-bit addressing is desired.

”EXERCISES
1.

Toggle into memory and run the programs written in Chap—
ter 2 for exercises 6 and 10.

Toggle the RIM Loader (Appendix E ) into memory using
the console switches. Verify the contents of memory with
the EXAM switch.

program to set the contents of locations 2000
through 2007 to the value of the switch register and then
Write

halt. Toggle it in and verify that it works.
4.

program to accept two numbers from the switch
register and add them displaying their contents in the acWrite

(Hint: precede each OSR instruction with a
HLT. After setting the switch register, activate the CONT
key.) Translate the program into octal and toggle it into

cumulator.

memory. Verify that it works properly.
4-24

editing

and debugging

INTRODUCTION
This

chapter introduces the reader to on—line operations with

Digital Computers. It describes how to load progIMS into core
both manually and by the use of available loaders, .how to‘ create
and edit programs, and how to debug and correct programs after
assembly or compilation. Most PDP-8/E systems require the use
of at least some of the programs in this chapter. The loaders——
RIM, Binary, Self-Starting Binary, and Hardware Bootstrap, the
Symbolic Editor, and the debugging programs—CDT and DDT,
are discussed thoroughly; The information on the Editor, ODT,
and DDT applies to these programs as they are provided on a
Paper Tape System. (Undercertain other systems, such as OS/S,
modifications may have been made to these programs either to meet

restrictions or to provide the user with an even more powerAny changes or modifications are described in the
apprOpriate manual )

core
-

ful version.

LOADERS
A loader is

short program

routine which, when in core,

enables the computer to acCept and store other programs. DEC
offers the user the following basic loaders:

5-1

any

Read—In-Mode (RIM) Loader—used to load into core
programs punched on paper tape in RIM format, in particu-

lar, the Binary Loader.
2.

Binary (BIN) Loader—used to load into core memory any
programs punched on paper tape in Binary format.

Self-Starting Binary (SS BIN) Loader—used to automati—
cally load or load and start a program in Binary format.
(SS BIN may be merged onto the beginning of a tape to
provide quick and easy loading.)

MI8—E

Hardware Bootstrap Option—a hardware option

purchased by the user containing a
bootstrap for
pre-loaded
a
particular configuration.
-

RIM Loader
.

When a computer in the PDP—8 series is first received by the

customer, its core‘memory. is completely demagnetized. With the
exception of the Hardware Bootstrap option (discussed later)
which may have been purchased with the system, the computer
“knows” nothing, not even how to accept input. However, it is

possible to manually load data directly into core using the programmer console switches.
The RIM Loader is the

first, and often only, program loaded

in this manner, and allows the computer to receive and store in
core

paper tape in RIM coded format. (This
otherstinary and ASCII—are described in

data punched

format, and

two

detail in Chapter 4.) The RIM Loader is used in particular to load

the Binary and Self-Starting Binary Loaders.
There are two RIM Loader programs—one for input from the

low-speed (Teletype) paper tape reader, and the other for input
from the high—speed reader. Table 5-1 lists the octali instructions
for these programs. The loading and verifying procedures are
detailed in the flowcharts in Figures 5-1 and 5-2. (These flowcharts are also contained in Appendix E.) After loading RIM, it
is a good programming practice to verify that all instructions have

been entered properly.

5-2

When loaded, the RIM loader occupies absolute locations 7756

through 7776.

INITIALIZE
I
SET ROTARY
SELECTOR SWITCH
TO MD

SET SWITCHES 6-8
TO DESIRED

INSTRUCTION FIEIJD*
ET SWITCHES

9-11

TO
DATA FIEL

DESIRED!’

SROULD

*DECTAPE USERS
LOAD RIM INTO FIELD O

PRESS
EXTD ADDR LOAD

SET SR
TO 7756

I
PRESS
ADDR LOAD

SET SR=
FIRST INSTRUCTION

LIFI‘ DEP

SET SR=
NEXT INSTRUCTICN

LIFT DEP

RIM IS LOADED

Figure 5-1

Loading the RIM Loader
'5-3

iNiTIALiZE

[

SET ROTARY
INDICATOR
SWITCH TO MD

SET SWITCHES
6-8 TO FIELD IN
WHICH RIM HAS

BEEN LOADED

PRESS
5x113 ADDR

LOAD

SET SR=7756

PRESS
ADDR LOAD

PRESS EXAM

ALL

CHEgKED

PRESS
ADDR LOAD

RIM IS LOADED
SET SR= CORRECT
INSTRUCTION

LIFT

DEP
,

Figure 5-2

Checking the RIM Loader
5-4

Table 5-1

RIM Loader Programs
‘

Instruction

Location

Low-Speed Reader

High-Speed Reader

6032

6014

7756

7757
7760

‘7761
7762
7763
7764

6031
5357
6036
7106'

_6011
-

7006

77657766

7510
5357'

7006

27767

6031

7770

[5367

‘

7771
7772
7773
7774
7775
7776

5357

6034
7420
3776

6016
7106
7006
7510
5374
7006
6011
5367
6016
7420
_

\3776
V

_3376
‘

5356
0000

3376
5357
0000

Binary Loader
The Binary Loader is a short utility program which, when in
core, instructs the computer to read-binary—coded data punched on
paper tape and store it in core memory. BIN is used primarily to
load DEC-supplied binary programs and binary tapes produced
by PDP—S/ E assembly programs such as PAL III and MACRO-8.
BIN is furnished to the programmer on punched paper tape in
~

RIM ‘coded format; therefore, RIM must be in core before BIN
can be loaded.

When loading BIN, the input device (low-speed or

high—speed reader) must be the same as that selected when loading
RIM, and RIM and BIN must be loaded into the same field.
Once stored in core, BIN resides 011 the last page of core, occupying absolute locations 7625 through 7752 and 7777 of the

field in which it was loaded. BIN was placed on the last

page of
all DEC’s

that it would alwaysbe available for use, as
software (with the exception 'of the Disk Monitor and OS/ 8) is
careful not to use this page. The programmer must be aware that
core so

if he writes a program whiCh uses the last page of core, BIN will

be destroyed when the program is run, and both RIM and BIN
must be reloaded before another program can be loaded into the
computer. Figure 5—3 details the method of loading BIN.
575

"-1 SEE FIGURE 5-1

LOAD RN

SET ROTARY

SELECTOR SWITCH
7T0 MD

J,
SET SWITGO‘E
5-8 To F|ELD
WMCH WNTAENS

RIM

l
SET swncngs
9-H TO HELD IN
wmcn BIN IS

TO BE LOADED

PUT am LOAOER
m

*IVITH LEADER/TRAILER

WEE THE READ D‘EAD

YES

Figure 5-3

Loading the Binary Loader
5-6

The programmer is now able to load
deseribed in Figure 5-4.

binary tapes Iusing the
'

method

Wm
SET ROTARY
SELECTW SWITCH
TO AC

i.
SET SMTCHES
6-8 To FIELD IN
WHICH BIN IS
LOADED

YES

Figure 5—4

QJECT TAPE
IS LOADED

Loading A Binary Tape Using BIN
5-7

0—me

,
_

RIM

)uul SEE FIGURES 54.5-2

SET ROTARY
SELECTOR SWITCH
TO AC

l
SET SWlTCHES
6-H TO FIELD
WHICH CONTAINS
RM
1

PRESS
EXTD AWR LOAD

SET 5R= 7756

PRESS ADDR LOAD

TURN TTY TO LINE

‘

PUT SSBIN

IN HSR

TSSBIN IN LSR

SET LSR TO START

PRESS
CLEAR AND CONT

PRESS CWT

Figure ‘5-5
,

Using 88‘ BIN with the RIM Ldader‘
5—8

Self-Starting Binary Loader
The Self—Starting Binary Loader reads binary format paper tapes
from either the high-speed or low-speed reader and, providing a
starting address has been specified, automatically starts the program at the completion of loading.
1

SS BIN itself is a RIM format program and is loaded with the
RIM Loader or the hardware bootstrap, generally as the first part
of a two-part tape. (The second part of this tape is the object
program or data to be loaded, and1s
separated from SS

physically

BIN by leader / tra11er code.)

Many DEC— supplied programs are now being distributed as
self—loading binaries that include a starting address for automatic,
starting and loading. However, SS BIN may be used independently,
in which case the binary object tape must be manually loaded, if
a starting address has been specified, it
will be automatically

started

The user may generate his own self-loading binaries; the pro-

cedures involved in this process and other detailed information
concerning SS BIN are contained in the library write-up DEC-8E-__
XBINA—A-D, which is available from the Software Distribution
Center.

SS BIN

occupies locations 7600 through 7755

and location:

7777 of the memory field into which it has been loaded. Figure
5-5 describes instructions for loading the SS BIN and object

‘

tape(s).
M18-E Hardware Bootstrap Loader
The MI8-E Hardware Bootstrap Loader is

available

hardware option

the PDP—S/E which provides a specific bootstrap
loader stored in read-only—memory as a RIM format program.
on

Using this option, the user can automatically bootstrap into core,
any DEC—supported system contained in any one of the configura—
tions listed in Table 5-2. (The Bootstrap Loader option and the
configuration are decided by the customer when he orders his
system.)
As can be seen from this table, the hardware bootstrap saves
the user the necessity of manually toggling the RIM loader into
memory (as described in Figure 5-1) and in most cases, provides
additional bootstrapping capabilities as well.
To operate the Bootstrap Loader, the user need only follow
the procedure outlined in Figure 5—6.
'

‘

5—9

Table 5-2

Hardware Bootstrap Loaders

Option Designation
MIS-E

RIM Program

Unencoded—The

user may specify any RIM
of
his
program
choosing providing it occupies
less than 32 words of core.
'

MI8-EA
M18-EB

Paper Tape—Provides the bootstrapping of the
low/high-speed RIM loader.

MIS-EC

DECtape—Provides the bootstrapping of a DECtape-based system.

‘

MI8-ED

RK8—Provides the bootstrapping of a disk-based

system.
MIS-EE

Typeset—Allows the bootstrapping of the PDP8/ E Typesetting system.

MIS-EF

EduSystem (low)——Provides the bootstrapping
a system in the
EduSystem series using the
low-speed reader.

MIS-EG

EduSystem (high)—Provides the bootstrapping
of

system in the EduSystem series using the

high-speed reader.
TD8-E

MIS-EH

(

DECtape—Ailows the bootstrapping of
TD8-E DECtape-based system.

START

MAKE SURE
COMPUTER AM).
.

TERMINAL ARE

ON—LINE

PLACE TAPE
ASSOCIATED WITH
SYSTEM IN POSITION

IF BOOTSTRAPPING
AN EDUSYSTEM,
SET SR TO
CORRECT ADDRESS

i
Figure 5-6

Using the Hardware Bootstrap
5-10

SYMBOLIC EDITOR
Introduction

The

Symbolic Editor (as used on a Paper Tape System) is
used to create and modify symbolic (source) program
tapes
from the Teletype keyboard Thus
tedious
task
of
the
preparing

source

program tapes off-line is eliminated.

The Symbolic Editor (hereafter, simply Editor), uses the Tele-

type keyboard as a. very sophisticated typewriter. That is, as the
program is typed, it is entered into core, where it can be
checked, corrected, and modified. Then, when the user is readyto

source

generate the source program tape, Editor will respond to the ap—
propriate command by producing a tape suitable for assembling
.

compiling into an object (binary) tape which will, in‘turn, run
'

on' the computer.

Editor is a very helpful tool; still, it must be told precisely what
to do.

The user directs its operations by typing certain commands

in the form of a single letter or a letter with arguments. All 00m—

mands are executed by typing the RETURN key directly after the

command

Editor occupies about 1000 locations of. core and leaves all but
the last page of core for the source program—allowing (in a 4K
core) approximately 60 lines of heavily commented text or about
340

lines of text without comments (about 420010 characters).

The source program is stored in the text buffer area of core. When
the text buffer is

‘

full, Editor rings the Teletype bell. The buffer

may then be enlarged, as explained later under Editing a Symbolic
Tape, or the contents of the buffer may be punched (dumped)
onto paper tape._ If punched on paper tape,

the Editor may be re-

(also explained later) and editing continues with a com—
pletely refreshed (clear) text buffer. The rest of the source program can then be placed into core and punched out so that the
entire source program is on one long tape, ready to be assembled
started

compiled.
Each line of text includes the terminating carriage

feed combination which is
lines in the bufier' are

produced by

return/line

the RETURN key. All

implicitly numbered in decimal, starting

with 1. This implicit enumeration is continually'updated'by Editor
to account for line insertions, moves, and deletions—a few of the

5-11

features available using Editor. For editing‘and listing purposes,
each line is referred to by its current implicit decimal line number.
Text may be entered into core using the Teletype or high-speed
paper tape reader (Editor does not distinguish paper tape from
keyboard input; it perceives only symbolic input). Loading and
-

operating instructions are contained toward the end of the chapter.
Modes of Gperation
To

distinguish between editing commands and the actual text

entered in the buffer, Editor operates either in command
mode'or text mode. In command mode all input typed on the
to be

keyboard is interpreted as commands to Editor to perform some
operation or allow some operation to be performed on the text
stored in the bufier. In text mode, all typed input is interpreted as
text to replace, be inserted into, or be appended to the contents
of the text buffer.
TRANSITION BETWEEN MODES

Immediately after being loaded into core memory and started,
Editor is in command mode; that is Editor is waiting for a command. The desired command code is then typed and terminated
by the RETURN key, which instructs the Editor to carry out the
command.
With Editor in text mode (through use of the Insert or Append

commands) corrections or insertions may be typed to the text. To
terminate text mode, a form feed, (CTRL/FORM combination)
‘or CTRL/G may be entered, instructing the Editor to «return to
command mode; Editor answers by ringing the Teletype bell to
indicate the transition back to command mode.
Command” Stmcture
A command directs Editor to perform a desired. operation. Each
command consists of a
or

single letter, preceded by zero, one, two

three arguments. The command letter tells Editor what to do;

the arguments usually specify which numbered line or lines of text

are affected. Commands to Editor must take one of the following

forms, where E represents any command letter.
,

NOTE

Except where specified, each line typed by
must be entered to the computer
by typing the RETURN key.
the

user

5—12

‘

Table 5-3

Command Structure

Command

Type Of Command

Format

Meaning

Perform operationE.

N0 Argument
One Argument

Perform Operation E on the ref-

nE
'

erenced line.
Two Arguments

Perform

m,nE

operation E on lines m
(m < 11).

through n, inclusive
Three Arguments

by MOVE command only
(move lines m through n to be-

m,n$jE»

Used

fore line j).

The arguments m and n, which refer to numbered lines in
memory, must be positive, and n must be greater than In. Two

arguments must be separated by'a comma, but no comma is allowed
between the arguments and the command.

Special Characters and Functions
A number of Teletype keys have special operating functions.
These keys and their associated functions are listed ‘below:
RETURN 'KEY

In both

command and text modes, typing the RETURN key

signals Editor to process the information just typed. In command
mode, it allows Editor to execute the command just typed. A com—
mand will not be executed until it is terminated by the RETURN
key (with the exception of 2, explained later). In text mode,
RETURN causes the line of text which it follows to be entered
'

into the text buffer. A typed line is not actually part of the buffer
until terminated by the RETURN key.

ERASE (CTRL/U)
The erase character (CTRL/U combination) is used for error
recoveries in both command and, text modes. It is generated by

holding down the CTRL key while typing a U and is not-echoed
on the Teletype. When used- in text mode, CT RL/ U cancels
everything to the left of itself back to the beginning of the line; Editor
performs a carriage return/line feed (CR/LF). The user then
continues typing on the next line. When used in command mode,
CTRL/ U cancels the entire command; Editor prints a ? and per—
5—13

forms a CR/LF.IThe erase character cannot cancel past a CR/LF
in either command or text mode. For example:

Command Mode:
A?

Text Mode:
THIS
HERE

TEXT

MODE

EXAMPLE

The Editor automatically performs

carriage return/line feed
after CTRL/ U is struck. In the first example above, CTRL/ U was
typed immediately after “A”, in the second example it was typed
immediately after “THIS”.
a

RUBOUT KEY
Rubout is used in
modes with

error

recovery in both command and text
When executing a READ command

exception.
from the paper tape reader, rubouts are ignored
below)
(explained
one

completely and do not go into the buffer. It is necessary for the
READ command to disable the rubout function since all tab characters on paper tape are, for timing purposes, followed by rubouts

which would destroy the tabs. Rubouts are not stored in the text

buffer but are inserted by Editor following all tab characters on
'

the output tape.

At any other time in text mode typing the RUBOUT key echoes

(\) and deletes the last typed character. Repeated
rubouts delete, from right to left up to, but not including, the CR/
LF‘ which separates the current line from the previous one. For
example:
a

backslash

THE

QUUICK\\\\ICK BROWN

FOX

will be entered in the bufferas:
THE

QUICK BROWN

FOX

When used in command mode, RUBOUT is equivalent to the

CTRL/ U and cancels the entire command; Editor then prints a ?,
performs a CR/LF, and waits for the user to type another command.

5-14

‘

.FORM FEED '(CTRL/FORM)

Form feed signals Editor to return to command mode. A form
feed character is generated by pressing the CTRL key While typing
the FORM

(/L) key. This combination is typed while in text

mode to indicate that the desired text has been entered and that

Editor should 'now return to command mode. Editor

‘

rings the

Teletype bell in response to a CTRL/FORM to indicate that it is
back in command mode. If Editor is already in command mode
when CTRL/FORM is typed, no bell will sound, CTRL/BELL
(/G) is equivalent to CTRL/FORM (except in the case of a
SEARCH command as explained later).
DOT (.)

Editor keeps track of the implicit decimal number of the. line on

which it is currently operating. At any given time the dot, which is
produced by typing the period key, stands 'for this number and
may be used as an argument to a command. For example:

means list the current

line, and

o'll.+lL

means

list the line preceding the current line, the current line, and

the line following it, then update the current line counter to the

decimal number: of the last line printed. The current line counter,

represented by the dot, is generally updated as' follows:
After a READ or APPEND command, dot is equal to the

.1.

number of the last linein the buffer
After an INSERT or CHANGE command, dot is equal to

2.
-

3.
,4.

the number of the last line entered.

After a LIST or SEARCH command, dot is equal to the
number of the last line listed.
After a DELETE command, dot is equaHo the number of
the line immediately after the deletion.

9.01

After a KILL command, dot is equal to 0.
After a GET command, dot is equal to the number of the
line

printed by

the GET.
5-15

After a MOVE command, dot is not updated and remains
whatever it was before the command.

SLASH (/)
The symbol slash (/ ) has a value equal to the decimal number

of the last line in the buffer. It may also be used as an argument
to a command. For example:

IGJIL

means list from line 10 to the end of the butter.
'0

LINE FEED KEY

Commands and lines of text are terminated by the RETURN

key which generates a carriage return and a line feed combination.
Line feed characters are completely ignored when input is on paper
tape. During output, Editor automatically punches a line feed following each carriage return.
Typing the LINE FEED while in command mode is equivalent
to typing:
‘

.+1L

and will cause Editor to print the line following the current one
and to increment the value of the current line counter

(dot) by

one.

ALT MODE KEY

Typing the ALT MODE key while in command mode will also
cause the line following the current line to be printed and the
current line counter (dot) to be incremented by one. If the current
line is also the last line in the buffer, typing either ALT MODE or
LINE FEED will cause a “Mo be typed by Editor indicating that
there is no next line. (Some Teletypes have an escape key (ESC)
in place of the ALT MODE; the function is. identical for both
ESCape and ALT MODE.)
RIGHT ANGLE BRACKET (>)

Typing the right angle bracket (>) while in command mode is
equivalent to typing:
.+lL

5-16

’

and will cause Editor to echo > and then print the line following
the current line. The value of the current line counter is increased

by one so that it refers to the last line printed.
LEFT ANGLE BRACKET

(<)

Typing the left angle bracket (<) while in command mode is
equivalent to typing:
o-lL

and will cause Editor to echo < and then print the line preceding
the current line. The value of the currentline counter is decreased

by one so that it refers to the last line printed.
EQUAL SIGN (2 )
The equal sign is used in conjunction with the line indicators,
dot (. ) or slash (/). When typed in command mode it causes
Editor to print the decimal valueof the argument preceding it. In
this way the number “of the current line may be found (.=XXX),
or the total number of lines in the buffer

( /: XXX) or the number

of some particular line (/— S—XXX ) may
counting from the beginning.
1

be determined without
~

COLON (z)
Colon is a lower case character with exactly the same function
as the

equal sign (2).

BLANK TAPE AND LEADER/TRAILER
Both blank ,tape and leader/trailer

(code 200) are completely

ignored on an input tape as are line feed characters and rubouts.
Line feeds andrubouts are automatically replaced wherever nec—
essary on output, whereas blank tape and leader/trailer

are

not.

TABULATION ( CTRL/ TAB)
Editor is written in such a way as to simulate tab stops at eight

Teletype paper. When the CTRL key
held down simultaneously, Editor produces a
tabulation. A tabulation consists of from one to eight spaces, de—
pending on the number needed to bring the carriage to the next
tab stop. Thus, the user may use Editor to produce neat columns
on the hard copy (printed page)

space intervals
and I key are

across

the

5-17

This tab function is used in connection with two switch register.

options (for input and output) )to allow the user to produce and
control tabulations in the text buffer during input and output
Operations (see Switch Register Options below). On input (under
a READ command) Editor can replace a group of two or more.
spaces with a tabulation if the user chooses to set bit 0. On output
't will produce ei er a. ta character followed by a rubout (for

timing purposes) or enough spaces to reach a tab stop, depending
on the settings of bit 1. Editor cannot output tab characters unless
tabulations have been entered in the buffer either from the keyboard or through'setting bit 0 on input.
NOTE
Location

0002

contains the

negative (2’s

complement) of the number of spaces used”
to simulate'tab stops. To change the tabula—
tion simply change the constant in location
0002 after loading the Editor.
Switch Register Options

register bits in conjunction With the
actual input and output commands to control the reading and
punching of paper tape. It is sometimes desirable to be able to
interrupt a command before it finishes. For example, if you mistakenly gave a LIST command in place of a PUNCH command
and you do not wish to wait) for the Teletype to list a large section
of text; bit 2 on the console switch register allows you to interrupt
any output command and return immediately to command mode.
Table 5—4 lists the switch register
bit options:
Editor

uses

five switch

Table 5-4-

Position

Bit
.

Switch Register Options
Action and Explanation

Read the input tape exactly as it is.

Read the input tape taking note of spaces.
Each time two or more successive spaces are
found, substitute in the butter altabulation for
that whole group of spaces. This option affects
only the READ command.

5-18

Table 5-4'

Switch Register Options (Cont)
Action and Explanation

Position

Bit
1
z

0-.

On punching (or listing) text from the buffer,
tabulations are to be interpreted as an appro—

priate number of spaces.‘

interpreted as a tab character
followed by a rubout (211;377).
Tabulations

are

Normal Operation. All output commands completed as specified.

Suppress list, punch, or search operation. If at
any time during execution of an output command this switch1s set to 1, output will cease,
Editor will return immediately to command
mode. (If this occurs while a line is being
searched, any modifications to the line made
during that search will be disregarded.) The
current line counter (.) Will be equal-to the
number of the line being printed or punched
at that time. Until the switch-is set to 0, any
further output command will be ignored.
10

Low-speed output. All punching will be done

via the Teletype punch.

High-speed output.\A11 punching will be done
via the high-speed punch.
11

READ command expects the source tape to be in the Teletype
reader. DO NOT use the APPEND command

Low-speed input. The

0
-

to read tapes.

High-speed input. The source tape will be read

from the high-speed reader.

Command Repertoire

Commands to the Editor are grouped under three general head-

ings: Input, Output, and Editing. Explanation of the three types
of commands is given in- the following sections. Each command

description will state if the Editor returns to command or text
mode after completing the operation specified by the command.
The Editor Will print an error message consisting of a question
mark whenever the user has requested nonexistent information or
5-19

‘

used an inconsistent or incorrect format in typing a command. For

example, if a command requires two arguments, and only one (or
none) is provided, the Editor will print ?, perform a carriage
return/ line feed, and ignore the command as typed. Similarly, if
a nonexistent command character is
typed, the error message ? will
be printed, followed by a carriage return/line feed; the command
will be ignored. (However, if an argument is provided for a com—
mand that does not require one, the argument will be ignored and
the normal'function of the command performed.) For example:

Message

Explanation

The butter is empty. The user is asking

fer nonexistent information.

7. SL

The arguments are in the wrong order.

?
.

nsmm

The Editor cannot list backwards.
This command requires two arguments

before the 33, only one was provided.

Nonexistent command letter.

INPUT COMMANDS

lnput commands allow text to be entered into the text buffer
either from the paper tape reader or the Teletype keyboard. The
Editor is in text mode until a form feed (CTRL/FORM) is
encountered.

Table 5-5

Input Commands

Command

Action and Explanation

READ a page of text from paper tape reader. Depending on the position of switch register bit 11, reading will
be done from either the high-speed or the Teletype
reader. The Editor will read information from the input
tape until a form feed character (CTRL/ FORM key
combination) is detected or until the Editor senses a
text buffer full condition. All incoming text except the
form feed is appended to the contents of the text buffer.
Information already in the buffer remains there.
In the case of input via the high-speed reader, the
end of the tape‘will be interpreted as a form feed if an

‘

actual form feed character does not appear on the tape;

5-20

Table 5-5: Input Commands (Cont.__)
Action and Explanation

Command

the Editor returns to command mode. In the case of
input via the Teletype reader, a' form feed must be
entered via the keyboard to return the Editor to command mode if an actual form feed character does not
appear on the tape. If this is not done, the READ command is still in effect and all Subsequent commands will
be interpreted erroneously as text and appended to

what was just read from tape.
Any rubout encountered during
'

READ command

will be ignored. (See RUBOUT.)
APPEND the incoming text from the Teletype keyboard
to the information already in the buffer (the buffer may
be empty initially thus allowing the user to create a new
file). The Editor will enter the text mode upon receiving
this command and the user may then type in any number of lines of text. The new text will be appended to
the information already in the butter, if any, until the
form" feed (CTRL/ FORM key combination) is typed.
As mentioned, by giving the APPEND command
with an empty buffer, a symbolic program may effectively be generated
by entering the program via
the keyboard.
Any rubout encountered during execution of an AP-

on-line

PEND command will actually delete the last typed character. Repeated rubouts will delete from right to left up
to but not beyond the beginning of. the current line.

The APPEND command must not be used to readpaper tapes from the Teletype reader since every rubout on the tape will delete a character.
nI

INSERT before line 11, the text entered from the Teletype keyboard. The- Editor enters text mode to accept
1n,put and the first

line typed

becomes the new line
11. Both the line
numbers of all lines
following the insertion are increased by the; number
of lines inserted; the value of the current line counter (.) is equal to the nUmber of the-last line .inserted.
command mode, the
To reenter
CTRL/ FORM
count and

(form feed)

the

combination must

be typed. This terall subsequent com-

minates text mode; if not typed,
mands will be interpreted erroneously as text and
entered in the program immediately after the intended insertion.
'

INSERT,

without" an argument, will insert text be-

fore line 1.
~5-21

NOTE
In

these

commands,

the

Editor

ignores

ASCII codes 340 through 376. These codes
include the codes for the lower case alphabet

(ASCII 341-372). The Editor returns

to command mode

only. after the detection

of a form feed or when Editor senses a buffer full condition
a

(see section on Editing

Symbolic Tape).

OUTPUT COMMANDS
‘

Output commands are subdivided into list and punch commands.
List commands will cause the printout on” the Teletype of all or
any part of the contents of the text butter to permit examination
of the text. Punch commands provide for the output of leader
trailer, form feeds, corrected text, or for the duplication of pages
of an input tape. List or punch commands do not affect the contents of the buffer.

List Commands
The following commands cause part or all of the contents of the
text buffer to be listed on the Teletype.

Table 5-6

List Commands

Command

Action and Explanation

LIST the entire page. This causes the Editor to
list the entire contents of the text buffer.

LISI‘ line It. This line will be printed, followed by
carriage return and a line feed.

m,nL‘

LIST lines m through n, inclusive (111 must be
less than 11). Lines m through It will be printed

onithe Teletype.
The Editor remains in command mode after a list command and
the value of the current line counter is updatedto be equal to the
number of the last line printed.

Punch Commands

The following commands control the punching onto paper tape
of leader/ trailer, text, and form feeds.

5-22

Note that the PUNCH and NEXT commands halt the computer

before executing the command. This is intended to let the luser'be
sure the control switches are set correctly before any tape is
»

punched. Pressing the CONTinue key on the console will cause
the Editor to proceed with the command. The Editor remains in
command mode after execution of any command which punches
‘

tape.
The Editor is designed to minimize the

possibility of illegal

or
.

meaningless characters being punched into a source tape, therefore the illegal (nonexistant) codes 340-376 and 140-177, and
most illegal control characters will not be punched. In this way a
tape containing illegal characters may be corrected by simply
reading it into the Editor and punching it out.
Depending on the position of switch register bit 10 (see Table
5-4) punching will be done by. either the Teletype punch or the
high-speed punch.
'

Table 5—7
Command

Punch

Action and Explanation

PUNCH the entire

PUNCH line 11 only.

contents of the text buffer.
_

PUNCH lines In through n, inclusive (where m must
be less than n).

m,nP
The above
the

text.

Commands

commands

do not

output

form feed character following

FORM FEED. This command causes. the punching.
of four blanks, a form feed character, and approxi--

mately two inches of blank tape. If using low-speed.
punch, turn punch 01? before typing command then
turn on immediately after typing carriage return.
T

TRAILER.~ This command causesabout fourinches
of blank tape to" be punched. ‘If using low-speedpunch, turn punch 01? before typing command then
after typing carriage return.
turn on

immediately

The F and T commands do not halt the computer before punching
tape. If using the low-speed punch, the user must therefore turn on the

punch immediately after typing thecarriage return. The codes for a
carriage return and line feed. may be punched-onto the tape in some
_

instances.

5-23

Table 5-7 Punch Commands (Cont._)

Command

Action and Explanation

NEXT. This is a utility command which combines
the functions of four commands. It punches the con:
tents of the bufler, punches some blank tape,

form

feed, more blank tape, kills the buffer, and reads in
the next page of text from the reader specified by
switch register bit 11 (i.e., it executes P, F, K, R).
Executes the above sequence 11 times. The Editor
halts only before the first punching. If n is greater
than the number of pages of input tape the command
will proceed in the specified sequence until it reads
the end of the input tape, then it will return to command mode. (If using Teletype reader, when tap’e
runs out type CT RL/ FORM to return to command

mode.)
NOTE

Output operations may be interrupted by
setting bit 2 of the switch register (see section on Switch Register Options).
EDITING COMMANDS
The following commands permit deletion, alteration, or expansion of text in the buffer.
Table 5-8

Editing Commands
Action and Explanation

Command

CHANGE line n. Line 11 is deleted, and thelEditor
enters text mode to accept input. The user may now
type in as many lines of text as he desires .in place

of the deleted line. Rubouts are recognized during
any CHANGE operation. If more than one line is

inserted, all subsequent lines will be automatically
renumbered and the line count will be

priately.
m,nC

CHANGE lines m

through

updated appro-

n, inclusive

(m must be

numerically less than 11). Lines m through 11 are deleted and the Editor enters text mode allowing the
type in any number of lines in their place.

user

All

subsequent lines will be automatically renum-

bered to account for the change and the
will be updated.

5-24

line count
_

Table 5-8

Editing Commands (Cont.)
Action and Explanation

Command

After any CHANGE

operatiOn, return to com-

mand mode is accomplished by typing a form feed
( CTRL/ FORM) to terminate input. After a
CHANGE, the value of the current line counter (.)
is equal to the number of the last line of the change.

DELETE line 11. Line n is removed from the text
buffer. The numbers of all succeeding lines are re-

duced by one, as is the line count.
DELETE lines In through 11, inclusive. The line fol-

m,nD

lowing n becomes the new line In and the rest of the
lines are renumbered accordingly. The value of the
current line counter (.) is equal to the number of
'

the line after the deleted line or lines. The Editor remains in command mode after all DELETE operations.
-

tag.

GET and list the next line which begins with a
.The Editor begins with the line

following the cur.-

tests for a line which does not bewith
a
tab, slash, or space. This will most often
gin
be a liner beginning with a tag. It might also be a line
containing an'origin. For example:
rent line and

this is the current line

TAD

DCA

/THIS
HERE:

COMMENT

this line would be printed
by the command G

:22

*5836

this line would be printed
next if .anéther G were

typed.
nG

GET and list the next line which'begins with a tag
starting the search with line n. The Editor begins
with line 11 and tests it and each succeeding line as
described above.
Both G and nG update the current line counter
after finding the specified line. However, if either
version of the GET command reaches the end of the
buffer before finding a line beginning with other than
a tab, slash, or
space, the current line counter retains
the value it had before the GET was issued, and a ?
'

typed to indicate that no tagged line was found.
5-25.

Table 5-8 Editing Commands (Cont)
Action and Explanation

Command

The Editor remains
in command mode after a GET
command.
’

KILL the entire page in the buffer. The values of the
special characters / and are set to zero. The Editor
remains in cemmand mode.
.

‘

MOVE lines m through 11 inclusive to before line i
(m must be numerically less than n and j may not be
in the range between m and n). Lines m through 11
are moved from their current position and inserted
before line j. The lines are renumbered after the
move is completed although the value of the current
line counter ‘(.) is unchanged. Moving lines does not
use any additional bufler space.
A line or group of lines may be moved to the end
of the buffer by Specifying j as /+1. For example:
1,10$/ +1M. Since the MOVE command requires
three arguments, it must have three arguments to
move even one line. This is done by specifying the

m,n$jM

‘

same

line number twice. Example:

5: 5 523M

This will move line 5 to before line 23. The Editor
remains in command mode after a MOVE command.
‘

SEARCH line n for the character specified after the
carriage return which enters the command. Allow
modification of line when this character is found.
The SEARCH command is one of the most useful
functions in the Editor. It is also structured somewhat difierently from the other Editor commands.
After terminating the command nS with a carriage
return the user has told the Editor to SEARCH line
11, but 'he hasn’t specified what to search for. The
Editor is, therefore, waiting for the user to type a
character. The character 'he types is taken as the
object of the search but is not echoed. The Editor
instead immediately begins printing the specified line.
After typing the character for which it is searching,
the Editor stops. All of the editing features are then
of
available to the user. He may proceed using
the following options:

any

1. Delete the entire printed portion of the line by
typing CTRL/ U (erase), (a carriage return/

line feed18 generated)

5-26

Table 5-8 Editing Commands (Cent)

Bit

Action and Explanation

Position
_

2. Delete the entire unprinted portion and terminate the line and the search

TURN key.
3. Delete from

by typing the RE-

right to left one of the
characters for each \‘ (rubout) typed.

4: Insert

characters

after

the last

one

printed
printed

simply by typing them.
5. Insert a carriage return/ line feed, thus dividing
the line into two, by typing the LINE FEED

key.

searching to the next occurrence of
the search character by typing CTRL/ FORM
When printing stops all options are again avail—
able.
7. Change the search character and continue
searching by typing CTRL/ BELL followed by
_the new search character.
Each time the Editor prints the character for
~which it is searching, printing stops and all or any
combinations of the above operation may be carried.
6. Continue

out.

m,nS

SEARCH lines

through n inclusive in the same

as described above. The search character is inafter
the carriage return and all of .the options
put
are available. The onlydifference is in option number 2; typing the RETURN key deletes the entire
unprinted portion of the line and terminates that.
line, but the search continues on the next line.
By typing CTRL/ BELL to change search characters, all editing of a single linelmay be done in one
pass. Clearly, typing CTRL/ BELL twice will cause»the search to proceed to termination, since the search
character will now be BELL which is not stored in
the buffer.

way

By typing S with _no arguments the entire buffer
may be searched for occurrences of a single character. It must be remembered, however, that as with

every CHANGE command, every SEARCH com»
mand uses additional buffer space for storage of the

5-27

Table 5-8

Command

Editing Commands (Cont.)
Action and Explanation

line. This is obviously necessary, since the program can have no prior knowledge of whether the
size of the line will be less than, greater than, or
new

equal to that of the old line, and it must therefore
assume that it will be larger. As the entire buffer is
searched, a new image of the text is created in core
that is guaranteed to occupy the same or less space
than previously, since all deleted spaces have been
removed. The Editor recognizes this and immediately
moves the text image back to the top of the butter
space. Thus, the only prerequisite to condensing the
text image is that there be enough core space left to
contain another image of the edited text. The options
available to the user

are

the

same

described for
'

m,nS.

Operating Procedures
After the Editor has been loaded,

it may be used to read into

the text buffer a page of the symbolic program to be corrected.
Corrections and additions may be either entered from the Teletype keyboard or inserted from paper tape via the reader. The

corrected lines, groups of lines, or
be listed or punched.

the entire page of text may then

The following pages describe the sequence of operations neces-

edit, and punch out a corrected symbolic program
tape; an example of Editor usage is given at the end of this section.
sary to load,

LOADING AND OPERATING THE SYMBOLIC EDITOR
The

Symbolic Editor

is loaded into

core

using

the

Binary

Loader (or SS BIN). The loading procedure is illustrated in the
flowchart in Figure 5-7. After loading, the user should select the

desired switch register settings as detailed in Table 5—4. Loading of
any symbolic tapes to be edited and all subsequent operations are

performed through the use of the Editor by giving appropriate
commands from the Teletype keyboard.
The Editor resides1n core in locations 200-1624.

5-28

LOAD BIN

LOAD EDITOR

{SEE FIGURE

5-3.

"-T-‘isEE FIGURE

5-4

-----

SET SWITCHES
6—8 TO FIELD
EDITOR IS IN

PRESS EXTD
ADDR LOAD

SET SR

0200

F—J

PREss‘ADOR LOAD

TURN TTYTO LINE

PRESS CLEAR
AND CONT

WITH CR/LF

SWITCH OPTIONS

(SEE TABLE 5-4)

EDITOR IS IN
COMMAND MODE

TYPE A AND
RETURN KEYS

TYPE SYMBOLIC
PROGRAM

I
WHEN DONE TYPE

CTRL/FORM
OR CTRL/G

$3532

IN COMMAND MODE

Figure 5-7

Loading the Editor and Génerating
Program On-Line
5—29

Symbolic

GENERATING A SYMBOLIC PROGRAM OFF-LINE
The flowchart in Figure 5—8 shows how to generate a symbolic
program ofi-line using the LT—33 low—speed punch. This procedure is generally much slower than using the symbolic Editor,
but in

(such as creating extremely short programs)
may prove adyantageous. Leader/trailer made up of 200 code
(rather than the blank tape produced by. the HERE IS key) may
be generated off-line by pressing (in order) the SHIFT, CTRL,
REPT and @ keys and holding all down simultaneously.
some

cases

(

INITIALIZE

)

TURN TTY
TO LOCAL

TYPE RETURN AND
LINE FEED KEYS

TYPE SYMKIJC
PROGRAM

10m
YES

TYPE RETURN AND
LINE FEED KEYS

HERE IS KEY

TURN LSP OFF

E
REMOVE TAPE

Figure 5-8

Generating a Symbolic Program Oif~Line
5-30

LOADING A SYMBOLIC TAPE USING THE EDITOR:
The flowchart in Figure_5—9 shows the user the method followed

loading a symbolic tape using either the. low-speed or highspeed reader. The Editor will continue reading a tape until a form
feed code is encountered (see Input Commands). Upon recog—‘
nizing the form, feed character, the Editor enters command mode
and rings the Teletype bell to indicate that it is ready to accept a
in

command.
CAUTION

When using the Teletype reader, if the form
feed code is encountered before the symbolic

tape has completely read in (as indicated by
the ringing of the bell), turn ofl the paper
tape reader. Otherwise, characters on tape
will be interpreted as commands to the Edi—
tar. The section of tape read in up to the
.form feed code should then .be edited first

before proceeding with the remainder of the

tape.

----‘-I|_SEE

LOAD EDITOR

LQW-

HIGHSPEED

SPEED

SELECT OPTION
IN TABLE 5-4

SELECT OPTION
5-4

-IN TABLE

TURN HSR ON

TURN

TTY To LINE

PUT TAPE 1N

HSR_

FIGURE 5-7

PUT TAPE IN
-

TYPE K

TYPEK

(To CLEAR BUFFER)
AND R COMMANDS

(To

SLEAR

BUFFER)

BELL

FENEG§31EFENDN°
N

TYPE CTRL/FORM

g
®
'

mnN LSR To on:

SYMBOLIC TAPE
IS LOADED

EDITOR IS IN
COMMAND MODE

Figure 5-9

Loading a Symbolic Tape Using Editor
5-3 1

RESTART PROCEDURE

If the user stops the computer for any reason, the Editor may
be restarted. The user has the option of either clearing the text

buffer or restarting so that the text in the butter is not disturbed.
1. To clear the bufier, place 0176 in the switch register; press
ADDR LOAD, CLEAR and CONT.
To restart without clearing the buffer, set 0177 in the switch

register; press ADDR LOAD, CLEAR and CONT.
2. Set 0200in the switch register.
3. Press ADDR, LOAD, CLEAR and CONT.
The Editor is restarted and in command mode.
EDITING A SYMBOLIC TAPE
The actual editing procedure depends, of course, on a particular

requirements. The general procedure is illustrated in the
example presented shortly. For input, editing, and output commands to the Editor, refer .to the detailed explanation of. the
command structure, command repertoire, and special characters
and functions under Operating Features, or see the corresponding
summaries of commands and special characters at the end of the
chapter. Observe also the following operating notes and precau-

user’s

tions.
1. Terminate each command to the Editor by typing the RETURN key. This directs the Editorto execute the command.
2. After a command to insert, change, or append text to a sym-

bolic program has been executed, the Editor remains in the
mode until the operator types the CTRL/FORM key
combination on the teletypewriter. This combination generates the form feed code, which tells the Editor to return to
text

command mode.
3. The Editor senses a bufier full condition (buffer capacity is

approximately 60 lines of generously commented text or 340
lines uncommented) when, after completing input of a text
line, it finds that characters have been packed in the last 128
locations in the text buffer. When this condition occurs, Editor rings the Teletype bell five times and exits to command

mode. The user then has a choice of deleting text and con-

tinuing editing as normal or attempting to input more than

5-32

200 additional characters. After each
alarm will precede

line, the buffer full

return to command mode. When no
'

characters can be packed, Editor will again ring five

times and exit from the input routine. Any further attempts
to input text Will be answered in the same manner until dele-

tions have been executed to make room for text input. Al-

though characters may be received through the input device,
they probably will not be appended as text.
If Editor runs out of buffer space while searching a line,
the unsearched portion of the line may be lost or the text line
counter may be incorrectly set during the buffer full exit so
that Editor thinks there is one more line of text in the buffer
than actually exists. Occurrence of the latter will cause an
error return after or during any output operation involving
the last line, (for example, an N operation will be terminated
as soon as

the text butler is punched). 'After the error return

the line counter will contain the correct value.
'

Users should note that all such problems may be avoided

by logically segmenting a program on paper tape into
“pages” of 50 to 60 lines. This is done by punching groups
a form feed character (see Output
of 50 lines followed
Commands).

anytime

by pressing the HALT~
key; to continue press the CONTinue key.

4. The Editor may be stopped at

PUNCHING THE CORRECTED SYMBOLIC TAPE

punching out the corrected symbolic tape
depends to some extent on the user’s requirements. The general
sequence is given below and in the flowchart in Figure 5-10.
”The procedure, for

1.- As desired, enter output commands to punch blank tape for

leader/ trailer purposes (T), form feeds (F), the appropriate
lines of text (m,nP) or the entire text bufler (P).
2. Following the Punch command, the computer will halt, giv-

ing the user the opportunity to‘ check the switch register (see
Table 5-4). and to turn on the appropriate punch if he has
not done so already. Punching is intiated by pressing the
CONTinue key on the programmer’s console.
5-33

‘

NOTE
If the low—speed punch is used, it should be

turned off during the typing of commands,
as

otherwise these codes will be punched on

the symbolictape.
3.

program does not delete it from
memory. The page remains in the text buffer in core until
the KILL command (K) is given to erase it. If it is desired

Punching the symbolic

to read another tape into the bufler,'the user must first de-

lete the entire page of text (K). Remember that the recommended page length,

delimited by the form feed, is ap—

proximately 60 lines of heavily commented text. However,
the Editor will accept more text if necessary.

COMPLETED
SCIJRCE PROGRAM
N TEXT BFFER
~

HGH'

SPEED

§w1ltfi

EGlSTER omou
(SEE TAKE 5-4)

LOW-

SPEED

SELECT SWITCH
REGISTER OPTION
(SEE TABLE 5-4)

TYPE T COMMAND
AND TURN
5? ON

SET HSP 0N

TYPE T WMMAND

AFTER LENIR
TAPE lS PUNCHED.
SET LSP OFF

l
TYPE PUNCH
COMMAND

(P,nP, 0Rm.nP)
SET LSP ON

TYPE F

7—1
TYPE F COHMANO
AND WICKLY
SET LSP ON

“Ir
SETLSPOFF

T m
AND TURN
LSP ON

COMMAND

TYPE T COMMAND

TEXT |S PUNCHED

AFTER TRAILER,
TURN PUNCH on

SET LSP OFF

REMOVE TAPE

Figure 5-10

Generating a Symbolic Tape Using Editor
5-34

Error Messages
The proper rules for giving» commands must be observed during

editing, as is explained under Operating Features. If commands
are given in an incorrect format or if arguments are either missing,’
erroneous, or extraneous, the Editor will respond by printing a
question mark. Notice that some commands can legitimately take
from zero to two arguments, and one takes three. In general, if an
argument is either missing or extraneous, the Editor prints 7 and
ignores the command. Similarly, if a negative argument is encoun—
tered or an illegitimate command string is typed, the Editor again
responds with the error message ?.
-

Example of Use
The following detailed example of the editing of a page of text
is intended to familiarize the user with the-basic operations of the
Symbolic Editor. Where details of the loading sequence and operating procedures are not shown, it is assumed that the user has
followed the Correct procedures previously explained.
This example concerns a program for adding up numbers stored
in locations 200,, through 2073 of the computer, with the answer
to be stored in lOcation 4108. The program is to start in location
600. The program listing18 shown below.

IADD UP NUMBERS
*6flfl
BEGN:

ITO

HLT

START

THE

HIT

PROGRAM:

"CONTINUE"

THE

CONSOLE

/THE NEXT

FIVE

INSTRUCTIONS

I-NIT.IALIZE

M16

ILOAD

DCA

COUNTR_

xpur

INTO

TAD

rwonuw

/LOAD

DCA

POINTR

/pur

INTO

TAD

ROUTINE

wirH THE NUMBER-~1n
COUNTER
wrru

THE

ACCUMULATOR

THE

ICLEAR

CLA

FIRST

ADDRESS

POINTER

,/
[THE

EEGN.

SEVEN

INSTRUCTIONS

POINTR

ARE

THE

PROGRAM

TAD

ISZ

POINTR

IINDEX

POINTER

rsz

COUNTR

IINDEX

COUNTER,

JMP

BEGN

/N0;CONTINUE

DOA

ANSWER

HLT
JMP

xaoo

IYESESTORE
IHALT

BEGN+l

5435

ITSELF

NUMBER

ADDING

ANSWER

ZERO?

’

/THE

THREE

REGISTERS

CONTAIN

THE

CUNSTANTS

MIG:

-!fl

INEGATIUE

Twouun.
ANSWER

2am

FIRST

TALLY

ADDRESS

NUMBER
BUFFER

are

/THE

COUNTR,

.PGINTR.

Two

REGISTERS

ARE

RESERVED

FUR

VARIABLES

Assume that this program has been assembled using the PAL
III

Symbolic Assembler.
printed the following:
DT

BEGN

“686

ADDRES

6616

”A

ANSWER

0612

0616

BEGN

On pass

1, however, the assembler

060$

BUFFER

COUNTR

662G

FIRST

0616

@616

Mlfl

$615

POINTR

062!

TWOHUN

6616

The message DT BEGN at 0606 signifies that the programmer

mistakenly used identical tags to specify two different addresses. An inspection of the program listing above shows that

has

the tag BEGN has, indeed, been duplicated. It appears in line 3 of
the listing as BEGN, HLT, then in line 14, starting with BEGN,

(Since the line numbers are implicit only, they
are not shown in the example; they may be obtained by counting
from the top down.)To correct the situation, the Symbolic Editor
was read in by the Binary Loader, as explained earlier under Loading Sequence. The symbolic tape to be corrected was then loaded
by the Editor using-the READ (R) command, and a series of
editing commands were given.
TAD I’ POINTR.

.
,

5-36

‘

14L
EEGN,

14c

TAD

/ADD

POINTR

NUMEER

.
,

ADDR,

IADD

PorNrR

TAD

JMP

BEGN

amp

ADDR

NEXT-NUMBER'

17L
.

INOBCONTINUE ADDING

170

13,18L

/No;coNr1NUE

/THE NEXT
ADDR.

SEVEN

INSTRUCTIONS

TAD

ISZ

POINTR

ISZ

COUNTR

ARE

/ADD

POINTR

TRE.pRoGRAm

ITSELF

NUMBER

/1NDEx

POINTER

/INDEX

COUNTER.

ZERO?

JMP

ADDR

DCA

INOICUNTINUE
_

25L
ANswER

ANswER

/YES;STURE

ANswER

are

ANSUER.

410
'

24L
TWOHUN,

2mg

FIRST

240

rUOHUN,

IFIRST

25g._

Apia-+21.

ADDRESS

BUFFER

M10,

—ia

1w0RUN,

zoo

ANSUER,

41g

BUFFER

ADDRESS

'INEGATIVE
IFIRST

TALLY

ADDRESS

NUMBER
IN

BUFFER

.
.

Having made the desired corrections, the programmer finally
asks the Editor to list the entire text by giving the LIST (L) com-mand, but still withholds the PUNCH command (P), pending
final corrections. A new program listing isprinted Out and the entire text is preserved in the butter. The programmer now punches
the entire text onto paper tape by giving the PUNCH (P) command and depressing CONTinue on the console. The text Will be
printed as follows on the Teleprinter (by theLIST command).
5—3'7

IADD

NUMBERS

#666
BEGN.
/To

HLT

START

THE

PROGRAM:

"CONTINUE"

HIT

CONSOLE

THE

I
.

ITHE NEXT

INSTRUCTIONS

FIVE

INITIALIZE

CLA

ICLEAR

TAD MIG

ILOAD

DCA

IPUT

INTO

COUNTR

ILOAD

DCA

IPUT

INTO

POINTR

ROUTINE

ACCUMULATOR

THE

TAD TWOHUN

THE

THE NUMBER

WITH

-Ifl

COUNTER

FIRST

WITH

ADDRESS

POINTER

/THE NEXT
ADDR:

SEVEN

INSTRUCTIONS

ARE

THE

PROGRAM

TAD

ISZ

POINTR

IINDEX

POINTER

ISZ

COUNTR

IINDEX

COUNTER:

JMP

ADDR

INDIGONTINUE

DCA

POINTR

IADD

ANSWMR

[YESJSTORE

ITSELF

NUMBER

ZERO?

ANSWER

HLT
JMP

IHALT

BEGN+I

[THE NEXT

THREE

REGISTERS

CONTAIN

-IG

INEGATIUE

TUOHUN:

260

IFIRST

MSUER:

41G

MIG:

THE

CONSTANTS
NUMBER

TALLY

ADDRESS

BUFFER

ITHE NEXT

COUNTR;

POINTR;

TWO

REGISTERS

ARE

RESERVED

FOR

VARIABLES

Once the program is corrected, it can be used as input to the
PAL III assembler. The pass 1 result of assembling this program
is:
ADDR

‘

0656

ANSWER

6617

BEGN

0600

COUNTR

@626

M18
POINTR'

062!

TVOHUN

0616

9615

Smmmnnyrfifihnnhoficikfikthpenuflnm
SPECIAL KEYS
Function

Key

RETURN key

Text Imode—-Enter the line in the text
bufier.

Command mode—Execute the command.

5—38

CTRL/ U keys

mode—Cancel the entire line of
text, continue typing on next line.
Command mode—Cancel command. Editor issues a ?
carriage return/ l1ne
feed.
Text

and

RUBOUT key

Text mode—Delete from right to left one

character for each rubouttyped. Does
past the beginning of the
line. Is not in effect during a READ

not delete

command
Command mode—Same as CTRL/ U
Form Feed. (CT RL/ FORM) Text. mode—End

input,

input, return to

Command mode.

CTRL/G

Text

mode—End

return to

command mode.
Dot (.)

Command

mode—Current. line

counter

used as argument alone or in combina-

tions with —I_— or

——

and a number (as
'

in'., .+.5L)
Slash (/ )

LINE FEED key

Command mode—Value equal to number of last line1n buffer. Used as argument (as in /— 5 ,/L.)
mode—Used in SEARCH command to insert a CR/LF combination
into the line being searched.
Command mode——Li'st the next line
Text

(equivalent to .+1L).
ALT MODE key
or

ESCape key

Command

modal-List

the

line

(equivalent to +1'L).

Right Angle Bracket (>)

Command mode—Same as ALT MODE

Left Angle Bracket (<)

Command mode-é—List the previous line

key
(equivalent to .—1L).

Equal Sign (=)

Command mode—«Used. in conjunction.
with
and / to obtain their value
.

(.227).

equal sign.

Colon (2)

Command mode—Same as

Tabulation ( CTRL/ TAB ).

Text: mode—Provides a tabulation which,
on output, is interpreted as spaces or a
tab character/rubout combination de-

pending on a switch option.
5-39

SWITCH OPTIONS
Switch

Position

Meaning
Read input tape as is
Convert spaces to tabulations on input

Output tabulations as spaces
Output tab character rubout combination for each

‘

tabulation
2

Normal operation

Suppress output

Low-speed output
High-speed output

1
0

Low-speed input
High-speed input

COMMAND SUMMARY
Command

Format(s)

READ

Meaning
Read incoming text from reader and ap-

pend to buffer until a form feed isencountered.
APPEND

‘

Append incoming text from keyboard to
any already in buffer until a form feed
'

is encOuntered.
LIST

List the entire buffer.

L‘

PUNCH

List line n.

m,nL

List lines m through 11 inclusive.

Upon striking CONTinue key on
console, punch the entire buffer.

Halt.

m,nP

Halt. Upon CONTinue, punch lines 111
through It inclusive.

Upon CONTinue, punch line 11.

Punch trailer, punch a form feed

(FORM FEED F

(214),

punch trailer.
TRAILER

Punch four inches of trailer.
Punch the entire buffer and a form feed,
Read the next page.

Kill the buffer

KILL.

and

Repeat the above sequences n times.

Kill the buffer.

5—40

FormattS)

Command

Meaning

Delete line n of the text.

-nD

DELETE

‘

Delete lines In through n inclusive.

m,nD

‘

INSERT

Insert before line 1 all the' text from the

keyboard until a form feed is entered.
Insert before line 11 until a form feed is
entered.

‘

CHANGE

m,nC

Delete line n, replace it with any number
of lines from the keyboard until a
form feed is entered.
Delete lines

through 11, replace from

keyboard as above until form feed is
entered.
'

MOVE

m,n$kM

Move lines 111 through 11 inclusive to be—
'

fore line k.

GET

Get and list the next" line beginning with

tag.

Get and list the next line which begins
with a tag (starting the search with
line n)
.

Search the entire butter for the character

specified (but not echoed) after the
carriage return. Allow modifiCation
when found.
118

Search line 11, as above, a110w modifica\

tion.

m,nS

Search lines 111 through 11 inclusive, allow
'

modification.

5-41

DEBUGGING PROGRAMS

Dynamic Debugging Technique (DDT) and Octal Debugging
Technique (ODT) are the two debugging programs for the
PDP-S/E. Dynamic debugging programs are service programs
which allow the programmer to run his binary‘program. on! the
computer and use the Teletype keyboard to control program
execution, examine registers, change their contents, and make
alterations to a program.
A symbolic program can be assembled correctly and still con-

tain

logical errors, i.e., errors which cause the program to do
something other than what is intended. The assembler checks for
certain syntax errors, but not logical errors. (Syntax errors include undefined tags, misspelled tags and incorrect formats, e.g.,
omission of a required operand.) Logical errors are detected only
when the program is running on the computer.

Debugging Without DDT or ODT
If the programmer feels sure that his program is correct and
ready for use, he can simply load the program and let it run until

it stops (if it stops). And if the program doesn’t produce the cor—
rect results, the programmer without a DDT program can use the

console switches to examine

‘

specific locations one-by—one to try
to find the error(s) throUgh interpreting the console lights. There
are two hazards to this approach. First, by the time the program
stops the error may have caused all pertinent information, including itself, to be altered or eliminated. Second, the program
may not stop at all; it might continue to run in an infinite loop,
and such loops are not always easy to detect.
Added to these problems are the difficulties of interpreting
binary console displays and translating them into symbolic expressions related to the user’s program listing. Further, adding
corrections to a prOgram in the form of patches (altered and
added instructions or routines) requires seemingly endless manip—
ulation of the console switches. In all

this, the chance of pro—
grammer error at the console is large and is likely to obscure any
real gain made from debugging.
The programmer can use the program assembly listing to men-

tally execute his program. This method is frequently used with
very short programs, very short only—~human memory cannot retain every step and instruction in even a fairly short program; it
cannot match a computer memory.

'5—42
‘

What is neededto conveniently and
'

accurately debug a user

program is a service program which will assume the tasks the programmer would have to perform if he used the console switches.
DDT and ODT are such debugging programs.
DEBUGGING WITH DDT

The Dynamic Debugging Technique for the PDP—S/E'facilitates
program debugging by alloWing the user to examine core memory
locations (registers) and change and correct their contents, place

and remove strategic halts and automatically restore and execute
the instructions replaced by the halts. Communication is-carried

Teletype keyboard using defined commands and the
symbolic language of. the source program or octal representation,
with DDT performing. all. translation to and from the binary rep-

out via the

resentation.

Tracking down a subtle error in a complex section of coding is
a laborious and frustrating job if done. by hand, but with the
breakpoint facility (explained later) of DDT, the user can interrupt the operation of his program at any point and examine the

state of the program and computer. In this way, sources of trouble
can be isolated and corrected.
_

By .the time the programmer is ready to start debugging a new
program at the'computer, he should have at the console:
,

1. The binary tape of the program to be debugged.
2. The symbol definition tape which was part of the assembly

output from pass 1.
3. A list of the symbols and their definitions.
4. A complete octal/ symbolic program listing.

‘

binary tape of the DDT program, which is loaded into
core
memory using the BIN Loader.

5. A

LOADING DDT
The BIN Loader- is used to load DDT and the

object program

(program to be debugged) into core. Refer to Figure 5— 3 for the
procedure used in loading BIN, and Figure 5-4 for details concerning using BIN to load binary format programs.
DDT requires locations 0004 and 5237—7577 (23418 loca—
tions): The permanent symbol table requires locations 5000—5237,
and 'the external symbol table is allotted locations 3030—5000
(250 symbol capacity). The starting address is 5400.
5—43

The flowchart in Figure 5—11 presents the basic loading pro—
cedure for both DDT and the binary tape of the program (to be

debugged.

‘

—————-———-——_

LOAD OBJECT

"""

infer/id
LOAD ODT

‘W
,______.____._____

—-—

—

fl SEE FIGURE 5-4

SET SR= 5400

PRESS ADDR LOAD

1
PRESS CLEAR
AND CONTINUE

YES

Figure 5-1 1

DDT AND OBJECT
TAPE ARE LOADED

Loading DDT and Object Program

The following are now in core memory:
1. The DDT program, which occupies upper memory between

registers 5240 and 75 77, inclusive.
2. The user’s program(s) which must not overlap the area occupied by DDT or its permanent symbol table.
3. A table of symbol definitions, extending downward from lo—
cation 5237 to 5004. This table includes the definitions for
all of the PDP-8 memory reference instructions, operate in-

structions,

ten

basic IOT

instructiOns, and

the combined

operations CIA and LAS.
Symbol Table Tapes
Part of the punched output of a MACRO—8 or PAL assembly
may be a tape containing the symbol definitions of the assembled
program The definitions from a symbol tape are entered into
5-44

the DDT external symbol table by the procedure outlined in Figure

5- 12.

Only the LT—33 reader may be used.

Reading will continue until the end of the tape is reached or
until a total of 250 symbol definitions have been read. If this
“maximum limit is reached, no further symbols may be added to
the table until some have been deleted, even if the limit is reached
in the middle of a tape. However, the user may proceed with debugging by typing EOT (CTRL/D), then turning the reader ofi
and pressing CONTinue. The remaining symbols left unread will
not be1n the table.

TURN TTY TO LINE

UT SYMBOL TABLE
TAPE IN LSR WITH
LEADER/TRAILER
OVER READ HEAD
‘

TYPE ALT MODE
AND THEN 3 KEYS

—_—-—.—_J

"“l

TYPE

SET LSR TO START

REngE' N

RELOAD DDT AND
OBJECT TAPE

TAPE STOPS

ISET

LSR T0 FREE

DEPRESS CONT

SET

OCTAL ADDRESS
TYPED IS LOWER
LIMIT OF
EXTERNAL SYMBOL
TABLE

‘

T5}? 3”

‘3

EXTERNAL
SYMBOL TABLE
LOADED

Figure 5-12

Loading External Symbol Table Tapes (LSR Only)
'

5-45

DEFINING NEW SYMBOLS

Often, during the course of a debugging run, the user will want
to add

symbols to the external table. This is especially so
when he adds a sequence of instructions to his program as a patch
elsewhere in memory. The patch is usually identified by asymbol
which is the address tag of the first instruction in the patch. In
order to use the symbol in subsequent debugging operations, he
must add its definition to the external table as explained in the
following flowchart:
new

“

WITH DDT-8 IN
COMMAND MODE

LOAD EXTERNAL
SYMBOL TABLE

—

___

SEE FIGURE 5 12
_

TLLI
SET SRIMOO

HU
TYPE

TYPE RETURN AND
LINE FEED KEYS

HU
TYPE NEW SYMBOL

TYPE 1 OR MORE
SPACES

I
TYPE OCTAL VALUE
OF DEFINITION OF
SYMBOL

MORE
SYMBOLS

‘7va RETURN

AND

LINE FEED KEYS

Ill
TYPE EOT (CTRL/D

PRESS CONT

ADDRESS TYPED IS
LOWER LIMIT OF
EXTERNAL SYMBOL
TABLE

E
NEW SYMBOLS
ARE ENTERED

Figure 5-13

Appending New Symbols to External Symbol Table
5-46

‘

To define the symbols PATCH-l and PATCHZ,
the operations will appear as
follows (Assume that the current
For example:

limit of the table1s 4775 ):

(CR/LF)
PATCH!

619

PATCHZ

628

4665
-

(CR/LF)
(CR/LE)
(BOT)
(Press CONTinue)
(new limit of the table)

If the user makes an error while typing a definition, he cannot
use *- to

eliminate .the information. The erroneous definition must

be entered.

A symbol already in the table may not

symbols can be added.

be redefined. Only new

NOTE

carriage return/line feed pairs may
not be inserted between definitions; they
will cause errors in subsequent table lookups
when DDT is operating.
Extra

A new or updated symbol tape may be created off-line using
the method described in Figure 5- 8 in the section

concerning the

Symbolic Editor
To completely expunge the external symbol table (for instance,
when starting a new debugging run
with DDT already1n memory),
the following command1s used:
{X

On receipt of this command, DDT removes all definitions in the

external table, types a carriage return and line feed, and prints the
new

lower limit of the (now empty) external symbol table. DDT

is then ready to accept another command. The permanent tableis

unaflected

5-47

Storage. Requirements
The operating portion of DDT occupies storage in upper memory from location 5240 to location 7577, inclusive. The permanent
symbol table extends downward in memory from location 5237 to

location 5004, inclusive. This table contains the definitions of the
mnemonics for all the basic memory reference instructions, the
operate class instructions of both Group 1 and Group 2, the com-

bined instruction CIA and LAS, the symbol I fortindirect address—

ing, and the basic IOT instructions: KCC, KRS, KRB, KSF, TSF,
TCF, TPC, TLS, ION, and IOF.'There is a list of all the symbols
and definitions in the permanent table at the end of this section
on DDT.

Space is reserved for the user’s symbol table immediately below
the permanent table. A maximum of 250 such external symbols
is allowed; hence if the user’s table is filled, the lower limit of
space occupied by DDT is 3030. However, space not used for external symbols is available to the user. Each new symbol defined
on-line uses locations in the external table.
During operation, DDT uses location 4 on page 0 for the break-

point link; thus this register is not available to the user.
Definitions
A symbol is a string of up to six letters and numerals, the first
of which must be

letter. The

following

are

legal symbols:

FIMAGE, K2, X464PQ, PMLA. The following are not accept‘

able:

‘

F2.8

Does not begin with a letter
Contains an illegal character

AN PRC

A space cannot be imbedded in a symbol

GANDALF

More than six characters

4WD

string of up to four octal digits (integers).
Hence, a number may have a maximum value of 77773. The digits
8 and 9, however, may be used only as characters in a symbol.
An expression is a symbol, an integer, or a sequence of. symbols
and integers separated by any of the following operators:
A number is

+
—

An operator designating addition (arithmetic plus).
An operator designating subtraction (arithmetic minus).

5-48

space
t

An operator which indicates that the remainder of the
expression is to be treated as the address part of an instruction

All

Other characters, except those for DDT Control commands,

are

illegal.

If two or more spaces appear in succession, all. but the first are

ignored Thus, TAD TEM and TAD TEM are identical expres-

'sions.

DDT will respdnd to an extra carriage return (CR) with a car-

riage return/line feed combination (CR/LF); the extra CR’s are
otherwise ignored
The following errors will cause DDT to print a question mark
the point of the
(?) .andignore all the information typed
error and the

between

previous tab or CR.

1. Undefined symbol; illegal symbol.
2.

Illegal character.

3. Undefined control command.
’

Cross—page addressing.

Mode Control

Any expression containing a symbol is symbolic; an expression
containing only integers is octal. The programmer is free to use

whichever mode of DDTis most convenient for the information he
i

is typing. On output, DDT will print exclusively in one mode or
the other, as determined by one of the commands described below.

NOTE
When DDT is first set into

operation, the

output mode is symbolic. “[” corresponds
to typing the ALT MODE (or ESC) key.

Command

Meaning

‘

This command causes DDT to print any subsequent
item of information as an octal integer. Typed input
may be symbolic or octal. If LOC: 2642 thenthe
following commands will both print the same answer:
LDC/1263
2642/1263

5-49

This command .causes DDT to print- any subsequent

item of information-as a symbolic expression. Typed

input may be symbolic or octal. If LOC ‘= 2642 and
the contents of LOC = TAD ‘DATA+4, then:
LOC/TAD DATA+0604
2642/TAD

DATA+6554

If the user wishes to find (the octal value of a symbolic ~expres—

typed by himself or by DDT without changing the output
‘mode, he may use the following command:
sion

Typed immediately after a symbolic expression, this
will cause DDT to print the value of the expression
as an octal

integer. For example:

LOC=2642
LOC/TAD

DATA+6$04

=1263

In the second example above, the prevailing output
mode is symbolic and remains so after the use of the

equal sign.

OUTPUT
When operating in symbolic mode, DDT will

always attempt.
to make a symbolic expression out of the contents of an opened
register, regardless of whether the contents are intended toibe such
or not. For example, if register DATA contains the number 6115,
opening the register will result in the following line:
DATA/lOTfGllS

The user can use the equal sign to ascertain the octal value:
DATA/IOT+@115

=6115

Program Examination and Modification
The commands and operations in Table 5-9 allow the user to
examine and change the contents of any register in the PDP-8/E
core memory.

5-50

NOTE

Be careful not toopen and modify any reg-

ister within the DDT symbol table or pro-

itself, DDT does not protect itself
against such intrusions, whichwill inevitably

gram

cause errors in

operation.

Table 5-9 DDT Commands

Command

Meaning
This is the register examination character. Typed
immediately after an expression, it causes DDT
to print the contents of the register whose address
is specified by that expression. For example, if
the user types:
'

LOCI

DDT will type out the contents of LDC,” thus:
LOG/TAD

nATA+snaa

The

may' now change the contents of the

user

‘

carriage return
(CR)

DATA+GBG4

JMP

LOC+IQ

This causes DDT to close the opened register
after making the specified changes (if any) in its‘
contents. For example:
LOG/TAD

DATA+BGG4

JMP

LOC+lfl

Typing additional CR’s will have no effect on the
operation of DDT.
line feed

If, after examining and/ or modifying the

(LF)

tents of

con-

location, the user wishes to open the
location, he types a line feed instead of a CR. The Open register is closed, and
DDT then opens the next location, printing the
a

next sequence

5-51

’

Table 5-9 DDT

Command

Commands (Cont.)
Meaning

‘

address, a back slash to indicate that the register
was not opened by the user, the contents of the
new register, and another five spaces. For example, assume that after examining and changirg the
contents of LOC, the user wishes to examine
the contents of LOC-l—l:
LDC/TAD

DATA+aaaa

LOC+l/DCA

Loc+1e

amp

(1J0

DATA

The register LOC+1 is now open.

Line feed may be used at any time, eVen 'if the
last location examined has been closed or if other
operations have intervened. For example, if the
following sequence of operations occurs:
LDC/TAD

DATA+93$fl

JMP

.+19

DDT will still open register LOC—H. The break-

point address (explained shortly) has no effect
on the counter within DDT which keeps track of
the last opened register.
If instead of changing the c0ntents of

T (up arrow)
’

the user wishes to examine the register addressed

by those contents, he types T, as follows:
LDC/TAD

DATA+GG$4

DATA+AIOPR+337

The

Note that this operation
unmodified locations. If the

typing sOme modifying
'

open.

use

with

types it after
information, the location
user

addressed will be the onewhich is changed. For
example, if the following sequence occurs:
LOG/TAD

DATA+0664

JMP

Loc+1ni

the information will be placed in DATA+4, so
that the‘next line, printed by DDT, will look like

this:
DATA+4\JMP

LOC+GGIQ

5-52

Table 5-9 DDT Commands (Cont)

Command

Meaning
The register LOC will not be changed.
An indirect address modifier will not be

interrupted by the T operation. If, for example, the
register LOC contained TAD I DATA+4, and
the user typed Tas in the previous example, DDT
would still open the register DATA+4.
The dot is used as a symbol whose value is the
address of the last previous register opened, and
can be used in several ways:

(dot)

1. To check the

results of a modification.

LOC/TAD’DATA+06@4

LOC+16

JMP

LOC+GGIG=

-/JMP

‘

2. To refer to the currently Open register.
LOC/TAD

DATA+GGG4

JMP

.+l@

To execute any command starting at an address relative to the last opened register.
LOC/TAD

ornaments

.+-1a

amp

C'SEG
<—

( back arrow)

An error may be deleted by typing a back arrow.
All information between the <- and the previous

tab or CR is ignored; DDT responds by printing

a tab. For example:
LOC/TAD DATA+$ZB4

JMP

LC“

JMP

CROSS-PAGE ADDRESSES
When the

user

types

~+lfl

.
.

instruction to be

placed in

open
the
address
of
that
instruction
must
be
in
the
same
register,
page
the address which contains the open register. If such a crosspage address is attempted, DDT will signal an error by typing ?
as

and will ignore the information. For example: if LOC : 2642 and

XPAG ~= 2770, the following sequence would result in an error
indication :

LOC/TAD DATA‘Gflfla

DCA

XPAG+20

5-53

The expression XPAG+20 is equal to 3010, which is outside
the page containing LOC. The location LOC will be closed without modification.

Conversely, an expression containing symbols defined outside
the page is acceptable if its value is in the current page. For ex2642 and XPAG
3010, the following sequence
ample if LOC
20 has a value Which brings it within
is acceptable, since XPAG
=

the current page:

DCA

LOG/TAD DATA+6804

XEAG-2G

USING COMBINED OPERATE OR IOT INSTRUCTIONS

Except for CIA and LAS, combined Operate and IOT instructions

are

not. defined in the DDT

permanent symbol table. To,

enter such instructions into an open register, the combination must

contain no more than two mnemonics, the second of which must

be CLA. Any other combination will be treated as an error, and
the information will be

ignored. For example, the following at-

tempt is an error:
XPAG/CLA

CLA

CMA

This attempt is correct:
XPAG/CLA

CMA

CLA

If the desired combination does not include CLA, the user may
do one of two things; He can define the combined operation as a
new symbol whose value is the combined operation code. For ex-

ample, the operation CLL RAR can be defined as a symbol, say,
CLAR, whose value is 7110.

Alternatively, the user may enter the combined operation as an
expression containing the symbol OPR. For example, the opera—
tion CLL RAR can be entered as OPR+110. The user may simuse the symbol IOT in entering new I/ O combinations.

ilarly

SPECIAL LOCATIONS
There are five registers within DDT which hold information of
interest to the user. These registers may be opened and their contents changed.

5—54

open any of the following special locations, type the ALT

MODE

(or ESCape) key followed by the name of

the location.-

DDT- will print a backslash_:(\) followed by the _.contents of the
location. The contents may be altered; at this point in' the

special

same way as any ordinary location.
Location

Meaning

contents of the
When a breakpoint is encountered,
accumulator, C(AC), at that point are placed in this

the.

C(L),

Where L‘When a breakpointis encountered, the
means the link bit, at that point are placed1n this reg-~

ister.
I

This register contains the address of the lower limit of
a word
C(L) = 0001.

search. Initially,

This register contains the address of the upper limit of.
a word search. Initially, C(U)— 5000.

—

‘

This register contains the mask usedm a word search,
Initially, C(M) = 7777.
~

The use of these registers is explained in the following pages.

Program Execution and Control

The commands described1n Table 5- 10 allow the user to control

the

execution

of his program.

Table 5-10 DDT Execution Commands

Command

k[G

Meaning
This

command causes DDT to begin the execution of

starting with the instruction in the
is specified by the expression k.
address
whose
register
If a breakpoint (see below) has been requested, it is
inserted just before control is passed to the
prothe user’s program,

gram. For example, if the user types:
BGINEG

5-55

user’s

Table 5-10 (Cont) DDT Execution Commands

Command:

Meaning
DDT will transfer control to location BGIN. LikeWisei:
-“'
G
FILIv

-will cause the. user’s program to start in the fifth regis.ter preceding the one labeled F ILI.

Using [G without an argument is an error. DDT will
ignore the command, and type '.7 to indicate the mistake.
'

k[B

This causes DDT to insert a breakpoint at the location

specified by the expression k. The breakpoint is not
placed immediately, however. When this command is
typed, DDT stores the value of the address indicated by
k. Then, when the user next types either a [G or a [C
(see below) command, the breakpoint is placed just
>

before control passes to the user’s program. At that
time the sequence of operations performed by DDT is
as follows:
1. The contents of location k are saved in

special

stitutes the instruction, JMP I 4 Location 4 contains the address of a special breakpoint handling

subroutine within DDT.
3. After the breakpoint has been placed, DDT
passes
control to the user’s program.

When, during execution, the ‘user’s program encouncontaining the breakpoint, control is
immediately passed (via location 4)‘ to the breakpoint
subroutine in DDT. The C (AC) and C(L) at the point
of the interruption are saved in the special registers A
and Y, respectively. DDT then prints out the address
of the register containing the breakpoint, followed by a
ters the location

right parenthesis and the contents of A as an octal number. Control has now returned to DDT, and the user is
free to examine and modify his program.
Only one breakpoint may be in effect at one time. As
soon as the user requests a new breakpoint using the B
command, any previous existing breakpointis removed.
To eliminate the

breakpoint entirely, the command is

typed without an argument, thus:
[B-

5-56

Table 5-10 (Cont) DDT Execution Commands

Command

Meaning

When the breakpoint is removed, the original contents
of the break location are restored.
After the breakpoint has occurred and the user has

‘

examined his program and made the changes he wishes,
he can cause his program to continue from the point of
the break by means of the following command:

This continue command causes DDT first to execute the
‘instruction which was originally in the break location,
and then pass control to the next location in the user’s
program. The breakpoint remains in effect.

The following example illustrates the use of the three commands

just described. Explanations are to the right of the commands.
FILI§7EB
V

BGINEG

Breakpoint inserted at location FILI+7.
Program execution is initiated at BGIN; program runs until breakpoint location is encountered.

FILI+GGG7)7721

DDT prints the address of the break location
and the contents of. the AC at the time of the

break; note that location FILI+7 is not opened.
The user performs such

ification as he desires
[C

examination and mod-

The user’s program continues,

beginning with
the execution of the- instruction originally in'
FILI+7; the breakpoint remains in effect.
Often the user would like to place

breakpoint at a location

within a loop in his program. Since loops can run to thousands of

repetitions, some means must be available to prevent a break‘from
occurring every time the location is encountered. This is done
using the [C command; after the breakpoint is encountered the
first time, the user can specify how many times the loop must be
executed before another break is to occur, as follows: After the
5-57

‘

first breakpoint

occurrence, the user wishes to wait for 2503 repe-

titions before the next break.

The breakpoint is inserted.

FILI +7 t B
‘

User program execution begins.

BGIN t G
0

F11. r mac 7 ) 7 721

The first breakpoint occurs.

zsgtc

The program continues; the next breakpoint
will not occur until the location FILI+7 has

been encountered 250 times.

Finneganasaa

The next break occurs after 250 times through
the loop.

RESTRICTIONS ON USE OF BREAKPOINTS
The user must not place

breakpoint at any of the following

places in his program:
1. Within any section of the program which operates with the
program interrupt enabled.
2. At any location that contains an instruction which is modified during the course of the program. For example, the program contains
structions:

152

TAD

sequence which includes the

following in-

A breakpoint may not be inserted at location B.

When the user’s program comes to a halt, control may be
returned to DDT by setting the switch register to 5400 and

pressing ADDR LOAD, CLEAR and CONTinue.
3. In a location containing a subroutine jump (J MS) which is
followed by one or more arguments for that subroutine.
A breakpoint maybe inserted at the point of a subroutine call
if the JMS instruction is not followed by any subrOutine arguments, but the breakpoint may not be removed until control has

returned from the subroutine to the calling program.

WORD SEARCHES
The searching operations are used to determine if a given quan-

tity is present in any of the locations of a particular section of
memory. The search is, initiated by the following command:
'

5-58

Command

Meaning

DDT will perform a word search and print the ad-

k[W

dress and contents of every register in the desired
section of memory whose contents are equal to the:

oi the expression k. If the expression k is
omitted, a search for the quantity 0000 masked by

value

C(M) is assumed.

The conditions for any search are set by the following criteria:

1. The contents of every location searched are masked by the

special register M, using the Boolean AND
operation. The resulting logical product is then compared.
with the value of k. If the two quantities are identical, the
address and contents of the examined location are printed
at the Teletype.
contents of the

2. The search is conducted over that section of memory whose

lower limit is given by C(L), and whose upper limit is given

by C(U), except for the special Case described in the next
paragraph.

3. If C(M)

7777 and the expression k contains any symbol

in its address 'part

(for instance, ISZ FILI+5; FILI is the

symbol), the search will be conducted only on the page for
which that symbol is defined, regardless of the search limits
specified by C(L) and C( U). For any other case; including
that where the address tag of k is defined for page 0, the
search is conducted according to the limits set.

'A search never alters the contents of any location examined.

Addresses and location contents

are

printed as symbolic ex-

pressibns or octal integers, according to the mode at the time of
the search.
5-59

'For example: Search locations 2000 to 4000 for all occurrences of an ISZ instruction.

[L\00@l
[U\5G00

2066
4ZGB

[M\7777
ISZEU

7060

The addresses rather than tags are printed when symbols are'not
defined.

2632\ISZ

2135

2953\152 2135
0317

2111\152

The search will continue until all registers containing an ISZ are
found. Note that the setting of the mask limits the investigation to

the first three bits of each register, so that only instruction codes
are considered.

Third example: Obtain a dump of any section of memory. The
search is conducted between the limits set, and the addresses and
contents of all registers in the searched section are printed.

EL\BGEI
[U\5065

2600
3668
3

[M\7777

2660\0069
2601\CLA

2602\TAD

2616

The search will'conti'nu'e to the specified limit,

printing the con-

tents of every register. Note the following points: The mask is set
to 0 to insure that results of every comparison

are

the same, i.e.,

0. The search is conducted for all registers containing 0,

that

the results of each comparison are equal to the desired quantity, 0.

Always remember that the contents of the registers themselves are
not altered.

5-60

PUNCHING BINARY TAPES
_

After making the desired corrections and changes, the user may

punch out a new binary tape of his program. This allows the debugged program to be used immediately, without waiting for the
programmer to incorporate the corrections in the source program
and then reassemble‘the program. The punching procedure given
in Figure 5-l4 may be used for either the Teletype console punch
.or the optional high—speed punch.

The punching procedure uses the

Command.

following commands:
Meaning

This command is used to obtaina segment of leader-

trailer.
This command

a;b[P

causes

punch a block of

DDT to

binary tape with the information contained in the
memory designated by the expressions a (lower limit) and b (upper limit), inclusive.

section of

core

a and b may be any kind of acceptable terms (refer
to Definitions at the beginning of this
.

This command is used at the end of

[E
.

section).

punching

opera—
tions and causes DDT to punch a checksurn block,
'

followed by a length of trailer tape.

NOTE
The user should not try to punch the section
‘of memory between 5000 and 7600 which
contains DDT.

If the user wishes to restart DDT before he has punched a Com-

plete tape (i.e., betWeen data blocks)

he must set the console

switches‘to 5401 to preserve the checksum. Subsequent restarts
must also be set to 5401 until the checksum- block has been
'

punched.

,
.
_
.
.

At any other time DDT may be restarted at location 5400.
-

5-61

‘

START

LOW-

SPEED'

‘SETSR snow 0
TURN HSP

TYPE

SET SR BIT OTO‘

TURN LSP OFF

TYPE

PRESS com

TURN LSP 0N

WHEN PUNCHING
IS COMPLETED.
TYPE a;b[P

PRESS

PRESS CONT

WHEN PUNCHlNG
IS COMPLETED,
TURN LSP OFF

com

TYPE ugbtP

PUNCH
MORE BLOCKS

TURN LSP ON

[

TYPE CE

PRESS CONT

PR ES S CONT

WHEN PUNCHING
IS COMPLETED ,
TURN LSP OFF

Yé”§8M$”L'E°T*E"B°
REMOVE

TAPE.

PUNCH
MORE BLOCKS

PRESS CONT

Figure 5—14

Punching Binary Tapes (DDT)
'

5-62

Example Program Debugged
I

The following is the third pass assembly listing of a program
which adds- five numbers and stores their result in the variable
SUM. The programmer writes and assembles such a program (or

complex), then loads the binaryvprogram into core along
with DDT and the symbol table which was the output of assembly
pmsL
one more

*200

0200

7200

0201

1213

0202

3215

DCA

INDEx

0203

1214

TAD

ADDR

'0204

3225

DCA

POINTER

0205

3216

DOA

SUM

0206
0207

1625

TAD

2215

0210

5206

OOMPSM,

OLA

/INIT1ALIZE

LOOP

/SET

SUM

ZERO

/ADD

THE

INTERGERS

LOOP

FINISHED

TAD

LOOP,

POINTR‘

‘

ISZ

INDEX

/15

JMP

LOOP

/N0

DCA

SUM

0211

3216

0212

7402

0213

7773

N:~1

-5

/MINUS

0214

0216

ADDR.

LIST‘I

/ADDRESS

0215

0000

INDEX:

0216

0000

SUM:

0217

0001

LIST,

0220

0002

.0221

0003

0222

0004

0223

0005

'HLT

/YES

0224

0017

0225

0000

INTEGERS
LIST

/c0UNTER

FOR

LOOP

/RESULT

GOES

HERE

/LIST

NUMBERS

17
L

POINTR;

/AUTO INDEX POINTER
/TO LIST

In order to have DDT read in the symbol table,‘ the

programmer

the ALT MODE and R keys

(which echo as [R). DDT
typed
echoes the symbol table as shown below, following which it prints
the address of the lowest memory location which is occupied by a

symbol definition. The programmer is now'ready to begin debugging the program.
'

[R
ADDR

0214

COMPSM

0200

INDEX

0215

LIST

0217

LOOP

02E6

0213

POINTR

@225

SUM

@216

4750

5—63

The user notices at this point that POINTR was stored in loca—
tion 225 instead of location 17, the * was left out where word 224

presently is. To correct this, the interaction with the computer
looks as follows:
LOOP/TAD

.IIAD

o-2/DCA

POINR

TAD

@8l7
POINTR

DCA

LOOP+ICB

COMPSMEG

LO0P+0G01)@@@l
cc

Loop+unn1>aons
[c

LO0P+@001)@6@6

[C
Loop+noni)o@12
N-itB

'tc
[Loop+non4>nnna

SUM/AND 00]"

=Q017

The user typed:
LOOP/

which DDT returned the

symbolic contents of the location
labeled LOOP. The user changed POINTR to 17 and closed the
location with the RETURN key. To check that the change had
been made, the user typed:

where the dot indicates the current location, the slash (as above)
allows the user to investigate the contents of the location. Since the
contents were recorded, the user typed:
0-2

which refers to location 204 on the octal/symbolic listing. POINTR

is changed to 17 in this instruction as well.
The user then inserts a breakpoint at location 207 on the listing

(addressed by indicating LOOP+1). To begin execution of the
program the user types:

COMPSMCG

5—64

COMPSM is the starting address 0f the program. [G is the command to DDT which says to transfer control to that

location.

DDT prints:

L00p+aaai)aan1
When the breakpoint Occurs, DDT saves the contents 'of the AC.
It then prints the address of the breakpoint,

right parenthesis,

and the saved contents of the”, AC. The programmer sees that this

is the correct value after the first time

through the loop, so he

issues the command‘[C which causes the program to cycle through

the loop until it enc0unters the breakpoint again. Each time the
user

verifies the contents of the AC and has the program continue

executing. Rather than check every cycle through the loop ‘( which
in this case is short, but might be' very long), the user moves the
breakpoint to the HLT instruction. The contents of the AC at that
point was zero, which1s not important
To check the
value of SUM upon program completion, the user.
types:
SUM/

and the computer returns what it attempts to express as a symbolic

instruction. When the user types the equal sign (= ), DDT returns
the value of the symbolic instruction in octal.
This debugging session was very simple; the error was obvious.
This is seldom the case, and with long or complex programs sev—
eral debugging runs. may be required. Being able to debug a pro—
gram using symbolic expressions shortens the time required to
arrive at a correct, workable program.
Command Summary
'

Command

Action

Separation character.

Space

Arithmetic plus.

Arithmetic minus.

—-

‘

Location examination character. When it follows the

address location, it causes the
and its contents printed.
_

5-65

carriage return

Make

line feed

Make modifications, if any, close location, and open

modifications, if any, and close location.
’

next sequential location.

When it immediately follows a location printout,-it
causes the location addressed therein to be opened.

Type last quantity as an octal integer.
Current location.

Delete the line currently being typed.
Sets DDT to print in symbolic mode.
Sets DDT to print in octal mode.
Word search for all occurrences of the expression N
masked with C(M).

N'[W

Insert a breakpoint at the location specified by k. If
address is specified, remove any breakpoint.

k[B

n[ C

Continue from a breakpoint n times
I n is absent, it is assumed to be 1.

k[G

Go to the location specified by k.

automatically.

Action

Command

Read symbol tablelinto external symbol table or define
symbols on line.

Punch leader-trailer code.

Punch‘ binary tape from memory bounded by the ad—

a;b‘[P

dresses a and b.

Punch end of tape: checksum and trailer.

following commands will open certain locations in DDT

The

whose contents are available to the user.
Word Opened

Command

Accumulator storage (at breakpoints).

Link storage (at breakpoints).

[Y
I

Mark used in search.

Lower limit of search.

Upper limit of search.

[U
‘

5—66

Internal Symbol Table
AND
TAD
ISZ
DCA

JMS

=
=

7040
.7020
._:
:
7010,

CMA'

1000
2000

:
:

6001

=
'

=
:
=

:
‘:
=

KSF

CLL

6002
6031
7100

CML
RAR

RALV

7004

RTR
RTL

,7012

‘

14000

CLA

3000
5000
6000
7000
7200
6032
6034
6036
6041
6042
6044
6046

'22

ICYF
OPR

KRS
KRB
TSF
TCF
TPC
TLS
ION
IOF

‘

IMP

KCC

7006

V7001-

7500

7440

7510'

’

’
‘

’

“

IAN:
SMA
SZA
SPA
SNA
SNL
SZL
SKP
OSR
HLT
CIA
LAS
I

7450
7.420
7430
7410
7404
7402

7041

=
=
,

‘

__——
=

’

7604
400
.

5-67

DEBUGGING WITH CDT
The Octal Debugging Technique is a debugging program which

facilitates communication with and alteration of the object program. Communication is directed from the Teletype keyboard

using octal numbers. ODT has the same capabilities as DDT except that the programmer must reference his program. using its
octal representation instead of mnemonic symbols, and ODT commands are formulated differently.
ODT occupies 6008 consecutive locations and one location on
page zero, and can be loaded into either lower (starting address
1000) or upper (starting address 7000) core memory, depending
on

where the user’s program resides. That is, if the user program

residesin the first few pages of memory, then ODT should be
loaded in the upper pages of memory, and vice versa. As with
DDT, the user program cannot occupy (overlay) any location
used by ODT, including the breakpoint location (location 0004 on
page

zero). The programmer will probably discover CDT to be

more useful

and convenient than DDT once he has adjusted to the

octal notation.“

Features
ODT features include location examination and modification;
binary punching (to the Teletype or‘high-speed punch) of user

designated blocks of memory; octal core dumps to the Teletype
using the word search mechanism, as in DDT; and instruction
breakpoints to return control to OFF (breakpoints). ODT makes
no use of the program interrupt facility and will not operate out—
side of the core memory bank in which it is reading.
The breakpoint is one of ODT’s most useful features. When de—
bugging a program, it is often desirable to allow the program to
run normally up to a predetermined point, at which the programmer may examine and possibly modify the contents of the accumulator (AC), the link (L), or various instructions or storage

locations within his program, depending on the results he finds.
To accomplish this, ODT acts as a monitor to the user program.
The

user

decides how far he wishes the program to run and

ODT inserts an instruction in the USEr’s program which, when encountered, causes control to transfer back to ODT. ODT imme-

diately preserves in designated storage locations

the contents of

the AC and L at the break. It then prints out the location at which
5-68

the break occurred, as well as the contents of the AC at that point.

ODT will then allow examination and modification of any location
of the user’s program (or those locations containing the AC and
L). The user may also move the breakpoint, and request that ODT
continue running his program. This will caUse ODT to restore the

trapped instruction and continue in the

AC and L, execute the

user’s program until the breakpoint is

again encountered or the

program is terminated normally.

Using ODT

When the programmer is ready to start debugging a new program at the computer, he "should have at the console:

The binary tape of the new program.

-2.

A complete octal/ symbolic program listing.

A- binary tape of the ODT program

(either high or low

‘

version).
To begin the debugging run, first be sure that the BIN Loader is

in core memory, then load the CDT binary tape followed by the

binary tape of the user program (see Figures 5-3 and.5-4 for a
description of loading procedures with the BIN Loader).
-

Operation and Storage
STORAGE REQUIREMENTS
ODT can be run in a standard-4K PDP—S series computer and
requires 600 (Octal) consecutive core locations and one location

(0004) on page zero.As distributed by the Software Distribution
Center, it resides in memory between 7000 and 7577 (1000 and
1577 for the low version). ODT is page-relocatable.
The source tape can be re—origined to the start of any memory
page except page zero and assembled to reside in-the three pages
following that location, assuming they are all in the same memory
'

bank.
.

ODT uses location 4 on page zero‘as an intercom location between itself and the user’s program when executing a

breakpoint.

If the user wishes to change the location of the intercom word, he
may do so by changing the value of ZPAT in'the source and reassembling. The intercom locationmust'remain on page zero.
.

5-69

LOADING AND CALLING PROCEDURES
The user should note that ODT cannot be called as a subroutine.

ODT is normally distributed as a binary tape with the source
listing available on request from the DEC Software Distribution Cent-'
er and

is loaded with the BIN Loader as described in Figure 5-15.

Load the binary tape of the program to be debugged in the same
manner as

ODT was loaded. Be sure the two do not overlap.

STARTING PROCEDURE FOR ODT

The

starting

address of DDT is the address of the

symbol

START. For standard library versions the high version starts at

7000 and the low at 1000.
Set the starting address in the switch register. Press ADDRess

LOAD, CLEAR and CONTinue. ODT will issue a carriage return
and line feed to indicate that it is now running and awaiting com—
mands from the keyboard.
To restart ODT without clearing the checksum, set the address

of START + 1 (7001 high version or 1001 low version) into
the switch register and press ADDRess LOAD, CLEAR and
’

CONTinue on the computer console.

LOAD

OBAIECT

LOAD ODT

SEE FIGURE 5-4

SEE FIGURE 5—4

---—

SETSR-TOOO

I sersa-rooorl

'
lPRESS

ADDR LOAD

PRESS CLEAR
AND CONT!“

OOTlSLOADED
,

mgr:

Figure 5-15 Loading DDT and the Object Tape
5-70

Commands
SPECIAL

CHARACTERS

Slash(/)——'0pen Preceding Location

The location examination character (/) causes the location ad-

dressed by the octal number preceding the slash to be opened and
its contents printed in octal. The open location can then be mod—
ified by typing the desired octal number and closing thelOcation.

Any octal number from 1 to 4' digits in length is legal inpfit.
‘Typing a fifth digit is an error and will cause the entire modification to be ignored and'a question mark to be printed by ODT.
Typing / with no preceding argument causes the latest named loca—
tion to be opened (again). Typing 0/ is interpreted as / with no
argument. For example:
‘

ace/6e46
460/6646

2468?

4916/6646

12345?

I6046

Return—Close Location
If the use has typed a valid octal number after

the content of

location printed by ODT, typing the RETURN key
1s

causes

the

binary value of that number to replace the original contents of the
opened location and the location to be closed. If nothing has been
typed by the user, the location is closed but the— content of the
location is not changed. For example:
400/6646

location 400‘. is unchanged.

4916/6846 2345
,2345 6046

location 400 1s changed to contam 2345.

replace 6046' in location 400.

Typing another command will also close an opened register. For

example:
ace/ecu 461/663]
“BB/6M6 “”2346

2346

location 400 is closed and unchanged and
401 is opened and changed to 2346.~

Line Feed—Close Location, Open Next Location

The LINE FEED key has the same effect'as the RETURN key,
'

but, in addition, the next sequential locatiOn is opened and-its con—
tents printed. For example:
5-71

location 400 is closed unchanged and 401 is

zine/sane
64$!

“”2

’693‘
152‘“

1234

opened. User types change, 401 is closed
containing 1234 and 402 is opened.

1(Shift/N )——-Close Location, Take Contents as Memory Reference
and Open Same

The up arrow will close an open location just as will the RETURN key. Further, it will interpret the contents of the location as
a

memory reference instruction, open the location referenced and

print its contents. For example:
i

404/3270 v
347“ ’“2 I 2

3270
5%“

symbolically is “DCA, this page,
relative location 70,” so ODT opens location 470.

s-{Shi}tz‘/0) Close Location, Open Indirectly
The back

arrow

will also close the currently open location and

then interpret its contents

the address of the location whose

contents it is to print and open for modification. For example:
365/5760

336,6

l0426

0426

15261

‘-

ILLEGAL CHARACTERS

Any character that is neither a valid control character nor an
octal digit, or is the fifth octal digit in a series, causes the current
line to be ignored and a question mark printed. For example:
‘

ODT opens no location.

4 xr
an?

466/4 671
I467!

67K?

ODT ignores modification and closes
location 406.

CONTROL COMMANDS

nnnnG—Trans-fer Control to User at Location nnnn
'

Clear the AC then go to the location specified before the G. All

indicators and registers will be initialized and the breakpoint, if
any, will be inserted. Typing G alone will cause a jump to location 0.

5-72

n'nrmB—Set Breakpoint at User Location nmm
'

InstruCts CDT to establish a breakpoint at the location spec-

ified before the B. If B is typed alone, ODT removes any previ-

ously established breakpoint and restores the original contents of
the break location. A breakpoint may be changed to another location whenever ODTIs in control, by simply typing nnt where
nnnn is the new location. Only one breakpoint may be in effect at
one time; therefore, requeSting a new breakpoint removes any previously existing one.
A restriction in this regardis that a breakpoint may not be set
on any of the floating-point instructions which appear as. arguments of a J MS. For example:
—

TAD

‘

Den
ans
FADD

The

Breakpomt legal here.
Breakpoint illegal here.

breakpoint (B) command does not make the actual ex-

change of ODT instruction for user instruction, it only sets up the
mechanism for‘doing so. The actual exchange does not occur until
a “go to” or a “proceed from
breakpoint? command is executed.

When, during execution, the user’s program encounters the loca—
tion containing the breakpoint, control passes immediately to ODT
(via location 0004). The C(AC) and‘C(L)' at the point of the
interruption are saved in special locations accessible to ODT. The
user

instruction that the breakpoint was replacing is restored, be—_

fore the address of the trap and the content of the AC are printed.

The restored instruction has not been executed at this time. It will
not

be executed until the‘ ‘proceed from breakpoint” Command is

given. Any user location, including those containing the stored AC
and Link, can now be modified in the usual manner. The break—

point can also be moved or removed at this time.
An example of
usage follows the section “Cont-'
inue and Iterate

breakpoint
Loop.”

A—Open C(AC)
and C(L) are
When the breakpoint is encountered the
saved for later restoration. Typing A after having encountered a
breakpoint, opens for modification the location in which the AC
-

C(AC)

was

saved and, prints its contents. This location may, now be mod—
I

5-73

ified in the normal manner (see Slash) and the modification will
be restored to the AC when the “proceed from breakpoint” com-

mand is given.
A (line feed)——0pen C(L)
After opening the AC storage location, typing the LINE FEED

key closes the AC storage location, then opens the Link storage
[location for modification and prints its contents. The Link location
may now be modified as usual (see Slash) and that modification
Will be restored to the Link when the “proceed from the breakpoint” command is given.
C—Proceed (Continue) From a Breakpoint

Typing C, after having encountered a breakpoint, causes CDT
to insert the latest
specified breakpoint (if any), restore the contents of the AC and Link, execute the instruction trapped by the
previous breakpoint, and transfer control back to the user program
at the appropriate location. The user program then runs until the
breakpoint is again encountered.
NOTE
If a breakpoint set by ODT is not encoun—
is

tered while ODT
(user’s) program,
causes

running the object

the

the break to

instruction

occur

moved from the user’s

which

will not be

re—

program.“

nnnnC—Continue and .Iterate Loop nnnn Times Before Break
The programmer may wish to establish the breakpoint at some
location within a loop of his program. Since loops often run to
many iterations, some means must be available to prevent a break
from occurring each time the break location is encountered. This

(where nnnn is an octal number). After
having encountered the breakpoint for the first time, this command
is the function of: nnnnC

specifies how many additional times the loop is to be iterated before another break is to occur. The break operations have been
described previously in the section on the B command,
Given the following program, which increases the value of the

AC by increments of 1, the use of the Breakpoint command may
be illustrated.

‘

5-74

#200

0200

7300

'0201

0204
0205

1206
2207
5202
5201
.402

0206
0207

0000.

0202

0203

.B.

001

‘

ONE;
ONT,

CLA CLL
TAD ONE
ISZ GMT

amp

JMP
HLT
I
0

A
~

A
B

cut
ONE

0201
0202
020?
0206

02013
2000
0201

(0000

c.
0201
C

(0001

0201

(0002

4C
02%!

(6007

CDT has been loaded and started A breakpoint. is inserted at
location 0201 and execution stops here showing "the AC initially
set-to 0000.. The 'use of the Proceed; command (C) executes the
f program until the breakpoint is again encountered (after one com—
plete loop) and-shows the AC to contain a value of'_0001. Again
execution continues, incrementing the AC to 0002. At this point,
the command 4C-is used, allowing execution of the loop to continue 4 more times (followingthe— initial encounter) before stop,ping» atthc breakpoint. The contents of the AC have now been
,

incremented to 0007.

M—‘Open Search Mask
-

Typing M causes CDT to open for modification the location
containing the current value of the search mask and print its contents. Initially the mask is set to 7777-. It’m‘ay be changed by opening the mask location and typing the desired value after the value
printed by ODT, then closing the location;
M Line Feed—Open lower search limit

The word immediately following the mask storage location contains the location at which the search is to begin? Typing the LINE
FEED key to close the mask location causes the lower search limit
to be opened for modification and its contents printed. Initially the

5—75

lower search limit is set to 0001. It may be

changed by typing the

desired lower limit after that printed by ODT, then closing the
location.
M Line Fcede—Open upper search limit

The next sequential word contains the location with which the

search is to terminate. Typing the LINE FEED key to close the
lower search limit causes the upper search limit to be opened for

'modification and its contents printed.

Initially, the upper search
limit is the beginning of DDT itself, 7000 (1000 for low version).
It may also be changed by typing the desired upper search limit

after the one printed by ODT, then closing the location with the

‘-

RETURN key.

nnnnW—Word Search

The command nnnnW (where nnnn is an octal number) 'will
cause CDT to conduct a search of a defined section of core, using
the mask and the loWer and upper limits which the user has spec—

ificd, as indicated abo"e. Word searching with ODT is similar to
word searching with DDT. The searching operations are used to

‘

determine if a given quantity is present in
any of the locations of a
particular section of memory.
The searchIS conducted as follows: ODT masks the expression

preceding the W and saves the result as
the quantity for which it is searching. (All masking is done by performing a Boolean AND between the contents ofthe mask word,
C(M), and the word containing the instruction to be masked.)
ODT then masks each location within the user’s specified limits
and compares the result to the quantity for which it is searching.
If the two quantities are identical, the address and the actual unmasked contents of the matching location are printed and the
search continues until the upper limit1s reached.
A search never alters the contents of any location. For example:
search locations 3000 to 4000 for all ISZ instructions, regardless
of what location they refer to (i.e. search for all locations begin—ning with an octal 2).
nnnn which the user types

M7777

Change the mask to 7000, open lower

7am

search limit.
7745.3/aea1
'

3mm

Change the lower limit to 3000, open
upper limit5-76

74 54/7666

Change the upper limit to 4000, close

4666
'

location.

2.666»:

2666
3657

/2467

3124

/2632
l2152

4666

Imtiate the search for ISZ instructlons‘.

/2561

These are 4 ISZ mstructlons in this
section of core.

PUNCH COMMANDS
‘

.T—Punch Leader Tape

ODT capable of producing leader (code 200) tape on—line.
IS

ThisIS done by typing T and then turning ON the punch When
enough leader has been punched, turn off the punch and hit the

key on the computer console.- It is imperative that the
punch be turned OFF before typing again on the Teletype keyboard, since anything typed will~ also be punched if the punch is
left on. To issue any furthen commands reload the starting address
(1000 or 7000), then press the ‘ADDR LOAD, CLEAR and
CONTinue keys on the computer console.

HALT

nnnn,mmmmP—Punch Binar'y

To punch a binary core image of a particular section
-

of core, the

above commandis used Where nnnn is the initial (octal) address
and mmmm is thefinal (octal) address of the section of core to be

punched. The computer will halt (with 7402' displayed) to allow
the user to turn ON the punch. Pressing the CONTinue key on
the console initiates the actual punching of the block. The punch—
ing terminates without having punched a checksum, to allow subsequent blocks to be punched and to allow an all inclusive 'checksum to be punched at the end by a separate command. This procedureis optional, however,
and the user may punch individually
checksummed blocks.

low-speed

or
Binary tapes may be punched using either the
high-speed punches. If using the low-speed punch, it is imperative
that the punch be turned OFF before typing commands, since the
keyboard and punch are linked. Using the high-speed punch requires switch manipulation. The flowchart in Figure 5—16 details
the procedures involved in punching binary tapes.
i

E—Punch Checksum and Trailer

Given the E command, ODT will halt to allow the punch to be
turned on. Pressing the CONTinue key on the console will cause it

5—77

TYPE PUNCH

"MAID

"nun-,mmmP

RELOAD OOT

SET SR=723| (H1614)
SRI123! (LOW)

SET SR- 6026

TTY $5.55 CR/LF

SET 541-722!“

5338122“

PfiESS MIR LOAD
CLEAR AM) CONT

VID'EN EWGH

LEADER?“
a RAISE MLT

SET WWHIGH)
SRt 1203(LON)

Figure 5-16

Punching Binary Tapes (ODT)

5-7 8

SET sn=7222musm

WHEN SUFFIOENT
TRAILER
TURN LSP o'FF

Vsa=1222uow
DDR LOAD

PRESS ADDR LOAD

SET SR=6046

CLEAR AND CONT

SET SR=6041

RAISE DEP

SET SR=7DOOCHIGH)

ODT

SET SR=7231 (HIGH)
SR= 123! (LOW)

HIGH 0R Low

PRESS ADDR LOAD
CLEAR AND CONT

SET 5811000

5R=1000(LOW)

SET SR=7000

PRESS ADDR .LDAD

PRESS
CLEAR AND CONT

TTY ISSUES

CR/LF
LIFT DEP

om [S m
COMMAND MODE

Figure 5-16

Pufiching Binary Tapes (ODT) (cbnt)

5-79.

to punch the accumulated checksum for the preceding block(s) of

binary output followed by trailer (code 200) tape. When a sufficient length of trailer has been output, turn OFF the punch and
press the HALT key on the console. To'continue using ODT, reload the starting address (1000 or 7000), then press the ADD
LOAD, CLEAR and CONTinue keys on the console.
The binary tape produced by ODT can now be loaded into core
and run. However, the change-s shOuld'be made to the symbolic
source tapes as soon as

possible.

Additional Techniques
TTY I/O—FLAG
Sometimes the program

being debugged may require that the

Teletype (TTY) flag be up before it can continue output, i.e., the
program output routine will be coded as follows:
rsr

JMP

-l

TLS

Since ODT normally leaves the TTY flag in an off state, the
above coding will cause the program to loop at the JMP.-1. To
avoid this, ODT may be modified to leave the TTY flag in the on
state when transferring control through either a “go to” or a “con-

tinue” command. This modification is

location XCONT—3
make the actual

XCONT—3

(normally
change, load

and modify it

accomplished by changing

7341)

ODT as

(7000) To
usual Open location

NOP

as follows:

7341/6642, 76%
l3“"6““2 7‘5““

(RETURN key) for high version
.

(RETURN key) for low version'

CURRENT LOCATION
_

The address of the current location or last location examined
is remembered by DDT and remains the same, even after the com-

mands G, C, B, T, E, and P. This location may be opened formspection merely by typing the slash ( / ) character.

5-80

PROGRAMS WRITTEN IN ODT COMMANDS

ODT will also correctly read tapes prepared ofl-line(e.g., a tape
punched with 1021/115717775 will cause location 1021 to be
opened and changed to 1157; then the memory reference address

opened and changed to 7775. This procedure will
work with breakpoints, continues, punch commands, etc. Thus, deébugging programs may be read into CDT, to execute the program,
list locations of interest, modify locations, etc.
157 will be

lNTERRUPT PROGRAM DEBUGGING
‘ODT executes an IOF when a breakpoint is encounteIed. (It
does not do this when more iterations remain in an nnnnC com-

mand.) This is done so that an interrupt will not occur when ODT
prints the breakpoint information. ODT thus protects itself against
'

spurious interrupts and may be used safelyin debugging programs
that turn on the interrupt mode.

However, the user must remember that ODT does not know

whether the interrupt was on when the breakpoint was encountered,
and hence it does not turn on the interrupt when'transferring control back to the program after

receiving a “go” or a “continue”
I

command.

OCTAL DUMP

By setting the search mask to zero and typing W, all locations
between the search limits will be printed on the Teletype. An
Octal Memory Dump program (DEC-8I-RZPA—D) is available
from the Software Distribution Center upon request.
INDIRECT REFERENCES
When an indirect memory reference instruction is encountered,
the actual address may be opened by typing t arid

(SHIFT/N

and SHIFT/O, respectively).

Errors
_

The only legal inputs

are

control characters and octal

digits.

Any other character will cause the character 'or line to be ignored
and a question mark to be printed by ODT. Typing G alone is an
error.

It must be preceded by an address to which control will be

transferred. This will elicit no question mark also if not preceded

by an address, but will cause control to be transferred to location 0.
r

5-81

Typing any punch command with the punch ON is an error and
will cause ASCII characters to be punched on the binary tape.
.

This means. the tape cannot be loaded and run properly.

Programming Notes Summary
ODT will not operate outside of the memory field in which it is
located.

ODT must begin at the start of a memory. page (other than page

zero) and must be completely contained in one memory field.
ODT will not turn on the. program interrupt, since it does not
know if the user’s program is using the interrupt. It does, however,
turn off the interrupt when a breakpoint is encountered, to prevent

spurious interrupts.
The user’s program must not use of reference any core locations-

occupied or used by ODT, and vice versa.
Register ZPAT is used as an intercom location by ODT when
executing a breakpoint. In library distributed versions ZPAT
0004. This location must be left free by the user since it is filled
.

with an address within ODT which is used to transfer control between the user program and ODT.

Breakpoints are fully invisible to “open location” commands;
however, breakpoints may not be placed in locations which the
user program will modify in the course of execution. or the breakpoint will be destroyed.
If a trap set by ODT is not encountered by the user’s program,
the breakpoint instruction will not be removed.
ODT can be used to debug programs using floating—point instructions, since the intercom location is 0004, and since breakpoints may be set on a 1 MS with arguments following.
This version of DDT will operate on a Teletype with an ALT
MODE key or an ESCape key.
To restart ODT without clearing the checksum, see the section

Starting Procedures for CDT.
The high-speed punch may be used by patching three locations
after typing the punch command. See the section on Binary Tapes
from the High-Speed Punch.
on

5-82

Command Summary
nnnn/

Open location designated by the octal number
nnnn.

Reopen latest opened location.

RETURN key

Close previously opened location.

LINE FEED key

Close location and open the next sequential one
for modification.

T (SHIFT/ N)

Close location, take contents of that location as
a

memory/reference and open it.

(SHIFT/O)

Close location, open

indirectly.

Illegal character

Current line typed
types ? (CR/LP).

nnnnG

Transfer program control to location nnnn.

nnnnB

Establish

Remove the breakpoint.

user

ignored,

breakpoint at location 'nnnn.

Open for modification the location in which the
contents of AC were stOred when the
was encountered.

Proceed from a
nnnnC

ODT

breakpoint

breakpoint.

Continue from a breakpoint and iterate past the

breakpoint nnnn times before interrupting the
user’s program at the breakpoint location.
Open the search mask.

LINE FEED key

Open lower search limit.

LINE FEED key

Open upper search limit.

nnnnW

Search the portion of core as defined by the upper and lower limits for the octal'value nnnn.

Punch leader.

nnnn;mmmmP

Punch a binary core image defined by the limits
nnnn and mmmm.

Punch checksum and trailer.

5-83

prggrgmmlng
_

iprogramming
nput'./out.r.>.ul'.w~
dectape

programming
'

Floating-point
packages :

input/output

programming
INTRODUCTION

Programming a computer to do calculations is of little use unless
there is some means of obtaining the result of the calculations from
the machine. In. most applications, it is also necessary to ~supply
the computer with data before calculations may be performed.‘ A
programmer must be able to translate information efficiently between the computer and the peripheral devices that supply input or
serve as a means of

There are three methods for the transfer of

;

output.

information between

input/ output (I/O) devices and PDP—S series computers The.
first two methods provide for computer control over the transfer.
One such method is programmed transfer, in which. instructions
.to accept or transmit information are included at some point in the
”program. Programmed transfers are program initiated and executed

under program control

Information may also be transferred Via program interrupt, 1a
standard feature of PDP—S series computers that allows I/O de-

vices to signal the computer when they are ready to transfer information. The computer interrupts its normal flow, jumps to a
:

special routine which processes the information, and then returns
tothe point at which the main program was interrupted. Program

interrupt transfers are device initiated and executed under program
control.
_

Both programmed transfers and program interrupt transfers use
a

the accumulator as the bufi‘er, or storage area, for all data transfers. Since data may be transferred only between the accumulator
and the device, only one'12—bit word at a time may be transferred

by programmed transfer or by program interrupt.
The third method of data transfer is the data break. A data
6-1

break is

essentially device controlled and allows for direct exchange of large quantities of information between the I/O device
and core memory. It differs from the previous two types of trans—
fer in that there are no program instructions to handle the actual

transfer, and the accumulator is not used as a buffer. Data break
transfers are device initiated‘and device controlled.

PROGRAMMED DATA TRANSFERS

Programmed transfers of information are accomplished by pro—
gram instructions. The instructions used are similar to the operate
microinstructions in that there is no need to specify an address in

memory. All programmed transfers occur directly between the
accumulator and the I/O device. Since many different devices

could be connected to one computer and each device might transfer information at

some

time, the instruction'5 must identify the

proper device for each transfer. It must also
nature of the function to be performed.

specify the exact

IOT Instruction Format

The instructions used to

perform pregrammed data transfers

called input/output transfer (IOT) instructions. An IOT instruction is a 12-bit word that has the following format:
are

BIT POSITION.

0
‘

‘

OPERATION

fl
1

J k

0
‘

1
J

L
V

CODE—J

DEVICE SELECT lON CODE
OPERATION SPECtFlCATlON BITS

An IOT instruction is divided into three parts:

operation code,

device selection code, and operation specification bits. The first
three bits of the instruction contain the operation code. These bits
are always set to 63 (110) to specify an IOT instruction.
The .next six bits of the instruction contain the device selection

code. This code is transmitted to all peripheral equipment whenever the IOT instruction is executed. A device selector within each

peripheral monitors the device codes. When the selector recognizes
6-2

code as that device’s assigned code; the device

three bits of the IOT instruction.

accepts the last
‘

The last three IOT instruction bits are the operation specifica-

‘tion bits, which‘may be set to specify up to eight functions or

combinations of functions. If a device is capable of performing
more functions than can be coded into the three operation specifi—
cation bits, then more than one device code must
be assigned to
that device.

Checking Ready Status
The computer operates much faster than'most peripheral devices. For this reason, I/O routines must check, the—status of a

device before executing an IOT instruction to ensurethat the device is not still performing a previous operation. Device status .is

signalled through a system of one-bit registers called flags. Every
I/O device has a device flag which .is set to l as soon as the device
finishes a‘current operation and is ready to begin a new operation.
If the flag is cleared (set to 0), the device is still performing" the
operation specified by the last IOT instruction received. Ready
status is usually checked by means of a skip-on—flag IOT instruc—
tion, such that the computer does not skip out of a waiting loop
until the I/O device1s ready to begin a new operation.
Instruction Uses
In general, for each I/O device there are at least three instruc—
tions:
13.
'

An instruction to transfer information and/ or operate the
device.

ready

An instrUCtion to test the
status of the device and
skip on the ready (or not—ready) status of the device.

An instruction to clear or set the device flag.

These instructions

may be microprogrammed. In particular, the
instructions to clear the flag and to
operate the device are often

combined.

Specific instructions for various devices are presented1n the fol—
lowing sections. The Teletype unit is described1n depth to illustrate
the fundamentals of programming data transfers. The general techniques developed for the Teletype unit may be extended to other
I

I/O devices.
_

6-3

ASCII Code
The ASCII code

(American Standard Code for Information In-

:terchange)15 describedin Appendix B. Many of the programs pre—
sented in this chapter use ASCII code to transmit information to
..the PDP- 8/ E Note that the ASCII code for octal digits 0 through
7is the sum of the
digit plus 2608.
PROGRAMMING THE TELETYPE UNIT
One of the most

I/O devices is the Teletype unit,
which consists of a keyboard, printer, paper tape reader, and
paper tape punch. The Teletype unit can use either the keyboard
or the paper tape reader to provide input information to the computer and either the printer or the paper tape punch to accept
output information from the computer. The Teletype is therefore
assigned two device codes. Functioning as an input device, the
keyboard/reader is assigned the device code 038, and functioning
as an output device, the printer/punch is assigned the device
common

code 043.

’Keyboard/ Reader Instructions
Figure 6-1 shows the format for the keyboard/ reader IOT instruction. Table 6-1 lists the mnemonic

bits 9,10 and 11.

instructions used to set

iio'ooopli‘fim
J

DEVlCE CODE'l03)
KRS

KCC

KSF

Figure 6-1
.

Keyboard / Reader Instruction Format

Figure 62 shows a program using the keyboard/reader IOT
instructions to read one ASCII character from the keyboard or
paper tape reader. This program does- not print the character on
the teleprinter; It merely stores the ASCII code for the character
in core memory location STORE.
6—4

Table 6—1

Keyboard/ Reader Instructions

‘
I

Mnemonic
KCF

Octal

Operation

6030

Clear

keyboard / reader flag without oper-

ating the device.
KSF

Skip the next instruction if the keyboard/
reader flag is a 1.

6031

‘

Clear the accumulatdr and the
reader flag.

6032

KCC

6034

KRS

keyboard/

Read a character from the keyboard/ reader
bufler. The keyboard / reader flag is set when
the Operation is completed.

.
.

6035

KIE

Enable the

keyboard/reader to cause pro-

gram interrupts if accumulator bit 11 is a 1.
Disable the keyboard/reader from causing
interrupts if accumulator bit 11 is a zero.1

KRB

Clear the accumulator and the keyboard/
reader flag, and read a character from the

6036

keyboard/reader bufler. This instruction is
-a microprogrammed combination of KCC
and KRS.

*ZOO

INPUT,

KCC
JMS
DCA

LISN
STORE

HLT—

LISN,

KSF

ETORE,

JMP
KRB

.-L

JMP

LISN

Figure 6-2

1 Use

/CLEAR KEYBOARD FLAG
/ENTER SUBROUTINE
/STORE ASCII CHARACTER
/HALT UPON COMPLETION
/LISN SUBROUTINE
/KEYBOARD FLAG RAISED YET?
/NO: CHECK AGAIN
/YES: READ THE CHARACTER
/RETURN TO MAINLINE
/CHARACTER STORAGE

Coding to Accept One ASCII Character

of this instruction will be discussed later in this chapter.

6-5

The program begins with

KCC instruction. In general, any
program should begin by clearing the flags of all I/O devices to.
a

be used later in the program. If the above program is started at

location 0200, it will proceed to the KSF, JMP.—-1 loop, and continue looping indefinitely until a key on the Teletype unit is pressed
or a paper tape is loaded into the reader. As soon as the ASCII

code for a character has been assembled in the keyboard/reader

keyboard/reader flag is set and the program
skips out of the waiting loop. The content of the bufler is then
transferred into the accumulator, and the keyboard/ reader butter
and flag are cleared.

butler register, the

Printer/ Punch Instructions
Figure 6-3 shows the format for the printer/ punch IOT instructions. Table 6-2 lists the mnemonic instructions used to set bits
9, IO and 11.

1
k

1’

0
J

0
1

L
V

QRERATION CODE (El—J

‘

0
1

1
1

DEVICE CODE (24)
TPC
TCF

TSF

Figure 6-3

Printer/ Punch Instruction Format

The program presented in Figure 6-4 prints out one ASCII
character which is stored in core memory location HOLD. It be-

gins by clearing the accumulator and executing a TLS instruction.
Thishas no effect on the printer/punch, since 000 is the ASCII
code for a blank, but it serves to clear the printer/punch butter
and then raise the device flag. If this instruction had not been in;

cluded, the flag would never be raised and the program would
remain in the TSF, JMP.—1 loop indefinitely. Instead, the flag is
set as soon as execution of the first TLS
instruction is complete,
6—6

Table 6-2

Printer/Punch

Instructions

M‘nemonic

Octal

TFL

6040

Set the printer/ punch flag.

TSF

6041

Skip the next instruction
punch flagIS a 1.

Operation

if the

printer/

TC-F-

TPC

‘

6042

Clear the printer/ punch flag.

6044

Load the contents of accUmulator bit positions 5 11 into the printer/ punch buffer and

operate

the

pr1nter/ punch

The

printer/

punch flagis set when the operation is com—
pleted.
.

6045

TSK

Skip the next sequential instruction if either
the printer/ punch interrupt request flag or
the keyboard/ reader interrupt request flag
is set.2

6046

TLS

Clear the printer/ punch flag, load the contents of accumulator bit positions 5-11 into

printer/ punch buffer and operate the
printer/ punch. This instruction is a microprogrammed combination of TCF and TPC.
the

*200

OUTPUT,

CLA
TLS
TAD
JMS

CLL‘
HOLD
TYPE

HLT_
TYPE,

TSF
JMP
TLS

.‘1

CLA

CLL
I TYPE

JMP

:OLD,

‘

24$

Figure 6-4

/CLEAR ACCUMULATOR AND LINK
/RAISE PRINTER FLAG
/GET THE CHARACTER
/ENTER SUBROUTINE
/HALT UPON COMPLETION
/TYPE SUBROUTINE
/PRINTER FLAG RAISED YET?
/N0: CHECK AGAIN
/YES: PRINT THE CHARACTER
/CLEAR ACCUMULATOR AND LINK
/RETURN TO MAINLINE
/STORED ASCII CHARACTER

Coding to Print One ASCII Character

2 Use of this instruction will be

discussed later in this chapter.

6-7

permitting the program to escape the skip loop and execute the
second TLS instruction. Finally, the program again clears the accumulator. It is advisable to clear the accumulator at the end of

any subroutine, unless meaningful data is contained in it.
Format Routines

Input and output routines are often written in the form of subroutines similar to the TYPE-subroutine in the previoiJs example.
Figure 6-5 presents a carriage return/ line feed subroutine that calls
the TYPE subroutine to execute a carriage return and line feed on
the teleprinter. Similar subroutines could be Written to tab space
the carriage a given number of spaces or to ring the bell of the
Teletype by using the respective codes» for these nonprinting characters. If such subroutines are commonly used in a program, they
should be placed on page, ‘0 (or a pointer to the subroutine should
be placed on page 0) to facilitate reaching the routine from all
memory locations.

‘*5@0

CRLF,

CLL

CLA

K215,
K212,
TYPE,

/CRLF SUBROUTINE
/CLEAR ACCUMULATOR AND LINK
~/RAISE PRINTER FLAG
/GET ASCII CARRIAGE RETURN
/PRINT IT
/GET ASCII LINE FEED
/PRINT IT
/RETURN TO MAINLINE
/ASCII CARRIAGE RETURN
/ASCII LINE FEED
/TYPE SUBROUTINE
/PRINTER FLAG RAISED YET?
/NO: CHECK AGAIN
/YES: TYPE THE CHARACTER
/CLEAR ACCUMULATOR AND LINK
IRETURN

0
‘

“

TLS
TAD
JMS
TAD
JMS
JMP
215
212
6
TSF
.JMP
TLS
CLA
JMP

‘

K215
TYPE
K2l2
TYPE
I CRLF

.-1

CLL
I TYPE

Figure 6—5

Carriage Return/Line Feed Subroutine

Text Routines
The examples in Figures 6-2 and 6—4 may be expanded. to ac-

cept and print

than

character.

Figures 6—6 and 6-7
illustrate one such expansion. These two programs are compatible
in that the characters accepted by the first program may be typed
more

one

6-8

out by running the second

program. The program in Figure 6-6

continuesto accept character input until a dollar sign ($) is typed
at the keyboard. It then stores 0000 in the next core location and

halts. The

program 'in Figure 6—7 types the characters whose

ASCII codes were stored by the first program, and halts when a

location with contents equal to zero is reached. Both programs use
.

beginning at 2000 as a storage bufier for the ASCII'
characters. The following flowcharts illustrate the techniques used
in the program coding.
locations

SET BUFFER POINTER
T0 FIRST LOCATION

EXECUTE

TLS

TO CLEAR BUFFER
-

CHECK FLAG
AGAIN

AND SET FLAG

IS
KEYBOARD
FLAG SET

SET BUFFER POINTER
T0 FIRST LOCATION

YES

ACCEPT ONE
CHARACTER

RETURN
CARRIAGE

STORE ONE
CHARACTER IN BUFFER
‘

GET NEXT
ASCII cone

‘
‘

PRINT THE
CHARACTER

INCREASE THE
BUFFER

VPOINTER
TYPE OUT
CHARACTER

IS THE
CHARACTER

A .3.

INCREASE BUFFER
POINTER

YES
'

STORE ZEROS
OVER '8'

6-9

*ZDD
CLL
TAD BUFF
DCA BUFFPT
KSF
JMP ;-l

CLA

START,
LISN,

KRB
TLS
DCA

TAD
TAD

BUFFPT
BUFFPT
MDOLAR
I

SNA
JMP

CLA CLL
DCA I BUFFPT

BUFF,
BUFFPT ,

/AND

LINK

INITIALIZE_POINTER

/KEYBOARD_STRUCK YET?
/NO: CHECK AGAIN
/YES: GET THE CHARACTER
/ACKNOWLEDGE IT ON PRINTER
/STORE THE CHARACTER
/CHECK FOR TERMINAL
/DOLLAR SIGN ($)
/IS CHARACTER A "$"?
/YES: STORE ZERO OVER $
/NO: INCREMENT POINTER
/GET ANOTHER CHARACTER
/CLEAR ACCUMULATOR
/STORE ZERO IN LAST
/BUFFER LOCATION AND HALT
/BUFFER BEGINS AT
/CORE LOCATION ZODD
/TWO'S COMPLEMENT OF @244
_

DONE
ISZ BUFFPT
JMP LISN

DONE,,

/CLEAR ACCUMULATOR AND
/SET UP BUFFER SPACE

HLT
2ZOD
D

MDOLAR, 7534
$

Figure 6-6

Program to Accept ASCII Characters

*38%

START,

CHRTYP,

CLA

CLL
TLS
TAD BUFF
DCA BUFFPT
JMS' CRLF
TAD I BUFFPT
SNA

HLT
JMS

TYPE

ISZ BUFFPT
JMP CHRTYP

CRLF,

JMS

K215
TYPE

TAD

K212

JMS
‘JM

TYPE
I CRLF

TAD

TYPE,

/PRINT

TSF
JMP

.‘1

TLS
CLA CLL
JMP I TYPE
2066

BUFF,
BUFFPT, D
215
K215,
212
K212,

Figure’ 6-7

/CLEAR ACCUMULATOR AND LINK
/RAISE PRINTER FLAG
/SET UP BUFFER SPACE AND
/INITIALIZE BUFFER POINTER
/RETURN CARRIAGE
/GET A CHARACTER
/IS IT ALL ZEROS?
/YES: HALT
/NO: TYPE THE CHARACTER
/INCREMENT BUFFER POINTER
/GET ANOTHER CHARACTER
ICRLF SUBROUTINE
/GET ASCII CARRIAGE RETURN
IT

/GET ASCII LINE FEED
/PRINT'IT
/AND RETURN
/TYPE SUBROUTINE
/PRINTER READY YET?
'lNO: CHECK AGAIN
/YES: TYPE CHARACTER
ICLEAR ACCUMULATOR
/AND RETURN
/BUFFER BEGINS AT
/CORE LOCATION ZDDO
/ASCII CARRIAGE RETURN
/ASCII LINE FEED

Program to Print Out ASCII Characters
6-10

The program to print ASCII characters may be specialized to‘

print a specific message, as in the program example of Figure 6-8.
This routine uses aut’oindex registers in place of the ISZ instruction. It types, “HELLOI”.

*3OO
O

HELLO,

CLA
TLS
TAD

CLL

CHARAC

DCA IRI
TAD M6
DCA COUNT
TAD I IRl
JMS TYPE
ISZ COUNTJMP NEXT
JMP I HELLO
O
TSF'

NEXT,

TYPE,

/FOR GETTING CHARACTERS
/SET UP COUNTER FOR
/TYPING CHARACTERS
/GET A CHARACTER
/TYPE IT
/DONE YET?
/NO} TYPE ANOTHER
/YES: RETURN TO MAINLINE
/TYPE SUBROUTINE
“/PRINTER FLAG RAISED YET?
/NO: CHECK AGAIN
/YES: PRINT A CHARACTER

JMP .-1
TLS
CLA.
JMP I TYPE

CHARAC,

.
'

510
SOS
314
514
517
241

/CLEAR ACCUMULATOR
/AND RETURN
/INITIAL VALUE OF IRl
/H
/E
/L
/L
/O
/!
/CHARACTER COUNT
/CHARACTER COUNTER
/AUTOINDEX REGISTER
'

-6

M6,
COUNT,-

IRIZIO

/MESSAGE SUBROUTINE
/CLEAR ACCUMULATOR AND LINK
/RAISE PRINTER FLAG
/SET UP AUTOINDEX REGISTER

.
_

Figure 6-8

Subroutine to Print the Message, “HELLO!”

Numeric Translation Routines

The ASCII code for a number must be converted to octal representation before the computer may use the number in calculations.

For example, 6 is represented by the ASCII code 266. When the

Teletype key for 6 is typed, the code 266 is transmitted to the computer upon execution of the next KRB instruction. ,Two methods
may be used to remove the 260 from
and obtain the octal number itself.

ASCII-coded number

One method is to‘ clear the first eight bits of the ASCII-coded
6-11

number by using the AND instruction and an appropriate mask.
If
»

178 is ANDed with the coded number, as shown below, the

octal value

of the number is recovered.

Instruction

Operation

Comment

000 010 110 110

ASCII Code 266 in accumulator,
MASK: 17.

AND MASK

000 000 001

111

000 000 000 110

accumulator after
AND instruction is executed.
Contents

The second method of stripping an ASCII-coded number is to
'

subtract 2603 from the character code. This is

accomplished by

TADing the two’s complement of 2608 to the coded number,

shown below.
Instruction

Operation.

Comment

TAD M260

000 010 110 110

ASCII Code 266 in accumulator.
M260: 75208 (2’s comp of 260)

111

101 010 000

000 000 000 110

Contents

accumulator

after

TAD instruction is executed.

Figure 6-9 shows two programs which accept and store an octal
digit, using the LISN subroutine presented in Figure 6-2. This process may be reversed to print out a digit which is stored in memory
by adding 2608 to the digit, as illustrated in the program of Figure
6-10. This program calls the TYPE subroutine of Figure 6—4 to
print out the binary number 7.

/USING THE AND
/INSTRUCTION

/USING

*zee

NUMIN,

KCC

/INSTRUCTION

NUMIN,
'

JMS LISN
AND MASK
DCA HOLD
HLI

HOLD,
MASK,
55

TAD

THE

ch
JMS LISN
TAD mesa
DCA HOLD
HLT

HOLD,
M260,

752m

Figure ”6.9 "Two Methods of Converting ASCII Code to Binary
6-12

*200

NUMOUT,

/CLEAR ACCUMULATOR AND LINK
/RAISE PRINTER FLAG
/GET NUMBER
/CONVERT T0 ASCII
/PR1NT THE NUMBER

CLA CLL
TLs
TAD NUMBER
.

TAD KESO

JMS TYPE
NUMBER,
K260,

/AND HALT
/NUMBER TO BE PRINTED

HLT
7
26E

Figure 6-10
All of the

Program to Type One Stored Digit

routines presented so far have been designed to handle

single-digit octal numbers. However, PDP-S/E core memory locations may contain octal numbers with up to four digits. The- program shown in Figure 6-1 prints out a four-digit octal number'which is stored in core memory. The program shown in Figure
6—13 accepts four octal digits from the Teletype keyboard, converts
them to an octal number, stores that number, and then loops back
to accept another four digits. Figure 6—11 is a flowchart illustrating
the procedure employed in these‘routines.
V

ACCEPT A DIGIT
PROM KEYBOARD

CLEAR A
STORAGE LOCATION

GET CONTENT OF

EXECUTE TLS
TO CLEAR BUFFER
AND SET FLAG

STORE DIGIT
TEMPORARILY

STORE": LOCAT’O"
IN AC

RECEIVED

BSIETTS 21:82:20

FOUR DlGITS
7

BRING STORED RESULT
INTO AC

A
.

YES

ADD 260

GET THE
NUMBER IN Ac

To THE Ac

—

BRING FIRST DlGlT
INTO AC

I
,

ROTATE THE NUMBER
1 PLACE LEFT

TTZEEDgf;

i
ADD THE CONTENTS

OF STORAGE
LOCATfoN

ROTATE LEFT
THREE PLACES

SUBTRACT

ADD NEXT

DIGIT

2603
ALL
DIGITS TYPED

I
.

STORE THE RESULT

TEMPORARILY

YES

ROTATE Ac THREE
PLACES LEFT

RETURN
CARRIAGE

PACKED
4 DlGlTS

STORE AC IN
STORAGE LOCATION
YES
STORE NUMBER
IN BUFFER

Figure 6-11

Flowchart for Figures 6-12 and
'

6-13

*ZOO

START,V CLA CLL

UNPACK,

TYPE,

CRLF,

DCA STORE
TLS
JMS CRLF
TAD M4
DCA DIGCTR
TAD NUMBER
RAL
TAD STORE
RAL
RTL
DCA STORE
TAD STORE
AND MASK7
TAD K269
JMS TYPE
ISZ DIGCTR
JMP UNPACK
JMS CRLF
HLT
O
TSF
JMP .‘1
TLS
CLA
JMP I TYPE
B
TAD K215
JMS TYPE
I

TAD K212
JMS TYPE
JMP I CRLF

NUMBER, 1254
MASK7, .7
M4,
DIGCTR,
STORE,
K212,
K215,
K252,

-4

z
0

212
215
26G

/CLEAR ACCUMULATOR AND LINK
/CLEAR STORAGE LOCATION
/RAISE PRINTER FLAG
/RETURN CARRIAGE
/SET LOCATION TO COUNT

/NUMBER OF DIGITS
/GET NUMBER TO BE TYPED
/ROTATE ONE PLACE LEFT
/ADD STORED LOCATION
/ROTATE THREE
/PLACES LEFT
/STORE ROTATED NUMBER
/MASK OUT
/FIRST 9 BITS
/ADD IN 26%
/AND TYPE A DIGIT
/TYPED FOUR DIGITS YET?
/NO: GO TYPE ANOTHER
IYES: RETURN CARRIAGE
/AND HALT
/TYPE SUBROUTINE
/PRINTER FLAG RAISED YET?
/NO: CHECK AGAIN
/YES: PRINT A CHARACTER
/CLEAR ACCUMULATOR
/AND RETURN
/CRLF SUBROUTINE
/GET ASCII CARRIAGE RETURN
/PRINT IT
/GET ASCII LINE FEED
/PRINT IT
/AND RETURN
/NUMBER TO BE PRINTED
/AND MASK
/DIGIT COUNT
/DIGIT COUNTER
/STORAGE LOCATION
/ASCII LINE FEED
/ASCII CARRIAGE RETURN

Figure 6-12

Program to Type a Four-Digit Number

6-14

*ZOO'
CLA

START,

/CLEAR ACCUMULATOR AND LINK
IRAISE PRINTER FLAG
/SET INDEX REGISTER TO
/STORE PACKED NUMBERS
/RETURN CARRIAGE
A/SET COUNTER FOR
/4 DIGITS
5/SET UP TEMPORARY
JEOR ASCII INPUT
A CHARACTER
/STORE IT TEMPORARILY

CLL

'TLS‘

TAD K1777
DCA IRl
JMS CRLF
NXTNUM, IAD M4
DCA1 :COUNTR
TAD KSSO'
DCA t"TEMP
NXIDIG, JMS LISN‘
DCA I TEMP
ISZ TEMP'
ISZ COUNTR‘=
JMP .NXTDIG
JMS CRLF
JMS PACK
DCA I IRI
-JMP NXTNUM
a
PACK,
DCA STORE
TAD M4
DCA COUNTR
TAD K556
.DCA TEMP
PAKDIG,'TAD STORE
"CLL RAL
RTL»
TAD 1
TAD MZSO
ADCA STORE
ISZ TEMP
ISZ' COUNTR’
.JMP PAKDIG
‘TAD STORE
JMPV I PACK

STORAGE

‘

IEME

CRLF,

.TAD K215
ers TYPE
‘TAD 1K212
JMS TYPE
JMP I CRLF

Figure 6-13

V's/GET

T“/INCREMENI-STPOINTER
‘/RECE1VED 4 DIGITS YET?_
I

T/NO: GET ANOTHER
/YES: RETURN.CARRIAGE
/PACK THE 4 DIGITS
,/5I0RE PACKED NUMBER
/GET A NEW NUMBER
/PACK SUBROUTINE
.ICLEAR STORAGE LOCATION
ISET COUNTER FOR
/4 DIGITS
‘ISET POINTER TO
IASCII INPUT CHARAC-TERS
/LOAD PARTIAL NUMBER
/ROTATE LEFT
"“/IHREE TIMES
/ADD NEXT STORED DIGIT
--/STRIP OFF THE 260
/STORE PARTIAL NUMBER
-

7/INCREMENT POINTER

‘

/PACKED 4 DIGITS'YET?
/NO: PACK NEXT DIGIT
/YES: TAKE PACKED NUMBER
/BACK TO MAINLINE
/CRLF SUBROUTINE
IGET ASCII CARRIAGE RETURN
/PRINT IT
/GET ASCII LINE FEED
IPRINT IT
/AND RETURN
-

Program to Pack and Store Four-Digit Numbers

6-15

LISN,

KSF
‘1”? 0'1
KRB 1'

TLS

TYPE,

JMP I LISN
a
TSF
J”? 0"]

TLS

Ki777,
M4,
couurn,
xsse,
TEMP,
STORE,
“250,
K215,
x212,

CLA
JMP I
1777
7774

TYPE

a
550
a
G

7520
215
212

IRI=13
$

/LISN SUBROUTINE
/KEYBOARD FLAG RAISED YET?
INO: CHECK AGAIN
IYES: READ A CHARACTER
/ECHO ON PRINTER
IAND RETURN

‘ITYRE SUBROUTINE
/PRINTER FLAG RAISED YET?
~/NO: CHECK AGAIN
/YES: PRINT A CHARACTER
/CLEAR ACCUMULATOR
/AND RETURN
/LAST LOC BEFORE BUFFER
IDIGIT COUNT
/DIGIT COUNTER
IBEGIN TEMP STORAGE
/TEMP STORAGE POINTER
IPACK ROUTINE STORAGE
IASCII CONVERSION CONSTANT
/ASCII CARRIAGE RETURN
IASCII LINE FEED
fAUTO-INDEX REGISTER

Figure 6-13 (cont) Program to Pack and Store
Four-Digit Numbers
'

The text and numeric translation routines described earlier are

combined in the following sample program, which accepts four-

digit octal numbers delimited by carriage returns and then sorts the
numbers into ascending numerical order and prints them, on the
teleprinter.
Any number of elements may be supplied. The end of input is
signalled by typing a. dollar sign ($). This program includes routines which ignore any non-octal digits of input; and echo a question mark. Onlyspositive octal digits are~accepted. The program
is presented in four illustrations.
6-16

ACCEPT ASCII
CODE FOR
ONE DIGIT

TYPE

'?"

AND IGNORE
THAT ENTRY

COUNT THE
NUMBERS

[5
NUMBER

NEGA?TIVE

IS i1
A NONOCTAL

CHARACTER
?

PUT THE
NUMBERS lN

INCREASING ORDER

HAVE FOUR
DIGITS
?

l
TYPE ONE

PACK AM)
STORE AS
ONE NUMBER

TYPED
4 mens
.

RETURN
CARRTAGE

"Figure 6-14 Flowchart for Sample Program

6-17

*200

START,.

ACCEPT,
‘

CLA
TLS

CLL

TAD BUFF
DCA BUFFPT
DCA AMOUNT
JMS CRLF
TAD M4
DCA DIGCTR
TAD TEMP!

DCA_TEMP

'NEWDIG,

CHECK,
‘

PACK,

DIGPAK,
,

JMS

DCA
TAD
~TAD
SNA
JMP

LISN
I TEMP
I TEMP

MDOLAR
CLA
ORDER
TA 1 TEMP
TAD M268
SPA
JMP ERROR
TAD M10
SMA CLA
JMP ERROR
182 TEMP
ISZ DIGCTR
JMP NEWDIG
TAD TEMP!
DCA TEMP
DCA HOLD
TAD M4
DCA DIGCTR
TAD HOLD‘
CLL RAL
RTL
TAD I TEMP

TAD7M266

DCA_HOLD
ISZ TEMP

ISZ’DIGCTR
JMP DIGPAK
TAD HOLD
DCA I BUFFPT
TAD I BUFFPT
TAD K4000
SMA CLA
JMP ERROR
152 AMOUNT
ISZ BUFFPT
JMP ACCEPT

Figure 6-15

ICLEAR ACCUMULATOR AND LINK
/RAISE PRINTER FLAG
/INITIALIZE STORAGE
/BUFFER POINTER
/AND ELEMENT COUNTER
/RETURN CARRIAGE
/INITIALIZE COUNTER
/FOR 4 DIGIT INPUT
/SET A POINTER T0
/TEMPORARY INPUT STORAGE
/GET A CHARACTER
/STORE IT
/CHECK FOR A
/TERMINAL $
/IS CHARACTER A $?
/YES: ORDER INPUT.
/NO: CHECK FOR
/OCTAL INPUT
/IS INPUT <'266?
/YES: ENTER ERROR ROUTINE
/NO: SUBTRACT 10
/IS INPUT > 26%?
/YES: ENTER ERROR ROUTINE
/NO: INCREMENT POINTER
/RECEIVED 4 DIGITS YET?
/NO: GET ANOTHER
/YES: SET POINTER TO
/STORAGE LOCATION
/CLEAR LOCATION HOLD
/SET COUNTER FOR
/4 DIGITS
/LOAD HOLD INTO AC
IROTATE INTO CLEARED LINK
/ROTATE TWICE MORE
IADD ONE ASCII CHARACTER
/SUBTRACT OUT THE 263
ISTORE IN LOCATION HOLD
/INCREMENT STORAGE POINTER
/PACKED 4 DIGITS YET?
/NO: PACK ANOTHER
/YES: STORE
/PACKED NUMBER
lCHECK FOR
/NEGATIVE INPUT
lIS ENTRY NEGATIVE?
/YES: TAKE ERROR BRANCH
/NO: COUNT THE ENTRIES
/INITIALIZE FOR A NEW ENTRY
/GET A NEW ENTRY
"

Initialization and Input Coding for Sample Program

6-181

ORDER,

TEST,

TAD AMOUNT
CIA
IAC
DCA TALLY
DCA FLAG
TAD BUFF
DCA X1
TAD BUFF
IAC
DCA X2
TAD I X2
CIA
TAD I X1
SMA SZA CLA
JMS REVERSE
ISZ X1
ISZ X2
ISZ TALLY
JMP TEST
TAD FLAG
SZA CLA
JMP

ORDER

JMP PRINT

‘

REVERSE,Z'
TAD
DCA
TAD
DCA
TAD
DCA
CLA
‘DCA
JMP

I X1
HOLD
I X21 X1
HOLD
I X2
CLL CMA
FLAG
I REVERSE

Figure 6-16

/SET UP A TALLY
/TO COUNT THE
/NUMBER OF
/COMPARISONS
/CLEAR THE FLAG
/SET XI POINTER TO
/FIRST BUFFER LOCATION
/SET X2 POINTER T0
/SECOND BUFFER
/LOCATION
»/GET X2 POINTER ENTRY
/FORM TWO'S COMPLEMENT
/ADD Xl POINTER ENTRY
/IS X1 ENTRY LARGER?
/YES: SWITCH X1 AND X2
/NO: INCREMENT X1 POINTER
/INCREMENT X2 POINTER
/COMPARED ALL ENTRIES?
/NO:.LOOP BACK
/YES: WAS SWITCH MADE
/0N LAST PASS?
/YES: MAKE ANOTHER PASS
/NO: TYPE THE ORDERED DATA
/SWITCH X1 AND X2
‘/GET X1 ENTRY
/STORE TEMPORARILY,
/GET X2 ENTRY
/STORE AT XI LOCATION
/GET Xl ENTRY
/STORE AT X2 LOCATION
/SET FLAG WHENEVER
"/A SWITCH IS MADE
/RETURN
‘

Order Routine Coding for Sample Program

6—19

PRINT,

JMS CRLF

/RETURN

ANOTHR,

MORE,

JMS
TAD
DCA
DCA
TAD
CLL
TAD

CRLF
M4

DIGCTR
HOLD
I BUFFPT
RAL
HOLD
'

NRAL
RTL

/RETURN THE CARRIAGE,
/GET DIGIT COUNT
/INITIALIZE DIGIT COUNTER
/CLEAR STORAGE LOCATION
/GET A CHARACTER
/ROTATE INTO CLEARED LINK

/ADD STORED LOCATION
/ROTATE LEFT

ISZ
JMP
ISZ BUFFPT
ISZ PRNTCT
JMP ANOTHR
JMS CRLF
JMP START

/THREE TIMES
/STORE ROTATED NUMBER
/GET STORED LOCATION
/MASK OUT FIRST 9 BITS
ICONVERT TO ASCII
/TYPE ONE DIGIT
/TYPED 4 DIGITS YET?
/NO: TYPE ANOTHER
/YES: INCREMENT POINTER
/TYPED ALL ENTRIES?
/NO: TYPE ANOTHER
/YES: RETURN CARRIAGE AND
/ACCEPT MORE INPUT

CLA
TAD QUEST
JMS TYPE
JMS ACCEPT

/ERROR ROUTINE
/GET ASCII FOR "7"
/PRINT QUESTION MARK
/DISREGARD ILLEGAL ENTRY

DCA

TAD
AND

TAD
JMS

ERROR,

THE

CARRIAGE
/INITIALIZE THE
/BUFFER POINTER
/INITIALIZE A COUNTER
/TO COUNT
/0UTPUT ELEMENTS

TAD BUFF
DCA BUFFPT
TAD AMOUNT
CIA
DCA PRNTCT

HOLD
HOLD
MASK?
K260
TYPE
DIGCTR
MORE

ENDZ.

Figure 6-17

Output Coding for Sample Program

6-20

*166

TYPE,‘

‘8

IJMP
TLS
CLA
JMP
0

CRLF,

0—1

TYPE

TAD K215
JMS TYPE
TAD K212
JMS'TYPE

JMP

CRLF

LISN,

I
-

KSF

‘JMP
KRB

.-l

TLS
JMP
END
Q

LISN-

BUFF,
BUFFPT,
7774
M4,
DIGCTR, O
TEMP1,.N.+2
TEMP,
@;O;0;0;@
MDOLAR, 7334'
-10
MIC,
K40@@,r AOOO
O
HOLD,
—zsa_
M260,
AMOUNT, O
O
FLAG,
O
TALLY,
O
.x1,
O
x2,
PRNTCT, O
7
MASK7,
‘

K260,
-K212,
K215,

.QUEST,
$

/TYPE SUBROUTINE
/PRINTER FLAG RAISED YET?
/NO: CHECK AGAIN
/YES: PRINT A CHARACTER
/CLEAR ACCUMULATOR‘
/AND'RETURN
ICARRIAGE RETURN/LINE FEED
/GET ASCII CARRIAGE RETURN
/PRINT IT
‘/GET ASCII LINE FEED
/PRINT IT
/AND RETURN
/LISN SUBROUTINE
/KEYBOARD FLAG RAISED YET?
/N0: CHECK AGAIN
/YES:.READ A CHARACTER
/ECHO ON PRINTER
/AND RETURN
/FIRST BUFFER LOCATION
IBUFFER POINTER
/DIGII COUNT
/DIGIT COUNTER.
/TEMP STORAGE POINTER
/TENP DIGIT STORAGE
/TwO'S COMP OF 244
/OCTAL -1O
/OCTAL AOOO
/PACK ROUTINE STORAGE
.‘ /ASCII CONVERSION FACTOR
/DATA ENTRY COUNTER
/SIGNALS DATA SWITCH
/COUNTS DATA COMPARASONS
/0RDER_ROUTINE
/POINTERS
/COUNT OUTPUT ELEMENTS
/ASCII CONVERSION MASK'
/ASCII DIGIT OFFSET
/ASCII LINE FEED
/ASCII CARRIAGE RETURN
/ASCII QUESTION MARK
.

TSF

26G
212

215
,277

’

Figure' 6-18

Subroutines and Constants for Sample Program

6-21

PROGRAM INTERRUPT FACILITY
The running time of programs using input and output routines

is primarily made up of the time spent waiting for an I/O device
to

accept or transmit information. Specifically; this time is spent

in loops such as:
TSF

/ SKIP ON FLAG

J MP .—1

Waiting loops waste a large amount of computer time. ~In those
cases where the computer can be doing something else while wait—
ing, these loops may be removed and useful routines included to
use the waiting time. This sharing of a computer between two tasks
is often accomplished through the program interrupt facility, which
is standard on all PDP—8 series computers. Theprogram interrupt
facility allows certain external conditions to interrupt the com:
puter program. It is used to speed the processing of I/O devices
or 'to

allow certain alarms to halt program execution and initiate

another routine.

’

Every device which is able to request a program interrupt contains a special one-bit register called the interrupt request flag.
This register normally contains a 0, but it is set to 1 whenever the
device requires servicing.
When the interrupt facility is enabled and any device flags an
interrupt request, the computer automatically disables its interrupt
system and executes a hardware JMS 0. This causes the contents
of the program counter to be stored in core memory location 0000
and the instruction in location 0001 to be executed. Location 0001

usually contains an effective 1 MP SERVE, where SERVE is the
entry address of an interrupt service routine. The interrupt service

routine calls I/O device service routines to correct the condition

which caused the interrupt, then re—enables the interrupt system

and executes a J MP I 0 instruction to resume program execution.
Table 6-3 lists the eight IOT instructions used to program the
‘

PDP-8/E for program interrupt operation.
.

If an interrupt occurs while another interrupt is being serviced,

the return address stored in location 0000 will be lost. This is pre-

vented by leaving the interrupt system disabled while the interrupt

6-22

‘

Table 6-3

Program Interrupt-IOTVInstr-uctions
I

Mnemonic

Octal-

Operation

SKON

6000

Skip 'the next instruction if the interrupt
system is on and turn the interrupt system
CE.

/
p

‘ICHQ

6001

Execute the next instruction, and then turn
the interrupt system on.

IOF

6002

Turn the

SRQ

6003

Skip

GTF

6004

Get flags. The link is loaded into accumulator bit position 0. Accumulator bit 2 is set
to a 1 if any device. is requesting an interrupt. Accumulator'bit 4 is set to a lifthe

interrupt system off.

the next instruction if one or
devices are requesting an interrupt.

interrupt system is enabled.3
'

RTF

6005

Restore
verse

flags. This instruction is the con-

of the GTF instruction. Execution of

RTE instruction is deferred until after
the next JMP or JMS instruction is exean

cuted. If accumulator bit 4 contains a 1,
this instruction enables the interrupt sys-V

tem.3

‘

SGT

6006

CAF

60073‘

3 The

the next instruction if the Greater
Than Flag is set. This instruction is implemented only if the KE8-E Extended
Arthmetic Element is installed.

Skip

Clear all flags. This instruction is the logical equivalent of operating the CLEAR
switch on the programmer’s console. It
should not be used while any I/O device
is active.

GTF and RTF instructions perform other operations on the remain-

ing accumulator bit positions if the KE8-E Extended Arithmetic Element
and/or KM8-E Extended Memory Control
Computer Handbook for further information.

6—23

are
'

installed. See the Small

‘

service routine is servicing an I/O device. The interrupt system
may then be re—enabled by an ION instruction immediately before
the J MP I 0 which terminates the interrupt service routine, or by
an
J
I

RTF instruction anywhere in the service routine. Execution of

the ION instruction will be deferred until the following instruction

has been executed, and execution of the RTF instruction will be
deferred until the next J MP or J MS instruction has been executed.
In this manner, an interrupt service routine protects the contents
of location 0000 by executing with the interrupt system disabled

and using deferred ION or RTF instructions to re—enable the in'

terrupt system as soon as mainline execution has resumed.Use of the interrupt system allows a mainline routine, referred
to as the background program, to execute without wasting a large
amount of time in waiting loops while I/O devices are assembling

and transmitting information. The interrupt service routine, called

foreground program, is entered automatically whenever an I/O
device requires servicing under program control.
a

Programming an Interrupt
The program presented in Figure 6-19 consists of a background
routine which rotates one bit through the accumulator endlessly,
and a foreground program, initiated by the interrupt service routine, which accepts and stores ASCII characters from the Teletype.
Upon receipt of the ASCII code for a period, the foreground program prints out the characters which have been stored.
The coding begins with an initialization routine which allocates
buffer space to store the incoming characters and sets the mode
for input. The program signals input mode by a value of MODE
=0 and output mode by a value of MODE-=1. Once the initialization routine has enabled the interrupt facility, the background
program is started.
The background program is a routine to rotate one bit through

the accumulator and link. The first instruction clears all bits ex-

cept bit 11. The program then counts through the two 152 loops,
after which it-rotates the bit one place left and then returns to the
count loops. With the console indicator select switch set to AC,

displays will exhibit a rapidly rotating
light while waiting for an interrupt to initiate the foreground
the accumulator and link

program.

6-24

An interrupt request will cause the

following operations automatically:

computer to perform

the

The interrupt system is disabled.

The content of the program

ory location 0000
The JMP I 2 instruction in location 0001 is executed. Loca-

counter is stored at core mem—

tion 0002 contains the entry address of the interrupt service

routine.
The interrupt service routine then performs the following op—
erations:
1.

The contents of the accumulator and link are stored.

The source of the interrupt is determined.

A JMP to either the keyboard input routine or the printer

output routine is executed.
The keyboard input routine is entered from the interrupt service

routine whenever the keyboard flag is set. The flag is cleared to
prevent further interruptions when the interrupt system is reenabled. If the mode is set for output, program control returns to
the background program. Otherwise,‘ the routine accepts a char—
_

(KRB), acknowledges receipt by printing the character on
the printer (TLS), and stores it in the buffer. No .KSF, J MP .-1
acter

loop-is necessary. The routine then checks for the ASCII code for
period, returning to the background program if the character
was not a period. Upon receipt of a period, the routine resets the
buffer and sets the mode for output. Program-control then returns
to the background program. Since
a TLS instruction was executed
previously, another interrupt will be requested as soon as the
printer becomes available, and the stored ASCII codes will be
typed out by the printer output routine.
a

The printer output routine is entered from the interrupt service

routine whenever the printer flag is set. This routine clears the
device flag and checks for output mode, then prints one character
from the buffer. No TSF, JMP .-1 loop is necessary. If the character is not

”

period, control returns to the background program
while the printer finishes typing the character. If the character is
a period, the routine resets the bulfer, sets the mode for
input,
and then returns to the background program.
a

6-25

The keyboard and printer service routines return control to the

background program Via the exit routin‘e. This routine restores the
content of the accumulator, which was previously saved by the interrupt‘service routine, then re-enables the interrupt system by
means of an RTF instruction, which also restores the link. The

RIF instruction does not take effect until after the IMP I 0 instruction, which terminates the exit routine, has been executed.
This allows background program execution to resume before another

interrupt can occur.

The constants used

by the various

routines conclude the listing.

cg:
SET MODE

FDR mpur

:3.“ +

RESTORE
A: AND L

CLEAR

LPRINTER

RESERVE
BUFFER SPACE
FOR CHARACTERS

l
TURN

mTERRUFT ON

WTERRUPT 0"

i
RETURN TO
BACKGROU'D
AN

READ THE
CHARACTER

TYPE ONE
CHARACTER

FROM BUFFER

ROTATE an
ENDLESSLY “mum
AcchLAToR

TYPE THE
CHARACTER

i
STORE ASCII
CODE EN

INTERRUPT

BUFFER

SAVE Ac

RESET BUFFER

AND L

POINTER

e
15 IT
A PERIOD

ENTER KEYWORD
SERVICE ROUTINE

CHANGE HOW

(KB)

TO INPUT
YES

RESET BUFFER

POINTE R

Figure 6-19A-

Interrupt Facility Program Flowchart
6—26

IFIRST INSTRUCTION
/AFTER AN INTERRUPT:
EISTORE RETURN ADDRESS
flJUMP TO SERVICE ROUTINE
IPOINTER TO SERVICE ROUTINE
/INITIALIZATION ROUTINE:
.

0‘

JMP I 2
SERVE

*EOE

START,

ICLEAR ACCUMULATOR AND LINK,
ISET MODE FOR INPUT
/INITIALIZE DATA
IBUFFER POINTER
ITURN INTERRUPT 0N
/BACKGROUND PROGRAM:
ISET ACCUMULATOR BIT ll

CLL
MODE
TAD K1777
DCA BUFFER
CLA
DCA

ION

ROTATE,'CLA' CLL

IAC
ISZ COUNT
JMP
ISZ
JMP
RAL
JMP ROTATE+1

/COUNT
/TWICE

SERVE,

DCA AC
GTF
DCA FLAGS

KSF
SKP
JMP KB
TSF
SKPJMP TP
CAP
JMP

Ex1T_

KB,

KCC

TAD
SZA
JMP
ISZ
KRB
TLS

MODE
CLA

EXIT
BUFFER

DCA I BUFFERTAD I BUFFER
TAD MPER'
SZA CLA

'JMP EXIT
TAD K1777
DCA BUFFER
CLA‘ CMA
DCA MODE
JMP EXIT

Figure 6-19B

/THROUGH
IDELAY LOOP
/ROTATE BIT LEFT
/RETURN TO DELAY LOOP
ISERVICE ROUTINE:
/SAVE ACCUMULATOR
/SAVE INTERRUPT‘
/FLAGS AND LINK
IKEYBOARD FLAG RAISED?
'/N0: CHECK PRINTER
lYES; SERVICE KEYBOARD
/PRINTER INTERRUPT?
INO: SKIP FOR EXIT
/YES: SERVICE PRINTER
/CLEAR ALL FLAGS
/AND RETURN
/KEYBOARD INPUT ROUTINE:
/CLEAR KEYBOARD FLAG
/CURRENTLY IN
IINPUT MODE?
/NO: RETURN TO BACKGROUND
/YES: INCREMENT POINTER
/READ THE CHARACTER
/ECHO 0N PRINTER
/STORE THE CHARACTER
/WAS THE
/CHARACTER A
/PERIOD?
/NO: RETURN TO BACKGROUND
/YES: RESET BUFFER
>/POINTER T0 TYPE
ITHE CHARACTERS
/SET MODE FOR OUTPUT AND
/RETURN TO BACKGROUND
-

Interrupt Facility Program Coding

6—27

TCF
TAD MODE
SNA CLA
JNP Exrr

IP,
_

ISZ

BUFFER

IAD I BUFFER
TLS
TAD MPER
SZA OLA
JMPvEXIT
DOA MODE
TAD K1777
DOA BUFFER
JNP EXIT

EXIT,

TAD FLAGS
RIF
OLA

TAD Ac

/PRINTER OUTPUT ROUTINE:
ICLEAR PRINTER FLAG
/CURRENTLY IN
/0UTPUT MODE?
JNOA RETURN TO BACKGROUND
/YES: INCREMENT POINTER
/GET CHARACTER FROM BUFFER
/PRINT IT
/WAS THE CHARACTER
/A PERIOD?
/NO: RETURN TO BACKGROUND
/YES: SET MODE FOR INPUT
/RESET BUFFER
'/POINTER AND
/RETURN TO-BACKGROUND
/EXIT ROUTINE:
/GET FLAGS AND LINK
/RESTORE FLAGS AND LINK
/CLEAR ACCUMULATOR

IRESTORE‘ACCUMULATOR

/RETURN TO BACKGROUND
./DELAY LOOP COUNTER
[MODE szIOR
ILAST LOO BEFORE BUFFER
/DAIA BUFFER POINTER
/SAVE ACCUMULATOR
/SAVE FLAGS AND LINK
INEGATIVE OF ASCII CODE

Figure 6-19B (cont)

Interrupt Facility Program Coding

JMP
O
O

COUNT,
MODE,
1777
x1777,
O
BUFFER,
G
AC,
O
FLAGS,
~25s
MPER,
5

'Multiple Device Intefrupt Programming
Many programming applications use the interrupt system to
service Several devices. For example, a PDP—S/E may use the
interrupt facility to control the operation of DECtape and DEC—
disk systems through a Teletype console. Systems of this type require a service routine that, determines the source of an interrupt
request (i. e., which device flag is set). Thefollowing instruction
sequence uses dummy skip instructions to determine which
device
requested an interrupt
6-28

/SKIP ON DEVICE A FLAG

DASF
SKP
IMP SERVA
DBSF
SKP
JMP SERVB

/ DEVICE A REQUESTED THE INTERRUPT
/ SKIP ON DEVICE B FLAG
'

/ DEVICE B

DNSF'

REQUESTED. THE INTERRUPT

/ SKIP ON DEVICE N FLAG
'
'

SKP

IMP SERVN

/ DEVICE N REQUESTED THE INTERRUPT

The

dummy instructions (DASF, DBSF, etc.) are skip-on—flag'
instructions for each of the devices in the interrupt system. Because of the predominance of SKP instructions, the instruction
sequence which determines the sourceof an interrupt request is
often called a skip chain.
A skip chain may be enlarged to test for almost any number
of device flags, provided that high-speed devices which retain information for a relatively short period of time are tested near the
top of the skip chain, so that the chain may be traversed and the
high—speed devices serviced before the information is lost. High—
speed. devices should never be required to wait for service while
a long skip chain is traversed. If several high—speed devices are
tested at the topof the chain, it may still be necessary to check
the timing constraints to ensure that the last high-speed device to
'

be tested will be serviced before any loss of information can occur.
If two

interrupts occur simultaneously, the high-speed device
will ‘be serviced first because it will be tested first in the skip
chain. The low-speed device may be serviced later in either of
I

two ways:

By terminating

the

high-speed

device service routine in

the usual manner, in which case the low—speed device will

request another interrupt as soon as the interrupt system
has been re-enabled and

resumed.“
2.

background program execution

By terminating the high-speed, device service routine with
a jump back into the
skip chain, without re—enabling the
interrupt .system. In this case, the skip chain must terminate
6—29

.—

with an ION, J MP I 0 instruction sequence to return con—
trol to the background program if no further interrupt re—
‘

quests are pending.

A Software Priority Interrupt System
The techniques described in the previous section may be used
to

guarantee that a high-speed device will be serviced before a

interrupts occur simultaneously. How—
ever, information will still be lost if the high-speed device requires

slower device when two

servicing and requests an interrupt while the low-speed device is
being serviced, because the interrupt system is disabled during
‘

this time.

An interrupt service routine may be written in such a way that

priority of device interrupts is established through software.
This may become necessary in a system that includes high—speed
devices which retain information for a short time and require im~
mediate attention. For example, a PDP-S/E system employing
Teletype and DECtape I/O may use a software priority interrupt
system which permits the Teletype service routines to be inter—
rupted whenever DECtape requests servicing. This is accom—
plished by modifying the keyboard and printer service routines to
permit the following sequence of operations:
a

Begin the keyboard and printer service routines by storing
the contents of location 0000, the accumulator, the link and

any other important registers in temporary storage locations.
Execute an ION instruction,

5-”

Clear the low-speed device flag. The ION instruction will

not take effect until this instruction has been executed.

Service the device. At this

point, the service routine may
be interrupted without danger of the background program.
return address being lost.
Terminate the service routine by restoring the contents of
the accumulator, the link and any other registers which were

stored in step 1, then executing a J‘MP I TEMP, where
TEMP is the location in which the return address from

location 0000 was stored.
A low—speed device service routine written in this manner operates with the

interrupt system enabled most of the time. The
6—30

"routine may be interrupted by an interrupt request from a higher
priority. device, and the second interrupt will not cause the con—
tents of location

0000 (or any other important registers) to be

lost.
In the example cited above, the DECtape and DECdisk service

routines would not ”re-enable the interrupt system until they are
ready to return control to the background program. However, the

“

Teletype routines re-enable the interrupt system almost. immedi—
ately. If DECtape or DECdisk interrupts one of the Teletype ser—
vice routines, the Teletype service routine is treatedlike a back—
ground program;.that is, execution of the Teletype service routine
is suspended until the high—speed device has been serviced. Control
then reverts back to the Teletype service routine, and finally back
to the mainline program.

Multiple Interrupt Demonstration Program
Figure 6-20- presents a demonstration program designed to use
the program interrupt facility. This program rotates a bit through
the accumulator, with the speed and direction [of rotation determined by switch register settings.~ Simultaneously," the pregram
accepts 4—digit, positive octal numbers from theTeletype, automatically terminating each .4-digit number with "a carriage return
and line feed'.‘ Upon receipt of a typed dollar sign (iii), the program
sorts the data into ascending numerical order and prints the or—
dered numbers on the Teletype.
This example illustrates the power of interrupt programming
because the computer appears to be performing two tasks at the
same time. The programmer knows that this is impossible, and
that the two tasks are actually sharing computer time; however,
the appearance indicates simultaneous action.

When an interrupt request occurs, the current value of the PC

is stored in location 0000 of page 0, while locations 000] and
0002 contain an indirect jump to the interrupt service routine. The

program constants which are stored on, page 0 beginning at location 0050 include four software switches that record conditions
within the program. The contents of location MODE indicates
whether the program is currently performing input or output func-

tions. Location SW1 indicates whether the next digit to be received
'

6—31

‘
'

/PAGE ZERO

/INTERRUPT
[HANDLER
/PACE ZERO CONSTANTS:
/INPUI=0
0UTPUT=-l

“fl*52
MODE,
"*‘sw1,1
Euswa,
(sws,
'

‘

Ac,

‘“FLAGS,'
PRINTR,
KEYBRD,
ORDPTR,
EXITPT,
BUFF,

BUFFPT,
DICCTR,
,TEMPI,
TEMP,

O
OLD # = l
LFz-l
DATAzl
/MODE BYPASS szTCH
/SAVE ACCUMULATOR
ISAVE INTERRUPT FLAGS
/POINTER To TP ROUTINE
/POINTER TO KB ROUTINE
/POINTER TO ORDER ROUTINE
/POINTER T gEXIT ROUTINE
IFIRST BUF ER LOCATION
/BUFFER POINTER
/-4 OCTAL
/DIGIT COUNTER
/TEMP STORAGE POINTER
/TEMP DIGIT STORAGE

INEU #
/CR=@

MDOLAR,
'MIB,
HOLD,

LHOLDL,
M266,
AMOUNT,
FLAG,
TALLY,
x1,
‘x2,
PRNTCT,
K7,
260
xzsg,,
212
x212,
215'
Eats,
277
QUEST,
4360
K4096,
.

Figure 6-20A

OCTAL
OCTAL
IPACX ROUTINE STORAGE
ISAVE LINK
/—26O OCTAL
/DATA COUNTER
/ORDER ROUTINE FLAG
/ORDER ROUTINE COUNTER
/ORDER ROUTINE POINTER
/ORDER ROUTINE POINTER
/OUTPUT ROUTINE COUNTER
/Asc11 CONSTANTS:
/ASCII CONVERSION FACTOR
/ASCII LINE FEED
/ASCII CARRIAGE RETURN
JASCII'QUESTION MARK
/4DOD OCTAL
/-244

x-IO

Multiple Interrupt Demonstratipn Program
,4

($32

TAD x215

CR,

aTLS

ACCUMULATOR

CLA £MA
DOA SW2
;A»H;
JMP I EXIIPI
TAD K212i

LF,-»

swstT,

‘TLS
CLA
TAD sws
SNA CLA
JMP swstT
DCA sws
DCA swz
JMP I EXITPT
CLA CLL IAc
DCA swz
JMP I EXITPT

Figure 6-20B

IGET ASCII CARRIAGE
RETURN
IPRINT IT
/LOAD -1 INTO
/SET swa FOR LINE FEED
=/ENTER EXIT ROUTINE
IGET ASCII LINE FEEO
/PRINT IT
/CLEAR ACCUMULATOR
/UEI MODE BYPASS szTCR
ISEI FOR BYPASS MODE?
/No: SET swz AND RETURN
/YES: TURN OFF MODE BYPASS
ISEI swa FOR CARRIAGE RETURN
/ENTER EXIT ROUTINE
-/LOAD 1 INTO ACCUMULATOR.
/SET swz FOR DATA
lENTER EXI T ROUTINE

Multiple Interrupt Demonstration Program

is the first digit of a new namber, to be packed into a new storage

location, or the neXt digit of a continuing number, to be packed
and stored with previous digits. Location SW2 indicates whether a

carriage return or line feed-should be printed to delimit input ele—
ments, and location SW3 regulates the printing of carriage returns,
line feeds and question marks while the program is in input mode.
A question mark is printed whenever a non—octal digit or nonpositive octal numberis received.
Other constants include location AC, in which the content of
the. accumulator issaved, during an interrupt, and location FLAGS,
in which the interrupt flags and the link are stored. Location BUFF
"contains the address of the first data buffer location, ’which .is the
core location
"

following the. last program instruction. Thus, the data
buffer is all of memory following the end of the program.
The 5-word-block beginning at location-TEMP is used to store
6—33

*ZOO

START,

10F
OLA
DCA
DCA
DCA
DOA

TAD

ROTATE,
BECIN,

GO,

INSTR,

CLL
MODE
SW1
SW2
SW5
BUFF

—

DCA BUFFPT
DCA AMOUNT
ION
OLA CLL CML
DCA SAVEAC
RAL
DOA SAUEL
TAD K7006
OSR
DCA COUNT
OSR
RAL
SZL CLA
JMS LEFT
JMS RIGHT
CLL
TAD SAVEL
RAR
TAD SAVEAC

HLT
ISZ COUNTR
INSTR+1
ISZ COUNT
JMP INSTR+l
JMP BEGIN

JMP

SA VEAC,

SAVEL,
KTOOO,
COUNTR,

/COUNTERS

COUNT,
LEFT,
ISZ LEFT
TAD KRAL
DCA INSTR

RIGHT,

KRAR,‘
KRAL,

/INTERRUPT SYSTEM OFF
V/DURING INITIALIZATION
/CLEAR ACCUMULATOR AND LINK
/SET INPUT MODE
/SET SW! FOR NEW ENTRY
/SET SW2 FOR CARRIAGE RETURN
/SET MODE BYPASS
/GET FIRST BUFFER LOCATION
r/INITIALIZE BUFFER POINTER
/AND ELEMENT COUNTER
IENABLE INTERRUPTS
/SET THE LINK
/CLEAR ACCUMULATOR STORAGE
/ROIATE LINK BIT LEFT
/SAVE LINK BIT
IGET OR MASK
/READ’SWITCH REGISTER
/INITIALIzE DELAY COUNTER
/READ SR BIT ZERO
/ROTATE INTO LINK
/wAS BIT ZERO SET?
{YES: ROTATE LEFT
INO: ROTATE RIGHT
ICLEAR THE LINK
/GET THE STORED LINK
/AND RESTORE IT
/GET STORED ACCUMULATOR
/OVERwRITTEN BY RAL OR RAR
/INCREMENT COUNTER
/UNTIL IT OVERFLONS
IINCREMENT COUNT
/UNTIL IT OVERFLOWS
/LOOP BACK AND REPEAT
/STORAGE FOR ACCUMULATOR
ISTORAGE FOR LINK
/SETS BITS 1-5
/ROTATE ROUTINE

JMP I LEFT
fl
TAD KRAR
DCA INSTR
JMP I RIGHT
RAR

RAL

Figure 6-200

/INSERT RAL INSTRUCTION
/INCRENENT RETURN ADDRESS
/CET RAL INSTRUCTION
/wRITE OVER RLT
/AND RETURN
/INSERT RAR INSTRUCTION
/GET RAR INSTRUCTION
waITE OVER RLT
/AND RETURN
[RAR INSTRUCTION
/RAL INSTRUCTION

Multiple Interrupt DemonsuafioniProgram
6-34

SERVE,
,

/SAME ACCUMULATOR

_DCA

'GTF
DCA

FLAGS’

IGET'FLAGS
ISAVE FLAGS AND LINK
/PRINTER FLAG RAISED?
/NO: CHECK KEYBOARD
IYES: SERVICE PRINTER
/KEYBOARD FLAG RAISED?
/NO: CLEAR ALL FLAGS
/YES: SERVICE KEYBOARD
IAND RESUME
/BACKGROUND EXECUTION
/0RDER ROUTINE
/GET NUMBER OF ENTRIES

TSF
SKP
JMP

KSF
SKP

PRINTR

I KEYBRD
CAF
JMP I EXITPT’
CLA CLL
TAD AMOUNT
CIA
IAc
DOA TALLY
DOA FLAG
TAD BUFF
DCA XI
TAD BUFF
IAC

JMP

ORDER,

IEST,""

REVERs,

‘

'/SET TALLY TO
ITERATIONS
/THRU ORDER LOOP
/CLEAR FLAG ON EACH PASS
IGET FIRST BUFFER ADDRESS
/INITIALIZE X1 POINTER
/FORM SECOND
/COUNT

[BUFFER

CLA

INCPTR,

CLL CMA
DOA FLAG
ISZ x1

152
182
JMP
TAD
SZA

X2
TALLY
TEST
FLAG
CLA

JMP

ORDER

CLA CMA
DCA’ MODE
TAD BUFF
DCA BUFFPT
DCA SW1
TAD AMOUNT
CIA
DCA PRNTCT
TLS
JMP I EXITPT
_

Figure 6-20D

ADDRESS

/INITIALIZE X2 POINTER
/GET X2 POINTER ENTRY
IFORM TNO'S COMPLEMENT
/ADD X1 POINTER ENTRY
/IS ACCUMULATOR POSITIVE?

DCA X2
TAD 1 x2_
CIA
TAD I x1
SPA SNA CLA
JMP INCPTR
TAD I x1_
DCA HOLD
TAD I x2
DCA 1 x1
TAD HOLD
DCA I-Xz

/NO: INCREMENT,POINTERS
/YES: SWITCH THE ENTRIES
/HOLD X1 ENTRY
/GET X2 ENTRY
/STORE AT XI LOCATION
IGET X1 ENTRY
/STORE AT X2 LOCATION
/LOAD *1 INTO FLAG TO
/SHOW SWITCH WAS MADE
~/INCREMENT X1 POINTER
/INCREMENT X2 POINTER
/COMPARED ALL ENTRIES?
/NO: LOOP BACK
/YES: WAS SWITCH MADE
/ON LAST PASS?
/YES{ CONTINUE ORDERING
/N0: FORM -1 AND
/SET MODE FOR OUTPUT
/GET FIRST'BUFFER ADDRESS
/INITIALIZE BUFFER POINTER
ISET SW1 FOR NEW NUMBER
/GET NUMBER OF ENTRIES
IFORM TWO'S COMPLEMENT

/INITIALIZE COUNTER
/RAISE PRINTER FLAG
IRESUME BACKGROUND

Multiple Interrupt Demonstration Program
6—35

successive digits of a 4-digit number. Location TEMP serves as a

pointer to the following four locations, and the content of location
TEMPl is used to reset location TEMP for the first digit of a new
number. Page 0 also contains three Subroutines which print carriage returns and line feeds, when indicated by the value of SW2,
and then reset SW2 to continue data I /0.
The program coding begins with an initialiation routine at loca-

tion 0200; This routine initializes the software switches and resets

the data buffer pointer and counter, then enables the interrupt
facility. At this point the background program is entered.
The ROTATE routine sets accumulator bit 11, then checks

*400

{KEYBOARD SERVICE ROUTINE
KCC

KB,

TAD MODE

CNTDIG,
’

CHECK,

SZA CLA
JMP EXIT
TAD SW1
SZA CLA
JMP CNTDIG
TAD M4
DCA DIGCTR
TAD TEMP!
DCA TEMP
KRS
DCA I TEMP
TAD I TEMP
TLS
TAD MDOLAR
SNA CLA
JMP I ORDPTR
TAD I TEMP
TAD M260

SPA
JMP
TAD
SPA

ERROR,

ERROR
MID

JMP LEGAL
CLA TAC
DCA SW5
DCA SW1
TLS
'JMP EXIT

Figure 6-20E

/CLEAR KEYBOARD FLAG
/CURRENTLY IN
/INPUT MODE?
/NO: RESUME BACKGROUND.
/YES: EXPECTING CONTINUED
INUMBER 0R NEW NUMBER?
ICONTINUED NUMBER
INEW ENTRY
'

IINITIALIZE DIGIT COUNTER
IAND TEMPORARY
IDIGIT STORAGE
/READ A CHARACTER
/STORE IT
/GET THE CHARACTER
IECHO ON PRINTER
’

ITS CHARACTER A
/DOLLAR SIGN (3)7
IYES: BEGIN ORDER ROUTINE
INO: GET THE CHARACTER
/SUBTRACT 260

/IS CHARACTER < 26%?
/YES: TAKE ERROR BRANCH
/NO: ADD TO
/IS CHARACTER > 267?
/N0: MUST BE LEGAL
/YES: FORM 1 AND
/SET SW3 FOR BYPASS
/RESET SW1
/RAISE PRINTER FLAG
/RESUME

BACKGROUND‘

Multiple Interrupt Demonstration Program
6-36

switch register bit 0 and rotates the accumulator'right, if bit 0 is

off, or left, is bit 0 is on. The appropriate instruction (RAL or
RAR) is written into location INSTR, which is arbitrarily assigned
an initial content" of 7402 (HLT). After determining the direction
of rotation, the ROTATEroutine loads the value of switch register
bits 1-11 into a counter and uses this value to‘control the speed
-

Increasing the value specified by switch register bits
1-11 decreases the speed of rotation.
of rotation.

LEGAL,

PACK,

DIGPAK,

CLA
DCA
ISZ
ISZ
amp
TAD

TEMP
DIGCTR
EXIT
TEMPl
DCA TEMP
DCA HOLD
TAD M4
DCA DIGCTR
TAD HOLD

RAL CLL.
RTL

NOTNEG,
DISALO,

/FORM -1 IN ACCUMULATOR
/SET SWl
/INCREMENT STORAGE POINTER
/INCREMENT DIGIT COUNTER

CMA
SW1

TAD I TEMP
TAD 'MBSB
DCA HOLD
ISZ TEMP
ISZ DIGCTR
JMP DIGPAK'
TAD HOLD
DCA I BUFFPT
TAD ‘I BUFFPT
TAD KADDD
SPA CLA
JMP NOTNEG
IAC
JMP DISALO
ISZ BUFFPT
132 AMOUNT
CLA CMA
DCA SW5
DCA SW1
TLS
JMP EXIT_

Figure 6-20F

"/RESUME BACKGROUND
/INITIALIZE TEMP
/STORAGE POINTER
/CLEAR STORAGE LOCATION
/INITIALIZE THE
/DIGTT COUNTER
/ADD STORED LOCATION
/ROTATE LEFT
/THREE TIMES
/ADD IN A DIGIT
/SUBTRACT THE 260
/STORE PARTIAL NUMBER
/INCREMENT DIGIT POINTER
/PACKED 4 DIGITS YET?
/NO: PACK ANOTHER
/YES: GET PACKED NUMBER
/STORE IN BUFFER
/GET STORED NUMBER
/ADD 408% OCTAL
IIS NUMBER NEGATIVE?
/NO: KEEP ZERO ACCUMULATOR
/YES: FORM 1 IN ACCUMULATOR
/TAKE ERROR BRANCH

/INCREMENT_POINTER
/AND COUNTER
/FORM '1 IN ACCUMULATOR
/SET MODE BYPASS
/RESET SW1
/RAISE PRINTER FLAG
/RESUME BACKGROUND

Multiple Interrupt Demonstration PrOgram
6-37

The interrupt service routine, which begins in location SERVE,

is simply a skip chain that directs control to the appropriate device
service routine- If an interrupt occurs when none of the devices
tested in the skip chain requested an interrupt, the routine clears
all flags and resumes background execution.
The ORDER routine is initiated whenever receipt of

dollar

sign indicates that input is complete. This routine uses techniques
introduced in Chapter 3 to sort the input data into ascending order.
The device service routines and the interrupt system exit routine
conclude the program listing.

TP,
.

SNA
JMP
TAD
SPA
JMP
TAD
TLS
CLA
DCA

MODCHK,
RETLF,

DATA,

TAD SW5

JMP
TAD
SNA
JMP
TAD
SNA
JMP
TAD
SPA
JMP
TAD
SZA
JMP
TAD
DCA

CLA

MODCHK
SW3
CLA
RETLF

QUEST
CMA
SW3

EXIT
MODE
CLA
EXIT
SW2
CLA
CR
SWZCLA

SW1
CLA
DIGTYP

M4
DIGCTR

DCA HOLD
DCA HOLDL
TAD I BUFFPT

Figure6-20G

/CLEAR PRINTER FLAG
/SW5 SET FOR
/MODE BYPASS?
INO: CHECK CURRENT MODE
/IS SW3 SET FOR
/A QUESTION MARK?
/NO: RETURN CARRIAGE
/YES: GET QUESTION MARK
/PRINT IT
/SET SW3 FOR
ICR AND LF, THEN
/RESUME BACKGROUND
ICURRENTLY IN
/INPUT OR OUTPUT MODE?
/INPUT: IGNORE REQUEST
/OUTPUT: BEGIN OUTPUT
/PRINT A
ICARRIAGE RETURN 0N
/FIRST PASS, AND
/LINE FEED
/ON SECOND PASS
/PRINT DATA ON THIRD PASS
/NEU NUMBER?
/NO: TYPE ANOTHER DIGIT
IYES: GET DIGIT COUNT
/RESET DIGIT COUNTER
/CLEAR TEMPORARY
/STORAGE LOCATIONS
/GET NEXT NUMBER TO PRINT

Multiple Interrupt Demonstration Program
’

6—38

TAD HOLDL
CLL RAL

_DIGTYP5
'

-TAD' HOLD
RAL
RTL
DCA HOLD
RAR
DCA HOLDL
TAD HOLD
AND

TAD
TLS
CLA
DCA
ISZ

.JMP
CLA
DCA
DCA
152
152
amp
CLA
DCA
DCA
CMA
DCA
TAD
DCA
DCA
JMP
CLA
TAD
RIF

RESTRI,

EXIT,

CLA
TAD
JMP

K266

CMA
SW1

DIGCTR
EXIT
SW1
SW2
BUFFPT
PRNICT
EXIT
CLL
MODE
swz

/GET‘LINK AND
i/RESTORE IT
/ROTATE
/STORED VALUE
/LEFT
‘

ITHREEZTIMES
/SAVE BIT THAT WAS
IROTATED INTO LINK
/GET ROTATED NUMBER
/MASK OFF FIRST 9 BITS
/CONVERT TO ASCII
/AND PRINT A DIGIT
/FORM 11 IN ACCUMULATOR
/SET SW1 TO CONTINUE NUMBER
/PRINTED 4 DIGITS YET?
/NO: RESUME BACKGROUND
/YES: SET SW1 TO
ISIGNAL A NEW NUMBER
/SET SU2.FOR CARRIAGE RETURN
/INCREMENT BUFFER POINTER
/AND ELEMENT COUNTER
/RESUME BACKGROUND
ICLEAR ACCUMULATOR AND LINK
/SET MODE FOR INPUT
'/SET SW2 FOR CARRIAGE RETURN
/SET SW3 FOR
/INPUT MODE BYPAS
IGET FIRST BUFFER LOCATION
/RESET BUFFER POINTER
1

sws
BUFF
BUFFPT

AMOUNT
EXIT
CLL

FLAGS

IAND ELEMENT COUNTER
/RESUME BACKGROUND
/CLEAR ACCUMULATOR AND
‘/GET INTERRUPT FLAGS

LINK

IRESTORE FLAGS

/CLEAR ACCUMULATOR
/RESTORE ACCUMULATOR
IRESUME BACKGROUND

ENDz.
.

Figure '6-20H Multiple Interrupt Demonstration Program

6-39

DATA BREAK
transfers of data, including program interrupt
transfers, pass through the accumulator. This requires that the
content of the accumulator be saved before the transfer is per-

Programmed

formed, and later restored. This type of transfer is often too slow
for uSe with extremely fast peripheral devices. Devices which op-

high speed, or which require very rapid response
from the computer, use the data break facility. The data break
permits an external device to insert or extract core memory words
directly, bypassing all program control. Because a computer proerate at very

cognizance of transfers made in this manner, it is
necessary to check for the presence of transferred data prior to
using it. The data break is particularly well-suited to I/O devices
gram has

‘

that transfer large amounts of data in block form, such as random
access

disk files, high-speed magnetic tape systems, or high-speed
'

drum memories.

Accessing Data
Before a peripheral storage device may accept data from the
computer, it must find the off-line location at which the data is to
be stored. In the same manner,

peripheral device cannot send

input data into the computer until it has found the location at
which the data is presently stored. The process of finding a stored
data‘ word, or the location at which a data word is to be stored, is
called accessing data. The time required for this process is the device access time. It is important to realize that even very fast
peripheral storage devices have an access time of many machine
cycles, so that many instructions could be executed in the time
required for the peripheral device to access one data word.

Single-Cycle Data Break

of two types: single-cycle and 3-cycle. In a
data break, the I/O device contains two registers

Data breaks

are

single—cycle
which specify the core memory location with which the next data
word is to be transferred (current address, or CA, register) and
the negative of the number of words that remain to be transferred
(word count, or WC, register)pIOT instructions initiate the transfer by setting the WC and CA registers. Other IOT instructions
indicate the direction of transfer and cause the device to perform
6—40

any “preliminary operations which may be necessary to accessthe
desired data. At‘ this point, prOgram execution continues and the

device assumes full control of the data transfer.

Figure 6—21 provides a diagram of the single-cycle data break.
After performing the necessary preliminary operations, the I/O
device accesses the first data word and signals a data break request. This causes the central processor to suspend operation by
disabling its major state generator and instruction register, while
loading its memory address register with the contents of the de—
vice CA register. The device then generates a-signal which either

'I
.

LOAD WC
REGISTER

ACCESS DATA
LOAD CA
REGISTER
V

[EXECUTE

REQUEST
DATA BREAK

IOT'S TO

AXCESS DATA
I

LOAD CA
NTO CPMA

ONE MACHINE
CYCLE STOLEN
H'TO PERFORM
THESE

’

OPERATIONS

LOAD DATA INTO
MB REGISTER

ACCEPT DATA FROM
MB REGISTERS

I”

I
NCREMENT
WC BICA
REGISTERS

CONTINUE

)
RAISE DEVICE FLAG

Figure 6-21

Single-Cycle Data Break
6-41

loads the memory buffer register with data or accepts data from
this register, depending upon the direction of data transfer. When
this operation is complete, the device increments its WC and CA

registers and returns control to the central processor, which con-

‘

tinues mainline execution.

Meanwhile, the I/O device begins to access the next data word
to be

transferred. This process continues until the device WC

Cycle Stealing
The process by which

cycle is “stolen” from the central
processor in order to transfer data directly to or from core mem—
ory is called a cycle-steal operation. Each single-cycle data break
causes the WC and CA registers to be incremented and transfers

’

one

word of data to or from core memory. The 1/0 device re-

quests a data break whenever it is ready to transfer a word, and

performs all transfers during cycles stolen from the central processor. Program execution continues between data breaks, while
the I/O device is accessing the next data word.
3-Cycle Data Break
Devices which operate by means of a 3-cycle data break do not
contain a WC register or a CA register. Instead, the addresses of
two core memory locations

are

hard-wired into the device, and

these core locations are used in place of the WC and CA registers.
When a 3-cyc1e data break is requested, the first cycle is used to

fetch the content of the core location designated as the WC register into the device, increment it, test for WC overflow, and re-

register is incremented in the
same manner during the second cycle, and loaded into the memory
address register. The actual data transfer occurs during the third
cycle.
In contrast to devices which use the single-cycle data break,
3-cyc1e devices increment the CA register before performing the
actual data transfer. For this reason, the CA register of a 3—cycle
device must always be loaded with one less than the address of
the first data buffer location. The WC register of a 3-cyc1e device
turn it to core memory. The CA

is loaded with the two’s complement of the number of words to
be transferred, just as with a single-cycle device.

6-42

the only other major difler—
ence between single-cycle and 3-cycle data breaks is that a singlecycle device contains its own 'WC and CA registers, which are
loaded by means of IOT instructions. A 3-cycle device uses two
core memory locations as its WC and CA registers, so that the
registers must be loaded by means of memory reference instrucFrom a programming viewpoint,

tions.

EXERCISES

Write a subroutine ALARM which rings the teleprinter bell

five times.

Write a format subroutine for the teleprinter to tab space
the teleprinter. carriage. The subroutine is entered with the

number of spaces to be tabbed in the accumulator.
3.
i

Write a program that'will type a heading at the top

of the

paper and then type the numbers 1 through 10 down the
left hand side of the page with a- period after each number.

Write a program which will accept a 2-digit octal number
from the Teletype keyboard and type “SQUAREDz” and
the value for the number squared, followed by “OCTAL”
and a carriage return and line feed.

.5.

Extend the program written in [Exercise 4 by adding routines

to disallow the input of an 8 or-9 and type out an

appropriate

message.

Combine the program of Exercise 5 with a bit—rotating pro-

gram to use the program interrupt facility.

6-43

dectdpe

Pr091'ammlng
INTRODUCTION
The

DECtape system is a standard option for PDP- 8 series

computers that serves as an auxiliary magnetic tape data storage

facility. DECtape provides a distinct advantage over conventional
magnetic tape because it stores information at fixed positions
which may be directly addressed, much like conventional magnetic
disk or drum devices. Yet unlike magnetic disk or drum devices,
DECtape is bidirectional and adaptable to a Wide variety of user
data formats. In this manner, DECtape combines the versatility
of

magnetic disk and drum storage with the convenience and econ-

omy of magnetic tape.

DATA BLOCKS

‘

DECtape is organized into a series of addressable
blocks. Each block consists of control words, used to identify the
A reel of

block, and data words, used for data storage. The data words con—
tain twelve bits each; however, every control word contains eighteen bits. Blocks are usually numbered in sequence, from O to
N—l, where N is the (octal) number of blocks on the tape. The
maximum number of addressable blocks per tape is 212, or 4096.
Every block contains ten control words. A block may contain
any number of data words, with the restriction that the number of
data words contained in each block must be an even multiple
of‘

three. If the number of b10cks on'a tape is known, the number of
data words per block may be found by the following formula:

where NW 2 Decimal number of words
212080
NB Z

+ 2

per block.
.
.

N“'+15

Decimal number of

blocks

per tape.

Disregard any fractional remainder.
7—1

DATA CHANNELS

DECtape is divided longitudinally into five pairs of identical
channels which run the entire length of the tape. Duplicating each
channel helps to minimize‘ any loss of information. The timing
the tape. The
mark—track channels establish the format of information contained

channels

are

used to reference fixed locations

in the data channels. The three pairs of data channels are arranged
way that no two identical channels are adjacent. The
arrangement of channels on the tape is shown in Figure 7-1.

in such

3/4 INCH MAGNETIC TAPE

\
\

”43‘

\
\

Figure 7-1

4r
).

$404”

"$2

2%‘3-

DECtape Data Channels

STANDARD DECTAPES

Figure 7-2 indicates that a standard, 260 foot reel of DECtape
contains 2702;; (or 147410) standard blocks and two end zones.
The figure also shows an enlarged diagram of one standard block,
which contains

2013 (or 129m) twelve-bit data words and 10

eighteen-bit control words. Note that only the top half of the tape
is shown. The five redundant channels have been omitted for

clarity.

7-2

ONE COMPLETE REEL-260 FT. -I474

STANDARD aI.CCKs

ONE STANDARD BLOCK-129 DATA WORDS AND 10 CONTROL w0RDs
TIMINGTRACK

‘E‘L

IIT F’T'I I

I I

WIII-II'TIIIIIIIIIIII

I I I II II I II I II
I II. I.
IIIIIIIII-III-I-IIII

‘
INFORMATION
TRACKS
.

2
3

‘

'IB-BIT .5

DATA

.
.

cCNTROI. wDRDs

wonos

12912 DATA

Iv—DNE DATA

WORD

LOCATDNS

WORD-4 LINES 0F Dan‘s

EACH-.1

TIMING TRACK
MARK TRACK
1

DATA
CHANNEL

l—ILINE—cl
.

Figure 7—2

StandIRTd DECtapC Format

lB-BII'

DATA

’

CONTROL w0RDs

woRDs

A further

enlargement shows the. structure of one data word.

again, the redundant channels have been omitted. If one
12-bit binary number is written into this data word in the forward
direction, the twelve bits will be stored sequentially in the numOnce

‘

bered locations shown.

DECtapes having more or less than 1291" data
words per block may be prepared to theuser’s specifications. As
Nonstandard

with standard

blocks, the number of data words per block must

‘

be an even multiple of three.

DECTAPE CONTROL UNIT
A

DECtape system consists of up to eight single DECtape

Transport Units (TU55) or four dual DECtape Transport Units
(TU56), all operated by one TC08 DECtape Control Unit. Data
is transferred between the computer and the DECtape Control
Unit by means of the 3-cyc1e data break facility, described in
Chapter 6. Transfers occur at a rate of one 12-bit word every 133
microseconds

(”i 30%). The DECtape Control Unit uses core

memory location. 7754 in field zero as its word count register and
core memory location 7755 in field zero as its current address
’

‘

TC08 DECtape comm! Unit
Core Address

7754
7755

Word Count (WC)
Current Address (CA)

DECTAPE STATUS REGISTERS
The DECtape Control Unit contains two 12-bit registers, des—
ignated Status Register A and Staus Register B, which may be

by IOT instructions. These registers consist of
various switches and indicators, as shown in Figure 7-3 and
Figure 7—4.

loaded and read

7-4

‘

BITPOSITION_:,O
DEOTAoE

LLJ

4'5

1__JL

TRANSPORTJ

TO BE USED

DIRECTION
O =FORWARD
I =REVERSE
MOTION
O=STOP
I =60
MODE
O =NORMAL
1 =cONTINuous

’FUNCTION

DOO=De =MOVE
DOI=18 =SEARCH
DIO=28 =READ DATA

‘

011=38 =READ ALL
I DO=43 =WRITE DATA
101:53 =WRITE ALL
1 IO=68 =WRITE TIMING AND MARK TRACKS

111:78 =UNUSED
INTERRUPT CONTROL
0= DISABLE DECTAPE FROM CAUSING INTERRUPTS
1= ENABLE DECTAPE TO CAUSE INTERRUPTS-

ERROR F'LAe CONTROL
G= CLEAR ALL ERROR FLAGS
1 =LEAVE ERROR FLAGS UNDISTURBED
.

DEC TAPE FLAG CONTROL
O = CLEAR DECTAPE FLAG
=LEAVE DECTAPE FLAG UNDISTURBED

Figure 7—3

Status Register A,

'IO

D12345673
ERROR

FLAG—f

MARK TRACK ERROR

END-OF-TAPE

SELECT ERROR
PARITY ERROR
TIMING ERROR
MEMORY FIELD
UNUSED

DEC TAPE FLAG (DTF')

Figure 7—4

Status Register B

‘75

Status Register B
Bits 0-5 of Status Register B provide six indicators which flag
error conditions

zeros,

that may occur. These indicators normally contain

indicating the absence of an error condition. The possible

errors, their abbreviations and causes are listed in Table 7-1. Note

that parity and mark-track errors are hardware malfunctions, while

timing and select errors are generally caused by the software.
Bits 6-8 of Status Register B specify the memory field from or
to

which data should be transferred. Bits 9 and 10 are not used.

Bit 11 is called the DECtape» Flag (D-TF). In general, a value of
0 in this bit at any given time indicates that a DECtape operation
-

is in progress. A value of 1 indicates that the current operation has
been completed.
Status Register A
Status Register A designates which DECtape transport is to be

(bits 0-2), in which direction it should operate (bit 3) and
whether operation should be commenced or discontinued (bit 4).
Bit 5 of Status Register A specifies the transfer mode, which
may be either normal or continuous. In normal mode, data trans-

used

fer continues until the end of a block is reached or a predetermined

number of words has been transferred, whichever occurs first. In
continuous mode data transfer continues, across block boundaries
if necessary, until WC overflow occurs, indicating that the required

number of words has been transferred.
Bits 6-8 of Status Register A designate the function to be per—
formed. The seven possible functions are summarized in Table 7—2.
The first four functions, MOVE, SEARCH, READ DATA and
WRITE DATA, are used exclusively for most applications.
The remaining three bits of Status Register A indicate whether
or not the interrupt system is to be wusedjj(b,it 59,), whether error

flags in States Register B are to be cleared (bit 10) and whether
the DECtape Flag is to be cleared (bit 11).
'

7-6

‘

Table 7-1

TC08 Error Codes

Bit
Posi-

tion

Designation

A value of Tim this bit indicates:

Error Flag

Detection of one or more of the follow-

ing errors.
1

Mark-Track Error

Erroneous information
the mark track.

End-of—Tape Error

The end zone on either end of the tape

read from

was

is over the recording heads.
'

Select Error

This

error

flagged

microseconds

after loading Status Register A to indicate one or more of the. following

conditions:
a. The unit select code is not assigned
to any transport or is assigned to
more than one transport.
b. A write function was specified with
_

the WRITE ENABLE/WRITE
LOCK switch in the WRITE LOCK

position.
c.

An unused function code was speci-

fied (111 in bits 6—8 of Status Regis‘

ter A).

‘

d. A write timing and mark track func,.

tion was specified with the WRTM/

NORMAL switch in the NORMAL

position.
4

Parity Error

Checksum Wasincorrect during a read
data function, or else WC overflow did
not occur at the end of the first block

read in continuous mode. This flag is

only set simultaneously with the DTF.
'

Timing Error

A program fault caused one of the folI

lowing conditions:
a.

A data break could not occur within

66 microseconds i30% of the data
break request.
b. The DTF was not cleared before the
_

control unit attempted to set it.
c.

A read data

write data function
after
the current block
specified
had been entered, preventing comor

was

plete data transfer.
7—7

Table 7-2

DECtape Functions
Possible

Function

Errors

Description

MOVE

Initiates motion of the

(000)

tape drive in the specified direc—
tion. Errors are inhibited, except

Select

specified

End—of-Tape

for end of tape errors. Used only

rewindltape. DTF, WC and
CA are never changed.
to

(001)

As the tape is moving in either
direction, the sensing of a block

Select

data transfer of
the block number. The CA is not

'Timing

mark

causes

End-of—Tape

Mark—Track

incremented.

Normal mode:

Continuous

DTF is set after

mode: WC is

each block
number iS'

incremented
after each
block number
is transferred.
DTF is set

transferred.

when WC over-

flow occurs.

m
READ DATA

(004)

data

Normal mode:
DTF is set at
the end of each

Continuous

block or when
WC overflow

overflow

T‘—

into memory.
Only data words are transferred.

Transfers

(002)

WRITE
DATA

mode: DTF is

Select
End—of-Tape
Timing
Parity
Mark-Track

set when WC

occurs.

Transfers data onto tape. When
WC overflow occurs in the middie of a block, the block is filled
out with zeros. Modes of opera-

Select

End-of-Tape
Timing
Mark-Track
'

the same as for
READ DATA function.
tion

are

7-8

the

Table 7-2

[(Cont’gd)

Possible
Function

Errors

Description

READ ALL

Transfers data into memory. The

Select

( 003)

entire content of the information

End-of—Tape
Timing

tracks

transferred.

including

Mark—Track

control words.1

Normal mode:
DTF is set after
each data word
is transferred.

Continuous
mode: DTF is
set When WC

overflow
occurs,

==fi=i

(005)

WRITE

Select
_

End-of-Tape
Timing
Mark—Track

Used

Select

by DECtape formatting
routines to write timing and

establish
of
the
blocks.1
length
change

AND MARK
TRACKS

Transfers data onto tape regardless of the data- format. Only
mark-track and end of tape errors are checked. Used to write
block numbers on tape. Modes
of operation are the same as for
the READ ALL function}.

WRITE
ALL

TIMING

mark-tracks, and

Timing

or
’

(0.06)

DECTAPE IOT
,

INSTRUCTIONS

The six basic IOT instructions used to program the PDP— 8 for

DECtape operation are listedin Table 7- 3.. A11 instruction to load
Status Register A is, in effect, an instruction to begin the operation
specified by the bit configuration loaded. Once begun, the operation
continues to execute until the DTF flags completion, andthe tape
remains in motion until a 1 is loaded into bit 3 of Status Register A.

These functions

are

normally used only by DECtape formatting routines

and diagnostics.

7—9

The memory field bits

(positions 6-8) of Status Register B are
loaded by means of a DTLB instruction. The remaining bits in
Status Register B are loaded by specifying proper values for bits
10 and 11 of Status Register A. Note that Status Register A is
loaded by exclusive ORing the contents of accumulator bits 0—9
into the register. Accumulator bits 10 and 11. are sampled directly,
without the exclusive OR operation, and used to set the Status
Register B flags.
Table 7-3

DECtape IOT Instructions

Octal
'

Mnemonic

Code

Operation

DTRA (Read Status

6761

The contents of Status Register A bits
0-9

are

ORed with

the contents of

accumulator bits 0-9, and the resultris

placed in accumulator bits 0-9.
DTCA (Clear Status

6762

;

DTXA (Load Status

6764
V

Status

Bits 0—9 in the accumulator are EX—
CLUSIVE ORed with bits 0—9 of
Status Register A and the result is
loaded into bits 0-9 of Status

A. Bits l0 and H in the accumulator
the

flags are
cleared if bit l0 is a 0. The DECtape
flag is cleared it bit H is a 0. The
are

sampled,

and

error

accumulator is cleared.

DTSF
on

677i

(Skip

Flags)

If either the error flag or the DECtape
flag is set the program counter is in—
cremented to skip the next sequential

instruction.
DTRB (Read Status

6772

The content of Status Register B is
ORed with the content of the accumu-

later and

the result is

placed

in the

accumulator.
DTLB (Load Status

~Register B)

6774

Bits 6-8 of the accUmulator are loaded
into bit positions 6-8 of Status Register B. The accumulator is cleared.
Error

flags and DECtape flag are un-

disturbed.

7-l0

PROGRAMMED DECTAPE OPERATION

using the TC08 DECtape system for data storage, a
reel of DECtape must be prerecorded to insert the timing track,
mark-track and block numbers. This function is performed in two
passes by a DECtape formatting program utilizing the WRTM
control operation. SuCCe‘ssful execution requires that the manual
switch on the DECtape Control Unit be placed in the WRTM
position during the first-pass. After one prerecording, a reel of DEC—
tape may be used indefinitely.
Prior to

Tape I/O operations are usually performed by a DECtape service routine, which uses the six DECtape IOTs to clear, read and
load the status registers. Since the actual data transfer occurs, by
means of the 3—cycle- data break, service routines often begin by
using memory reference, instructions'to set the word count and
current address registers. By loading an appropriate bit configuration into Status Register A, the SEARCH function is then initiated
to locate the block number selected for transfer.

While searching, the

DECtape control reads only block num-

bers. In normal mode the DTF is raised at every block number
and the block number is transferred into the address specified by

the CA register, which is not incremented during SEARCH. The
contents of this location may be monitored until the desired block

number is

transferred, at which time a READ DATA operation

must'be instituted

promptly (within 400 microseconds i30%)
by reloading Status Register A.',
‘

During the READ DATA operation,- the DECtape service routine monitors Status Register B constantly by means of a DTSF
(skip on flags) loop “to check for error conditions and determine
when the DTFflags completion of data transfer. Meanwhile, data
transfer continues without program control until the data block is

exhausted or WC overflow occurs. Either of these eventswill raise
the DTF (in normal mode), at which time the. service routine may
.

again lOad Status Register A, forcing a 0 into bit 4 to terminate
the operation.
With the interrupt facility enabled, the DECtape service routine

described abOVe may be designed to execute as a foreground program, and mainline operations in the background program may be
performed while the DECtape system is accessing data.
7—11

operations summarized above may be broken down as

The

follows:

with

the negative
1. Use an MRI to .load the WC register
(two’s complement) of the number of words to be trans‘

ferred.
2.

Use an MRI to load the CA register with one less than the
core

address with which the first transfer is to be made.

Successive transfers

will be made with successive core lo‘

cations.
3.

Use a DTLB instruction to load Status Register B bits 6-8
with the desired memory field designation.

Begin to search for the desired block number by using a
DTXA instruction to ”load Status Register A with an ap—
propriate bit configuration. This should also clear all flags.

Remain in
as

skip-on—flag loop and test each block number

it is transferred until the desired block number has been

transferred.
6.

Reload Status Register A to clear all flags and begin READ
'

DATA function.
7.

Remain in skip-on-flag loop until the DTF flags completion.

Load

1 into bit position 4 of Status

the tape.

‘

procedure summarized above is neither complete nor efficient, although it does illustrate the sequence of operations gen—
erally employed. Step 5 is ineflicient because it may be necessary
to search the entire length of a tape before the desired block number is transferred. Steps 5 and 7 are incomplete because they do
The

conditions, and any error condition will cause
the program to skip out of the waiting loop before the current

not test

for

error

operation is finished. Finally, this procedure makes no provision
for reversing the direction of tape motion, which may become necessary if either end zone is reached or if the desired block is initially accessed in the wrong direction. Techniques for dealing with

all of these conditions will be developed in the following sections.
7—12

‘

USE OF THE DECTAPE FLAG
The

DECtape flag may be tested to

determine the

status

(in

operation.
progress, or finished)
the flag is set when WC overflow occurs. If WC overflow has not
of the current

In normal mode,

occurred and the flag is set:

During SEARCH function:

A block mark has

just been

read and transferred to the address contained in the CA

During READ DATA function: Afull block of data has

just been transferred to core memory Via data break. The
control unit is still in READ DATA mode and the tape is
moving. The CA register contains the address of the next
sequential core location following the last one read into.
The nextdata block will be read into core beginning at this
location.

3.! During WRITE DATA function: A block of data has just
been written on tape from core memory Via data break. The
control unit is still in WRITE DATA mode, and the tape
is moving. The CA register contains the address of the next

sequential core location following the last one transferred.
The contents of. core beginning at this location will be writ—
'

ten into the next block on the tape.

In continuous mode, the
.

DECtape flag is set only when WC

overflow Occurs, regardless of the functiOn being performed.2 This

usually indicates that the current operation is complete and that
execution should now be terminated under program COntrol. Note
that the tape. does not stop moving automatically when the DTF
flags completion of the current operation; however, any error con—
dition except a parity error will cause the tape to stop.
2 If

transfer has continued across one or more block boundaries, the Parity

Error Flag is also set as soon as WC overflow occurs.

7—13

SELECTING DIRECTION
Data is normally read in the direction in which it was written;
If data is read in the wrong direction, the data buffer will be filled
in reverse order, with what was originally the first data word being

placed in the last buffer position, and so on._ Furthermore, since
the DECtape system breaks every word into four 3-bit increments
which it stores sequentially, each data word will be returned with
its bit positions changed as shown below:

0| 1|2|3|4|5|6|7|8|9|10|11
RETURNEDBITPOSITIONS:9|10|n|6|7|813|4|5I0] 1|2

ORIGINALBITPOSITIONS:

Finally, reading a data word in the wrong direction causes the
word to be returned with every bit complemented. Thus, an octal
1234 written in the

direction would be read

reverse

3456 in

the forward direction.

REVERSING DIRECTION

The DECtape transport requires at least one full standard block
to

reverse

direction and

cycle back up to reading speed. Most

programmers allow two full blocks .to assure successful turn
around. If block number 105 is to be read when block 200 is

positioned under the recording heads, for example, it is necessary.
to search in

the reverse direction for block 102. The tape may be

turned around just inside block

102, and it will cycle up to read-

ing speed before reaching block 105. In general, to read block N
forward, search in reverse for block N-3‘.
The

standard

DECtape

format

provides sufficient

space be—

tween the end data blocks and the end of the

tape to perform a
turn around in the end zone and successfully access either block 0
in the forward direction or block 2701 in reverse. Some bootstrap

loaders take

advantage of this by performing a MOVE reverse

until the end zone is reached, and then a READ forward to load
a

routine stored in block 0.
7-14

ACCESSING DATA BLOCKS

The most e'flicient method 'for accessing data blocks is sum—_
marized. below:
'

SEARCH forward in normal mode until the first block

number is transferred into core at the location
the CA register.

specified by

flags completion of this transfer, test the
block number to determine how many blocks must be bypassed and in which direction. Allow two extra blocks if a
turn around is. required.
Load the WC register'with the number of blocks which
must be bypassed, then SEARCH in continuous mode until
the DTF flags WC overflow. Turnaround, if necessary.
When the DTF

The next block encountered will be the desired block.

ALLOCATING

STORAGE AREAS
Any process. whichattempts to access single DECtape blocks
sequentially will be ineflicient, because it? may be necessary to
turn around twice to access each block. This problem develops
whenever large amounts of code or slow I/O operations must be

accessing adjacent blocks of DECtape. Under
these circumstances, the block being accessed may overshoot the
recording heads before the intermediate operation is Complete.
Stopping the tape to allow full execution of the intermediate process; is‘not a solution, because the tape requires two blocks to
cycle up to reading speed, so that it will still overshoot the block
being accessed.
An efficient way of solving this problem is to break up data
files and programs into large segments spanning many blocks,
executed between

and then alternate a recorded segment with a blank segment on
the tape. While a blank segment is passing under the recording

heads, it is possible to perform an intermediate process and com-

plete it in time to access the first block of the next recorded segBlank segments: may be recorded in a similar manner in
the reverse direction. The DECtape Copy program uses this type
ment.

'14 block segments

separated by 14 block
intervalsflt is very eflicient at reading and copying DECtapes.

of operation to

access

7—15

PROGRAMMING FOR ERROR CONDITIONS

generally desirable for every DECtape service routine to
provide an extensive method for handling error conditions. This
might include:
It is

Ni-i'
3.

Requesting operator intervention after a select error.
Counting changes in tape direction to guard against tape
rocking loops, which can occur when the desired block is
nonexistent or when the tape mechanism is malfunctioning.
Attempting to wade block a second time following a parity
error.

Informing the user when a nonrecoverable error condition
exists.

PROGRAMMING FOR INT ERRUPTS
The

‘

DECtape control unit contains a 1—bit register called the
DECtape Control Flag (DTCF, not to be confused with the DECtape fiag) which always contains the inclusive OR of the Error
Flag and the DECtape flag. It is this register which is actually
sampled by the DTSF skip-on-flag instruction.
If the interrupt facility is enabled (ION instruction) a 1 in bit
position 9 of Status Register A causes an interrupt to occur when—
ever an error condition or an operation—complete condition (or
both) is flagged by the DTCF. This makes it unnecessary for the
DECtape service routine to monitor Status Register B during
every operation bymeans of skip—on-flag waiting loops. The time
spent in waiting loops may be used to execute mainline instructions, provided that these instructions do not attempt to use data
before the DECtape service routine has had time to transfer it.
Because the DECtape system is comparatively fast, an interrupt
service routine will. normally test the DTCF~near the top of the
skip chain. This creates a problem when the DECtape system is
not in use. An interrupt from a lower priority device will eventually occur, in which case any bit configuration in Status Register A
will be interpreted as a SCICCt error when the DTCF is tested, and
the program may loop indefinitely. Such problems may be pre—
test with a NOP when—
vented by replacing the DTSF skip—onever DECtape15 not in use.

flag

7—16

IDTAPE

SUBROUTINE3

IDTAPE subroutine illustrates one method of programming for DECtape operation. IDTAPE operates with the interrupt
facility disabled. It performs data transfers in continuous mode, so
that multiple blocks of data may be transferred in a single operation. Searching may commence in either direction, to minimize
The

access

time. One full block is allowed for turn around when search-

ing in the reverse direction. IDTAPE reads and writes in the forward direction only
.

IDTAPE must be stored in field zero, but it will read or write
data into any specified memory field. It will not automatically
field boundaries. Errors flagged while searching cause
cross
I

IDTAPE

remain in the SEARCH

loop indefinitely. If the
specified block number is nonexistent, it hangs up in a tape rocking loop. Errors flagged during data transfer cause the branch
IDSERR to be taken with tape motion halted and Status Register
3 error flags set.
to

The IDTAPE

sequence is:

calling

JMS to IDTAPE, ie., indirect
0000 JMS (IDTAPE) Effective
JMS if IDTAPE is not
page
on

same

calling sequence
Bits 0—2, unit number
as

0001

WORD l,

Bit

3, start search (0: forward l=reverse).Bits 6—8, memory field for transfer.
Bit 10, error return (0: JMP WORD 5'
l'zJMP I WORD 5).
Bit 11, function .(OzREAD l:WRITE).

0002
0003

Block number for start of' transfer.

WORD 2,
WORD 3,

Two’s

complement

the

number of

words to transfer.

0004

0005

WORD 4,
WORD 5,

Memory address minus 1 of first transfer.
‘

Error return

address for error return

(to correspond to Bit 10 of WORD l).
0006

RETURN,

Transfer completed;
cleared.

return

with

On exit, the DECtape drive will halt.

This subroutine is not currently available from the DEC Software Distribution Center, and is included here for the user’s convenience.

7—17

*ZOO
/AN ORIGIN OF OZOO IS USED FOR TESTING
/HOWEVER THIS SUBROUTINE MAY OCCUPY THE FIRST
/113 (OCTAL) LOCATIONS OF ANY PAGE IN FIELD
/LOCATIONS 7754 AND 7755 ARE ALSO USED
/AND MASK MUST BE
ID7400,

ZERO

74GB

/FIRST

CLA
TAD
DCA
ISZ

TAD

IDD2OO,

AND

I IUIAP’E
IDCODE
IDTAPE
IDCODE
ID74OD

TAD IDOOIO
DTCA DTXA
DTLB
TAD IDWC
DCA I IDCA

IDSERR, RTL
RAL
CLA CML
TAD IDOZDO
.

SNL
TAD

IDCONT,

IDO4OO

DTXA

SPA
JMP IDSERR
DTRA

RTL
RTL
SZL CLA
TAD IDOOOZ
TAD I IDNC
CMA
TAD I IDTAPE
CMA
SZA CLA
JMP IDCONT
SZL
JMP IDCONT+I
ISZ IDTAPE
TAD

IDTAPE

DCA

IDWC

ISZ
TAD

IDTAPE
I IDTAPE
I IDCA

DCA

PAGE

/CLEAR ACCUMULATOR
/GET WORD 1
/STORE IT
/ADVANCE POINTER TO WORD 2
/GET NORD !
/MASK OFF BITS 4-11
/PUT INTO SEARCH MODE
/CLEAR AND LOAD STATUS A
/LOAD STATUS B FOR FIELD O
/GET-ADDR OF NO REGISTER
/INITIALIzE CA FOR SEARCH
/THEN FALL THRU ERROR
/ROUTINE TO START TAPE
/NORMALLY ENTERED NITH
/STATUS B IN ACCUMULATOR
/MOVE END ZONE FLAG
/INTO LINK
/COMPLEMENT LINK
/MASK STOP/GO BIT ON AND
/ENTER DECTAPE SEARCH LOOP
/TAPE IN END ZONE?
/YES: REVERSE DIRECTION
/NO: BEGIN SEARCH
/MONITOR FLAGS UNTIL
/A FLAG IS RAISED
/ERROR FLAG RAISED?
/YES: TAKE ERROR BRANCH
/NO: MUST BE DTF, SO GET
/DIRECTION BIT AND
IROTATE IT INTO THE LINK
/MOVING FORWARD?
/NO: GET "BLOCK TO FIND" '2
/YES: GET LAST BLOCK SEEN
/FORM ONE'S COMPLEMENT
/ADD IN BLOCK TO FIND
/TWO'S COMP MIGHT SET LINK
lBLOCK NUMBERS MATCH?
1N0: REENTER SEARCH LOOP
/YES: MOVING FORWARD?
/NO: TURN AROUND
/ADVANCE POINTER TO wORD 5
/GET wORD s
/LOAD wC REGISTER
/ADVANCE POINTER
/GET wORD 4
/LOAD CA REGISTER
.

DTSF DTRB
JMP .-l
’

NORD

/ENTER’SUBROUTINE

IDTAPE,

’

7—18

TAD

IDCODE

/GET WORD l
/LOAD STATUS B FIELD BITS
./SET BIT 11 FOR
/READ 0R WRITE THEN
-

DTLB
IAC‘
AND

IDCODE

RTL CLL

".RTL

/ROTATE INTO POSITION AND

/BUILD XOR MASK TO
/LOAD STATUS REGISTER A
/BEGIN READ/WRITE
~/MONITOR STATUS B UNTIL
/A FLAG IS RAISED
/ADVANCE T0 WORD 5
‘~/IS ERROR FLAG SET?.
/NO: ADVANCE TO WORD 6
/YES: KEEP ERROR RETURN
/GET INDIRECT RETURN BIT
/ROTATE INTO LINK
/INDIRECT RETURN?
/NO: MAKE NORMAL-RETURN
/YES: CHANGE INDIRECT RETURN
/TO DIRECT RETURN
/GET STATUS REGISTER A
/MASK STOP/GO BIT OFF‘
/PRESERVE ERROR FLAGS
./LOAD STATUS A TO
STOP TAPE
/RETURN
/POINTER TO WC REGISTER
/POINTER TO CA REGISTER
v/SETS BIT 8 FOR SEARCH
/SETS BIT 4 TO-REVERSE TAPE
/BUILDS XOR MASK
'

TAD IDOISO
DTXA
DTSF DTRB
.JMP 1-].
ISZ IDTAPE;
SMA
ISZ IDTAPE
SPA CLA
TAD IDCODE
RTR
SNL CLA
JMP .‘+3
TAD '1 IDTAPE
DCA IDTAPE
DTRA
AND IDOZOO
TAD IDOODZ
DTXA
‘IJMP I IDTAPE
7754
7755

Ich,
IDCA,
IDDZIO,1O

109400, 4am
ID013B,150
IDOO02,_2

iDCODE,

A/HANDY CONSTANT
/STORAGE FOR

WORD

DECTAPE SYSTEM SOFTWARE
DECtape Software, described in the following-sections, provides
the programmer with fOur major operational materials:
_

Input/output subroutines which may be included in larger

programs.
2. AROutines for copying and formatting DECtapes. These pro‘-

grams will duplicate non-standard tapes, write timing and
mark-track channels, insert block format information and

perform similar chores for the programmer.
_3.1 A
4.

library system for storing and

retrieving programs

DECtape.
08/8, which provides the most eflicient means of using both
DECtape and disk storage.

7—119

DECTAPAE SUBROUTINES
DECtape subroutines allow the programmer to read, write" and
using prewritten and tested subroutines which
execute with the interrtipt facility enabled, so that mainline instructions may be performed during tape operations. The DEC
library supplies these subroutines on one ASCII symbolic tape
which has no origin, and ends with the pseudo-op PAUSE. It must
be assembled with a user program, and the resulting binary tape
loaded with the Binary Loader. The following restrictions should
search, DECtapes

be observed:

The routines

are

designed to be used with standard tapes

(129 words per block). The last data word in each block is
not accessed, so that each block is effectively one core page

long.
The routines will read jor'write in the forward direction only.

Data may be transferred from or into any memory field,
but these routines will not transfer data across field bound—
'

aries.

storage locations.
They may be located on any page of core in field 0, except

The

routines

page

0, and they must all reside on the same page, filling

require 2003

consecutive

that'page completely.
An ION instruction will be executed by the routines upon
entry; however, the user is responsible for storing the contents of the accumulator and link when an interrupt occurs.

define a register called MCOM on page 0.
This register is initialized by the tape.routines and is used

The

user must

for communication with the interrupt system. If the DTF is
set on the occurrence of an interrupt, the accumulator must

be cleared and the instruction JMP I MCOM must be

executed to return control to, the tape routines.
7—20

DWAIT Subroutine

The DWAIT subroutine tests the motion bit

(bit 4) of Status
Register A. If the motion bit15 a 1, the routine cycles through this
test process until the motion bit is cleared, indicating that the tape

stopped and the previous operation is now complete. It then

has
‘

location following the JMS which called it.
DWAIT13 called automatically, whenever necessary, by
of the
all
DECtape subroutines.-

returns control to the

following

‘

DWAIT

Calling Sequence

JMS-I DWAITI

GOSUB,

Explanation

Where DWAITI contains the address of

subroutine DWAIT.

RETURN,

‘

After tape motion has stopped, DWAIT
returns control to the location

following

the JMS which called it”

SEARCH Subroutine
The SEARCH subroutine should only be used to position the
tape. It is called

automatically by the READ and WRITE sub—

routines. In the event that the tape is moving, the SEARCH subroutine will transfer control to the DWAIT subroutine, and con-

tape stops moving. If the user wishes
to continue mainline execution "immediately upOn completion of
the search, the completion return address in the folloWing calling
tinue to execute when the

sequence should be the address of the interrupt exit routine.

Calling Sequence
GOSUB,~

CLA CLL
TAD'BLOCK
'

Explanation

Where BLOCK contains the number
being sought. Any other
method of getting the block number
into the AC is'acceptable.
of the block

7-2 1

SEARCH
.

Calling Sequence

Explanation

JMS I SERCHI

Where SERCHI contains the address
of routine SEARCH. SERCHI may be
on, page 0 or the same page as the call0

ing sequence.
CMPADD

Where CMPADD is the completion
return
address. When the correct
block has been found, control transfers to the address contained on this
line with the tape stopped and the interrupt system ofi.

U000

Bits 0-2 contain the

DECtape trans-

port unit number. The rest of the word
contains

zeros.

For

example, using

unit 3 this would be 3000.
IRETRN

Where

IRETRN

intermediate

absolute return. When searching starts,
control is transferred to this line with
the interrupt on to allow multiprocessing while the tape is in motion. If multiprocessing is not _desired this line

should read “3MP .—-l” which causes
the program to idle in a closed loop
consisting of one instruction until the
requested block has been found and
an interrupt occurs.

READ and WRITE

Subroutines

The READ and WRITE subroutines will transfer any number

of memory pages to or from standard'blocks of DECtape, how—
ever only whole pages will be transferred onto
whole data blocks,

calling sequence specifies the starting
location, the’memoryffield, the starting block number and the tape
unit number. As with the SEARCH routine, the program waits
until any previous tape operation has been completed. These rou—
tines use the SEARCH subroutine to find a specified block num—
‘ber, however theydo nOt stop the tape until after execution is
complete.
and vice

versa.

The user’s

7—22

READ/WRITE

”

"
’

Calling Sequence.

JMS 1 READl

Explanation

gWhere READI contains the address of R128 and
WRITEI contains the address of W128

‘

JMS 1 WRITEl
C ORADD
’

1
_

Where CORADD is the address of the first
location on the page to be used for transfer.

core

UOFO

Where bits 0-2 contain the DECtape transport
unit number and bits 6-8 contain the memory field.
The remainder of the word contains zeros. (E.g..
for unit 3 and memory field 1 this would be 3010.)

—NUl\/l

Where —NUM is the negative (2’s complement)
of the number of successive blocks to be read or

;

written.
BLCK

Where BLCK is the number of the first block on
tape to be read or written

l-RETRN

an intermediate absolute re—
turn. When the routines start searching for the
block specified, control is transferred to this line
Where IRETRN is

multiprocessing while the tape is in moIf multiprocessing is not desired, this line

to allow

tion.

may contain

JMS to the DWAIT.routine.

DECtape Copy Program
The DECtape Copy program (DTCS) p1ovides a simple, ef—
ficient method for copying from one DECtape to another on
PDP—8 series coreruters. Features include the capability of handling nonstandard block lengths (up to 15501., 12- bit words per
data.
block) and the ability to reread and verify the
routine is internal and monitor independent.

copied

The

DTCS resides in memory between locations 0000 and 1547,
with the remainder of core divided into two buffers. The starting
address is 0200, and the routine may be restarted at this address

normally distributed as a binary tape and
loaded with the Binary Loader. Alternately, it may be stored on
DECtape or disk and loaded with the monitor system. Two DEC—
tapes must be mounted with WRITE LOCK enabled on the input
at any time. DTC8 is

tape.
7-23

Teletype input is required at the conclusion of each printed
message

generated by DTC8. The operator may type:

‘

,CTRL/C, which causes a branch to location 7600.
2. A string of octal digits (0 to 7), which will be interpreted
as an octal number. Only the rightmost four digits are preserved, so that 1234567 is interpreted as 4567.
3. RETURN, which terminates the input string If no digits
were typed, the input value15 zero.
4.

Any other

characters which

are

ignored

and

not

echoed.

The operator is expected to respond to the following messages
printed by the program:

DECTAPE COPY FROM UNIT

Type the input unit number (0-7). Only the last digit
typed is accepted. Terminate with acarriage return.
TO UNIT
’

the output unit number, which must be different
from the input unit number. Terminate with a carriage

Type
'

return.

FINAL BLOCK TO COPY

(OCTAL)

Type the octal block number of the last block to be copied.
If no digits are typed, the program will copy to the end
of the tape. If a number is typed, it must be greater than
or equal to the number of the first block to be‘copied.
Terminate with a carriage return.
PDP-8 WORDS PER BLOCK

nnn

The program types the octal number of 12-bit words in
each block. No‘ operator response is required.
VERIFY OUTPUT?

(OZYES, 1,:NO):

If a l is typed, the program does not reread and verify the

output copy. Terminate with a carriage return.
DONE

Program indicated completion. Type RETURN key
restart

the

program

CRTL/C

7600.
7—24.

branch to location

Any illegal Teletype input causes the program to restart
itself. CTRL/C may be typed any time after the start of the
program to cause a branch to location 7600.
A
'

parity error while reading from the input unit causes

the message:

PARITY ERROR

‘ON BLOCK

nnn

Copying continues, but when execution is finished the operator
“may attempt to‘copy the designated block separately.
A validation error is flagged when the information written
is not the same as the information read in. Three consecutive

validation errors cause the

This message

message:

WRITE ERRORS ON UNIT #n
can occur

only if validation was requested. Typing

the RETURN key causes the program to

attempt to reread and

validate the erroneous data.

A select error on either tape driVe causes the message:

SELECT ERROR ON. UNIT #n
After a select error, the operator should correct the error condition and then type the RETURN
to continue program

‘

key

execution.

Mounting an output tape with more than 15501.)

12—bit

words per block causes the message:
BLOCK LENGTH ERROR

After a block length error, the program automatically restarts.

DECtape Formatting Program
package is available to perform formatting and
maintenance operations on DECtapes. It provides for recording
0ftiming and mark-track channels and permits block formats to
be recorded for any block length. Patterns may be written in these
blocks, and then read out and verified. Specified areas of tape "may
also ,be “rocked” for specified periods of time. In this manner, a
reel of tape may be thoroughly checked before it is used for data
storage. For detailed information, refer to the TCOl—TUSS DECtape Formatter (DEC-08-EUFB) available from the DEC Software
.A

software

Distribution Center.
7-25

DECTAPE LIBRARY SYSTEM
The DECtape Library System permits the user to build a com-

plete file of his active programs and continuously update it. It is
capable of calling programs by name from the keyboard and
allowing for expansion of programs stored on tape. It conforms
to existing system conventions, in that all of core except for the
last memory'page is available to the programmer, and it permits
the Binary Loader to reside in core at all times. The Library
System fully restores the initial state of the computer when it
exits.

Library System may be illustrated as follows.
A program that will be run repeatedly is written in PDP—8/E
FORTRAN. At the keyboard, the operator may call the FORTRAN compiler from the library tape and compile his program,
obtaining an object program on paper tape. He may then call
the FORTRAN operating system from the library to load and
run his object program. Finally, the library program UPDATE may
be called. The operator defines a new program file, consisting of
his object program and the FORTRAN operating system, then
adds it to the library tape. The program is now available for easy
access on the library tape.
The minimum library system, called a skeleton library, occupies
the first 408 blocks of a standard DECtape. It contains the following routines:
One

use

for the

INDEX

‘—— Causes

the names of all routines on the library

tape to be printed on the Teletype.

UPDATE

-——-

Allows the user to add routines to the library

tape.
'

GETSYS

’DELETE

—

-—-

Used to "generate a new library tape.‘

Causes

specified routine to be deleted from
‘

the library tape.

ESCAPE

—

Causes the

library system to exit.
7—26

A prerecorded skeleton library tape may be obtained from the

DEC Software Distribution Center. This tape should be duplicated
with DTC8 (or else GETSYS should be used to generate a second

skeleton library tape) and the original should then be stored in a

safe place.
The Directory
The directoryIS part of the library system. It contains the names

of files on the library tape and all information thatIS required by

the library system to load, delete, or add named files. The direc-

tory contains 3481., usable locations, With each entry requiring a
minimum of

seven

locations.

entry is as follows:
_

The structure of

single directory

File name in trimmed ASCII with characters

First 3 Words:

packed

2 per word. For

example,

INDEX

would appear as:
1 1 16

0405
3000
_

/IN
/DE
/X

Starting DECtape block number for the file. 5
Starting core address of the file.
First in a list of core specifications for the file.
The format of these specificationsIS shown1n
Figure 7-5.

Fourth Word:

Fifth Word:
Sixth Word:

Last Word}

0000 to terminate list of core

there is

specifications. If

only one specification, the directory

entry is seven words long. This is the mini—
mum length for an entry.

‘l

23456'789'1011

W
FLAG BlTS ——J
PAGE. A

‘5-

PAGE 3

Figure 7-5

Format of Core Specifications

7—27

Bit 0:

If set to 0, the two 5-bit page-number specifica-.

tions

(page A and page B) designate discrete
single pages which belong to the file.
If set to

1, page A signifies the first page in a

series of contiguous pages and page B signifies
the last page in the series.
For example: 0247 or 00 00101 00111

means

that page 5 and page 7 belong to this file. ObserVe

that the user may not save the last page

of core (7600-7777) with one of his files.
Bit 1:

This is always set to the same

value as bit 0.
Five bit page number specification (Page A

Bits 2-6:

above).
Bits 7—11:

Five bit page number

specification (Page B

’

above).
There are spaces for almost 50]“ names in the directory. UPDATE will determine whether or not the directory is full and if
so, it will

print a message to this effect. There are 26408 usable
blocks on the library tape, which is more than adequate in view
of this limitation on the directory size.
'

Using the Library System
All skeleton library routines are called from the Teletype by
typing the name of the routine and then a carriage return. Some Of
the

routines

generate messages which request operator input.

Typing a RUBOUT causes any input on the current line to be
ignored and the line to be retyped.
Typing INDEX and RETURN causes a list of all routines stored
on the library tape to be generated on the Teletype.
The UPDATE routine

assumes

that

file to be added to the

library tape was in core before the library system Was loaded.
Typing UPDATE and RETURN generates the message:
NAME OF PROGRAM:

The operator must type. a name consisting of one to six charac—
ters,_ and then a carriage return. All characters, are legal except:

7—28

(@,T,TAB,FORNIFEED,amlLDflEFEED.UPDATENMH
now

type:

SArocrAny
The operator must type the Octal starting address of the file
being loaded. If the file does not have a proper starting address (as.

example, the floating point package), the starting address of
the system loade1 (7600) or the HLT in the BIN
(7700)

for

may be

Loader

specified UPDATE will now type:
PAGELOCATIONS:

The operator must specify the page locations in core memory at
which the file is

presently stored. This infOrmation may be pro-

vided to UPDATE in either of two forms: <XXXX> which means

page On which the octal address XXXX falls, or
<XXXX, XXXX> which means the page on which the first ad-

the

single

through and including the page on. which the second

dress falls

UPDATE accepts information of this type until a
semicolon (;) is received-Spaces, tabs, carriage returns, and line _'
feeds are ignored between location elements. For example, if a
program occupies locations 1—2354, 4600-7577, and 2400—2577,
UPDATE might'be told: <0,22’00> <2400> <4600,~7577>. The
numbers must be in ascending order, and any numbers lying within
the same page are considered equivalent. UPDATE will now add
the new file to the library, add an entry to the directo1y, and trans—
fer control to the file- loading
leaving the directory in
address falls.

core.

program

Typing DELETE and RETURN generates the following mes—
sage:

NAME OF FILE TO BE DELETED
The operator must type a name

(up to six characters) followed
by a carriage return. DELETE searches the directory for the designated name and generates an error message‘if it is not found or
if it is the nameof a system program. -If the name is found,
DELETE removes it from the directory, removes the named program from the tape, and repacks the tape. DELETE then transfers
control to the file—loading program, leaving the directory in core.
7—29

Typing GETSYS and RETURN generates the following'message:
SKELETON TAPE WILL BE CREATED ON UNIT #
The operator must type a single digit (1-7) and then a carriage
return. GETSYS expects a preformatted, standard DECtape to be

specified tape unit, with the WRITE ENABLE
/ WRITE LOCK switch in the WRITE ENABLE position. When
the new library system has been created, GETSYS transfers control
to the file—loading program, leaving the directory in core.
Typing ESCAPE and RETURN causes the library system to

mounted

the

restore all of core and exit. The condition ,of the computer should

be identical to. its condition before the library system was loaded.

example shown below, the operator used the Binary
Loader, to load the FORTRAN compiler; He then loaded the
library system, and the following interaction took place:
In the

‘

Typed by user.

INDEX
ESCAPE
UPDATE

Presently

gg‘fgg

stored

pro-

grams listed by system.

PALIII

UPDATE

£23m"

Zfifi’éfiflmg
1" DEX

User

supplies data requested by system.

:<0,74%>;

PAGE LOCATIONS

Typed by user.

Second INDEX.

‘

’

ESCAPE

gggg

Stored programs listed

by system-

csrsrs

‘PALIII

FRTRAN
*

DELETE

NAME OF FILE To BE DELETED: PALL
NAME OF FILE To BE DELETED:PALIII

Typed by'user.
User

typed

and

used

mistake
RUBOUT

1‘65"

INDEX

533252

Third INDEX.

DELETE
GETSYS
FRTRAN

ESCAPE

7—30

Theamountof time required to load a file from tape- into core
memory depends upon the file location on the tape. If}the file is
near the beginning, loading time will be about 8 seconds. UPDATE
executes, in 30 to 45 seconds. DELETE time varies too much to
'

make an estimate possible; it may take as long as several minutes.

GETSYS requires approximately 30 seconds.
The

system has one DECtape error halt at location 7670. No

recovery is possible at this point, and. any attempt to restart may
result in'destruction of the library tape data.

TC01 Bootstrap Loader

The library system bootstrap loader resides in the last page of
core

with the RIM and BIN loaders; however,— with the bootstrap

in core, only the RIM loader is operable. Since this bootstrap con-

sists of only a few instructions, it may be loaded from the switch

InstruCtion

7600

6224

7601

'6774
1221

7602
‘

4213

7603
7604

.1222

7605

1223

7606

7607

3355
4213

0000

7610
7611

*
,

7612

0000
0000

7613

0000

.7614

6766

7615

3354

7616

6771

7617

5216

7620

5613

7621

0600

7622

7577
_

7623

3
7-31

0220

explained in Appendix E. Check carefully to ensure that the bootstrap is loaded correctly. If it is not, the system tape may be destroyed by an attempt to load the system. For checking procedures,
See Figure 4—3..
To load the library system using the bootstrap loader, ensure
that the library system tape is mounted on a DECtape transport.
The tape may be wound to any point along its length, but at least
four turns of tape should be on either reel.
1. Set the transport unit selector dial to O (or 8).
2. Set the LOCAL/ OFF/ REMOTE rocker switch to

REMOTE.

3. Set the WRITE LOCK/WRITE ENABLE rocker switch to

WRITE ENABLE.
4. Set the starting address of the bootstrap (7600) into the con—

sole switch

load

into core.

When the system is loaded, the Teletype keyboard will be enabled,

awaiting operator commands.
TDs-E DECTAPE SUBROUTINE
A TD8—E DECtape system consists of up to four TU56 dual

DECtape transport units with one TD8—E DECtape control unit for
each dual transport unit. This system

is similar to

the TC08

DECtape system in that it uses the same hardware transport unit
the TC08. However, the TD8—E system
relies on programmed transfer for I/O operations, rather than the
and tape data format
3- cycle data

such

break, and requires that status monitoring operations

as error

sensing and checksum generation be performed by

software under program control.
A TD8— E DECtape system is best employed by
or

utilizing OS/ 8

the TD8— E DECtape Subroutine, which is a general data han-

dling routine with a standardized calling sequence that facilitates
tape l/O operations and provides full compatibility with OS/8
device handlers. This subroutine requires two adjacent pages of
memory, in any memory field, for each TU56 dual transport
to be serviced; Thus, operation of a maximum configuration system

core

requires that four copies of the subroutine ( with four different sets
of assembly parameters) reside in core.
7-32

Assembly Parameters

Values for five parameters must be

supplied

when the TD8- E

These parameters and the permissible,
values which may be supplied are listed below.
subroutine is assembled.

DRIVE
_

_~_
—

10
-20
3O

DECtape units 0 and-1
DECtape units 2 and 3
DECtape units 4 and 5
DECtape. units 6 and 7

.40

ORIGIN = nnnn

Specify an absolute origin, which will also
be the entry point for the lowest-numbered,
DECtape transport unit. The entry point for
the other unit will be ORIGIN + 4.

AFIELD = n'

Designate a field (0< AFIELD< 7) into
which the subroutine will be loaded.
'
'

MFIELD =n_0

Specify

10 times

AFIELD.

WDSBLK 2

the value designated for

Indicate the _octal number of data
block.

words per

Assume, for example, that an assembly of the subroutine to

handle DECtape units 0 and 1 is to be loaded into memory field
0, pages 24 and 25, and standard DECtape
format is to be used.

Input to the assembler would consist of:
DRIVE

ORIGIN

=—

5000

AFIELD

MFIELD

WDSBLK

201

This would be followed

by a source copy of the subroutine. If’
DECtape units 2 and 3 are to be used simultaneously, a second
assembly of the subroutine may be loaded into, say, memory field
1, pages 35 and 36. Assuming that standard format is also em—
ployed onthese tape drives, the assembly parameterswill be:

DRIVE

ORIGIN

7200

AFIELD

MFIELD

10.

WDSBLK

201

7-33

.In this manner, as many assemblies of the TD8-E Subroutine as
necessary may be loaded into any available core locations,
provided that each assembly occupies two contiguous pages. The
are

subroutine may be relocated temporarily during program execution,
if necessary, but it may not be called at any location except the one
for which it was assembled.

Calling Sequence
A call to the TD8—E Subroutine should have the following gen,

eral format:

GOSUB, CDF DATA

/DESIGNATE DATA FIELD:
/CURRENT FIELD
’

CIF MFIELD

/FIELD IN WHICH SUBROUTINE
/WAS LOADED

JMS ENTRY

/EITHER “ORIGIN” OR
/ “ORIGIN + 4”

ARGl

ARGZ

'ARG3
JMP

ERROR

/ERROR RETURN

/NORMAL RETURN

JMP CONT

The first argument which must be
format:

BIT POSITION:

O
t

1=WRITE

5
1x

BINARYNUMBER OF
BLOCKS T0 TRANSFER

supplied has

MEMORY FIELD OF DATA BUFFER

UNUSED
D = TRANSFER
VLORDS PER DECTAPE BLOCK
1 =TRANSFER WDSBLK -| WORDS PER IICTAPE BLOCK
0=SEARCH REVERSE INITIALLY
1 =SEARCH FORWARD INITIALLY

:WDSBLK"

7—34

8
J

the

'following
10

The second argument (ARGZ) contains the core memory address
of the data buffer. Data will be transferred to or from

sequential

locations, beginning at this address.
The third argument (ARG3) specifies the DECtape block number

core

at- which transfer should commence. Data will be transferred to or
-

from

sequential blocks until the number of blocks designated by

bits 1-5 of the first argument (ARGl) has been transferred. -_
If no errors

are

encountered, the subroutine takes the normal

return to the fifth location following the JMS which called it with

all tape motion stopped, the accumulator cleared and the instructiOn and .data fields equal to the value of DATA specified in the

rsubroutine calling sequence.» Any error condition sensed during a
data transfer operation causes the subroutine to take the error
return to the fourth location following the J MS which called it. An
error

return with the acCumulator cleared indicates

select error.

Parity, timing and checksum errors return a value of 4000 in the
”accumulator, as does any attempt to access a non-existent block
number.

7—35

‘

Floating-point
package's
'

INTRODUCTION

‘

Floating-point packages provide an easy means 0f performing
basic arithmetic operations, such as addition, subtraction, multi—
plication and division, using floating-point numbers, They also provide extended function capabilities for the‘
cOmputation of natural
logarithms, exponential functions, basic trigonometric functions
and the like. The floating—point package maintains a high degree
Of precision, and greatly facilitates I/O operations in floating—-point
notation. This is particularly useful for computations involving
numerous arithmetic operations on variables whose magnitudes
may vary widely. The floating—point package, or FPP, stores very
large orve‘ry small numbers by saving only the significant digits
and assigning an exponent to account for leading‘and trailing zeros.

There are’three floating-point packages

designed for use with
the PDP-S/E. The 23—bit Extended Arithmetic‘Element FloatingPoint Package (EAE FPP) may be employed on. any PDP—8/E
equipped with a -KE8-E Extended Arithmetic Element and an
LT33 Teletype. The'EAE FPP is described in detail in this chapter.
The 23-bit

Floating—Point Package (non-EAE FPP)

may be

any PDP—8 series computer with an LT33 Teletype.
This FPP is functionally equivalent to the EAE FPP in
re-

used

many

spects; in particular, the two are fully program compatible. Impor—
tant differences between the EAE FPP and the non—EAE FPP are

noted throughout this chapter.
8-1

The 27-bit

Floating-Point Package (27-bit FPP) is also designed for use on any PEP—8 series computer equipped with an
.LT33 Teletype. The 27-bit FPP provides extended precision for
computations that require accuracy in excess of 6 or 7 significant
digits. The 27—bit FPP is program compatible with both the EAE
FPP and the non—EAE FPP, and similar to these packages in
most respects. Differences are noted throughout the chapter.
Each floating-point package is supplied as a binary paper tape
and is loaded into

memory via the Binary (BIN) Loader (see
vAppendix E). Since a floating—point package is actually a collec—
tion of subroutines, the user must also load a program of his own-

which calls the

floating-point package (or one of the individual
subroutines) and tells it what operations to perform. The binary
tape of the user—program is normally loaded after that of the

package; this is particularly important if the user is not calling
part of the package (for example, the extended functions) and his
hunt-lave flu-2+
nrnnram
flue”
“J“
VVAA

u u”

narf
full.-

ASSEMBLY INSTRUCTIONS

available on three source
paper tapes which the user may assemble with PAL III (or the
equivalent) if he intends to alter the package. The source tapes
should be assembled in ascending numerical order: tape 1 first,
followed by tape 2 and tape 3. Tapes 1 and 2 end with the
pseudo—op PAUSE, whiletape' 3 ends with a dollar sign;
Each Floating-Point Package is also

‘

If the FPP is assembled with PALS, the user must define several
,

floating-point and PDP- 8/ E instructions which are used by the in—
terpreter but not containedin the PALS symbol table. The following paper tape should be prepared and used as the first tape of the
'

assembly, before tape 1 of the FPP:

8-2

non-EAE

FPP
(DEC-OS—NFPPA—A—PAI
PA2, PA3)
27-bit FPP
(DEC-OS-NFPEA—A—PAI
PA2, PA3)

EAE ‘FPP

(DEC-SE—NEAEA-A—PAI
PA2, and PA3)

FADD

1000

FIXMRI

2000

3000

FIXMRI FDIV

4000

FIXMRI FGET

5000

FIXMRI FDIV '_ : 4000
FIXMRI FGET‘ :. 5000

FIXMRI FPUT

6000

FIXMRI FPUT

,FIXMRI FADD
FIXMRI FSUB

‘

FIXMRI FMPY

1000

FIXMRI FSUB

2000

FIXMRI FMPY

3000

0000

FEXT

FNOR .,:'7000

FNOR

PAUSE

FEXT

SWP

7521'

CAM

7621

7501

7421

6006

MQA
MQL
SGT

6000

0000
7000

PAUSE

‘

Following assembly of the FPP, a user program may be assembled. The user program should begin With the following pseudo-instruction sequence, or a tape containing the following sequence
terminated with a PAUSE pseudo-op should be assembled before
the user program:

8-3

Assembling with PAL III

Assembling with PALS

FIXMRI

FJMP

0000

FIXMRI

FJMS

7000

FJMP

0000

FIXMRI FADD

1000

FISZ

:0000

FIXMRI FSUB

2000

FEXT‘

0000

FIXM‘RI FMPY

3000

0001

FIXMRI FDIV

4000

0002

FIXMRI FGET

5000

FSIN

0003

FIXMRI FPUT

6000

FCOS

0004

FIXMRI FJMS

7000

FATN

: 0005

FEXP

FLOG

FSQU
FSQR

F182

0000

0006

FEXT

0000

0007

FSQU

0001

0010

FSQR

0002

0011

FSIN

0003

0012

FCOS‘

—_—

0004

FFIX

0013

FATN

0005

PLOT

0014

FEXP

0006

7000

FLOG

0007

FCDF

7001

FNEG

0010

sto

:7002

FIN

:0011

FSWI

7003

FOUT

0012

FHLT

7004

FFIX

0013

FSMA

:7110

FLOT

:0014

ESZA

7050

FNOR

7000

7100

FCDF

7001

FSNA

7040

sto

7002

FNOP

:7010

FSWl

:7003

FSKP

7020

FHLT

7004

FSMA

7110

FSZA

7050

FSPA

7100

FSNA

7040

FNOP

7010

FSKP

7020

FNEG
FIN

FOUT

FNOR

FSPA

FLOATING POINT NOTATION
'A floating-point number may be written as a mantissa, which
consists of the floating-point number with its decimal point shifted
a

given number of places in either direction, and an exponent,
8—4

which indicates the number of places that the decimal point was

shifted and the direction of the'shift. A negative exponent corres-

ponds to a shift to the right, while a positive exponent corresponds
to a shift to the left. For example, the decimal number 12. 625
may
be expressed1n the following ways:
Mantissa

Exponent.

12.625

12.625 2 12.625 X 100
'

1.2625 X 101

0.12625 X 102

1265.0 x 10‘3

1.2625

0.12625
12625.0

2
-—3

A floating—point number which has been converted to a mantissaand an. exponent may be recovered by mutiplying the mantissa by
the Eth power of the radix (base) in use, where E is the exponent.

Normalization
A floating-point number may be represented in an infinite variety of ways, since the decimal point may be shifted any number

of places in either direction. If the decimal point is shifted until it
appears immediately to the left of the most significant digit, the
number is said to be normalized. The mantissa of a normalized

floating-point number may be stored as an integer, since the dec—
imal point is understood to appear to the left of the most signifi—
cant digit. In the discussion which follows, all floating—point num—
bers are assumed .to be expressed in normalized floating-point
notation, as indicated below.
Normalized Floating—Point Notation

‘Decimal
Number

‘

12.625

Mantissa

Exponent

.12625

0.0012

—15_30'.0
0.0

.12

—.153

-—2

(

——89.9

——.899

—0.0899

——.899

—1

8-5

When a floating-point number is expressed in normalized notation,
thP manticga ummllv
falls in the range:
We”,
-

“M”-..

-m

R‘41 S [ mantissa | < 1
where R is the radix of the number system‘used. "Thus, the abso—
lute value of the mantissa of a decimal floating-point number will

greater than or equal to 1/ 10.but less than 1 if the decimal

point is positioned to the left of the most significant digit, in ac—
cordance with the convention presented above. The only exception
to this rule is the floating-point number zero, which is defined to
have a mantissa and an exponent both equal to zero.
In computer applications, all numbers are manipulated and
stored in binary notation. The preceding discussion applies equally

'well to decimal or binary numbers, in that a binary number may
be converted to normalized floating—point notation by shifting the
decimal point to the left of the most significant digit and assigning
an

exponent equal to the (directed) number of places that the dec-

imal

point was shifted. Since the left-most significant digit will
always be a binary 1, the mantissa conforms to the convention:
1/2 S l mantissa l < 1
except for the special case of zero.
Number Representation

PDP-8

floating-point numbers are stored in three consecutive

12—bit core memory locations as follows.
EAE AND NON-EAE FPP

The first location, which has the lowest core address, contains
the exponent. The second word contains the twelve most significant
bits of the mantissa, called the high-order mantissa. The third
word, which has the highest core address, contains the last twelve

(least significant) bits of the mantissa, or the low—order mantissa.
As with one-word integers stored in the conventional manner, if
the exponent or mantissa is negative, its two’s complement is
used, thus the first bit of both the exponent and the high-order
mantissa may be considered as sign bits. The low—order mantissa
is unsigned, however, and should be considered an extension of
the high-order mantissa.

storage location of a floating—point number may be
tagged with a symbolic address, as shown in Figure 8~1. In this
The

core

’

8-6

example, the floating-point number 12.625 is stored incore location “FPNUM.” The tag FPNUM is associated with the core lo-

cation at which the exponent is stored, while the mantissa occupies

locations FPNUM+1 and FPNUM+2.

FPMM

r—SIGN

BIT

0
I

O
I

EXPONENT

HIGH-ORDER

FPNUM+10110.0I101.0

°,°.°MA~TI$3A

FPNUM+20

Figure. 8-1

olo

olo'o

LOW-ORDER

010

°.°.°MANTIS$A

Storage Allocation for a 23-bit Floating-Point Number

27-BIT FPP

The first word (lowest core address) contains the sign of the
mantissa, the exponent in bias 2008 notation, and the higher,

order 3 mantissa bits. The second two words are the middle order
and low order mantissa; respectively, in sign-magnitude notation;

HIGH ORDER 3
MANTISSA ans

EXP O NENT + 200 8
_

MIDDLE ORDER
MANTISSA
12

LOW ORDER
MANTISSA

Figure 8—2’

Storage Allocation for a 27-bit Floating-Point Number

The sign of the number is bit 0' of the first word. The value of
'

the exponent is obtained by subtracting 2003 from bits 1
8 of the first word.

USING

through

THE FLOATING-POINT PACKAGE

The Floating—Point Package contains subroutines which perform

floating-point operations using three core memory locations—44,
45 and 46, which are collectively designated as the floating accumulator, or FAC. Some of the subroutines require a floating-point
8-7

argument. The Floating Add Subroutine, for example,

must be

supplied with the address of a floating-point number which is to
be added to the content of the» FAC. Other floating-point subroutines do not require an argument because they operate on the
FAC directly. The Floating Square Root Subroutine, for example,
operates directly on the FACuy replacing the current value of
the FAC with the square root of this value. It does not require an
argument.
Most of the floating-point subroutines may be called by a user
program at any time, in precisely the same manner that user subroutines are called, subject to the restriction that the subroutine

call, the FPP itself and the argument (if required) must all reside
in the same memory field. In general, all FPP subroutines should
be entered with the hardware accumulator cleared.

FIOating Subtract Subroutine, for example, may be called
by an indirect JMS toits entry address. The location following the
The

subroutine call must contain the address of the floating—point argument to be subtracted from the FAC. Once this Operation has been

performed, the subroutine will return control to the core location
following the address of the argument (i.e. the second location following the JMS). The Floating Square Subroutine may also be
called by an indirect JMS to its entry address. Since this subroutine
does not require an argument, it will square the contents of the
FAC and return control to the location immediately following the
1 MS which called it.

When used in this manner, the FPP is being employed in single-

instruction mode. In this mode of operation, every call to an FPP
subroutine causes one floating—point operation to be performed on
the FAC. Table 8-1 lists the floating—point subroutines which may

be called in single-instruction mode, specifying which subroutines

reqUire'an argument-and which don’t, along With their'abs'Ol'ute'
entry addresses and a description of the operations they perform.
‘

NOTE

Subroutine addresses are constant for all three flbatingpoint packages. Thus, the user may easily upgrade from
NON-EAE to an EAE system if greater speed is required and from either 23- bit FPP to the 27- bit FPP
a

if greater accuracy is required.
8—8

Figure 8-3 is a program which calls the 23-bit floating-point.
single—instruction mode. This program calls the
Floating Input Subroutine to accept a floating-point number from
the Teletype, then calls” the Floating Multiply Subroutine to multiply the input by 10.0; Finally, the program stores the result of
these operations and halts.
subroutines in

* 260

xcc
TLS
ans
JMS

FINP
FMPYP

I
I

.TEN
JMS I FPUTP
STORE
HLT
6290

FINP,
,FMPYP,
FPUTP,
TEN,

66am.

7522

4
'

24mg
NOOO
D
O

STORE,

IINITIALIZE KEYBOARD
TELEPRINTER
/CALL INPUT SUBROUTINE
/CALL MULTIPLY SUBROUTINE
/ADDRESS OF OPERAND
ICALL PUT SUBROUTINE
ISTORAGE ADDRESS
/HALT UPON COMPLETION
/POINTER To INPUT ROUTINE
/POINTER TO MULTIPLY ROUTINE
/POINTER-TO PUT ROUTINE
/EXPONENT
/HIGR-ORDER MANTISSA
/LOU-ORDER MANTISSA
/5-UORD FLOATING-POINT
/STORAGE LOCATION

,/AND

Figure

8-3

Using the FPP in Single-Instruction Mode

The same program could‘be
FPP

by changing the constant TEN (necessary because of the

different
TEN;

converted for use under the 27-bit-

floating-point format) to look like:
2045
0000

0606

/EXPCNENT
/r-lIGH-0RDER MANTI SSA
/LOW- ORDER MANTI SSA

The user should note that in single-instruction mode, the hardware

AC must be zero when Calling a floating—point routine. The

contents of the link are not important.

8—9

Table 8-1 Floating-Point Subroutines
Subroutine and
Entry Address

Function

FLOATING ADD
7000

FLOATING SUBTRACT

Add the argument to the content of the
FAC and store this result in the FAC.

7 1 17

Subtract the argument from the content
of the PAC and store this result in the
FAC.

FLOATING ‘MULTIPLY
6600

Multiply the argument by the content of
the PAC and store this result in the FAC.

FLOATING gDIVIDE

.Divide the content of the PAC by the argument and store this result in the FAC.

6722

An error results from any attempt to di-

vide by zero.
FLOATING
7306

GET

‘

Load the argument into

the FAC.

FLOATING PUT

Replace the argument with the content

7322

of the FAC.

FLOATING INVERSE
SUBTRACT
6400

Subtract the content of the PAC from
the argument and store this result in the
FAC.

“

FLOATING

INVERSE

DIVIDE

Divide the content of the PAC into the

6412

argument and stOre this result in the FAC.

FLOATING
NORMALIZE
7265

Normalize the content of the PAC and
store this result in the FAC.

FLOATING “SQUARE
75 64

and

FLOATING SQUARE
ROOT
645 1

store

this result in the PAC.

Compute the positive square root of the
absolute value of the content of the FAQ
and store this result in the FAC.

FLOATING

SINE

‘

Multiply the content of the FAC by itself

5000

Compute the sine of the content of the
PAC (in radians) and store this result in
the FAC.

FLOATING COSINE
5053

Compute the cosine of the content of the
FAC and store this result in the FAC.

FLOATING

Compute the primary arctangent of the

ARCTANGENT

content of the PAC and store this result

5200

in the FAC.

‘

8-10

Table 8-1 (Cont) Floating-Point Subroutines
Subroutine and
Entry Address

Function
‘

Compute the exponential function of the

FLOAHHNC;
EXPIONENTIAL

content of the PAC and store thisresult

5135

in the FAC.

FLOATING
LOGA‘RITHM

Compute

the natural

of the

logarithm

content of the FAC and store this result

in the FAC. An error results if the con-

5263
'

tent of

the FAC is negative or zero.

FLOATDKENEGATE

Negate the" content of the PAC and store

7135

this result in the FAC.

FLQKHNGINPU
6200

'
-

‘

FLOArnu3OUTPUT

Accept a floating-point decimal number
from the Teletype keyboard, convert it to
a floating—point binary number, normalize
and truncate if necessary, and store
result in the FAC.

this

5600

Convert the content of the PAC to a decimal floating-point number and print this.
result on, the Teletype printer.

Fix
5500,

Compute the largest integer that is not
larger than the content of the PAC and
this result in the exponent word
(10c. 44). An error occurs if this result falls outside the range
-—2047 S X < 2047.

store

of the PAC

FLOAT
5533

Compute the floating-point number-that
is equal to the content of the exponent
word of the PAC (10c. 44) and store
this result-in the FAC.

The FPP also contains an interpreter, which is a subroutine that

decodes and executes floating—point pseudo-instructions. The in-

terpreter may be called in the same manner as any other subroutine, however the FPP loads a pointer to the interpreter into core
location 0007 of the memory field in which it resides, so that the
interpreter is most conveniently called by a J MS I 7 instruction.
To call the FPP from

which it

memory field other than the one in
effective JMS to location 7400 of the
a

resides, use an
field in which the package has been loaded. The following is an
8-11

example of a call to the floating-point package where multiple
{1an fiplflc nrp invnlvprl
”up“

HAUL)

“I. V

J-ll

U}.

V“.

N=MEMORY

FIELD

M=MEMORY

FIELD

59?
CIF
JMS

‘

CONTAINIVG

CALL

CWTAIVING FPP
{HAY SE CM! TTED IF
BE

MAY

0‘11 TTED

P7499

( PSEUDO

P7 409:

IFr

CUR. E31":

DF=N

CURRENT

IF=M

‘

INSTRUCTIONS)

7 409

floating data field is originally set to the hardware data
field upon entry to the interpreter. This may be changed via the
The

_FCDF instruction which has a format similar to the normal PDP-8
CDF instruction. The

floating data field is interpreted like the
normal data fieldin that it applies only to “1....1
me operanu of an indirectly addressed memory reference instruction (op codes 1- 6).
The interpreter essentially accepts all of the pseudo—instructions

following the JMS which called it as arguments. Beginning with
the first pseudo—instruction following the interpreter call, the in—
terpreter decodes each pseudo—instruction as an effective JMS to
the appropriate floating-point subroutine and passes the subroutine
an argument, if required. When used in this manner, the FPP is
being employed in interpretive mode. In the interpretive mode the
package can be called from any memory field, and the user can
access data in any memory field.

The pseudo-instructions which are interpreted as calls to those

subroutines that

require

argument

are

closely analogous to

standard memory referenceinstructions. The first three bits of the
'

pseudo-instruction specify which floating-point operation is to be
performed (i.e. which subroutine to call) while the last nine bits
specify the effective address of the argument according to the same
conventions used for effective address generation by standard
memory reference instructions. The pseudo-instructions which are
interpreted as calls to those subroutines that do not require an ar-

gument are. analogous to the operate microinstructions in that these

pseudo-instructions do not reference a core memory location.
8—12

The basic arithmetic operatiOns (FADD, FSUB, FMPY, FDIV)

require that both FAC and floating operand be normalized. All
four yield a normalized result.
NOTE

Pseudo-Instructions

are

the

‘

same

for each of the. three

FPP’s to facilitate. conversion of user programs from one
package to another. No alteration of the user program is
necessary to convert from the EAE FPP to the NONa user program
EAE FPP and vice versa. To

convert

from either 23- bit package to the 27- bit FPP,

floating-point

constants need be

changed.

only the

Table. 8-2 Floating-Point Pseudo-Instructions
Mnemonic

Octal

FEXT

0000

Operation

the interpreter and execute the next
normal machine
guential instruction

Exit

se-

instruction.
'

0001

Call the Floating Square Subroutine.

FSQR

0002

Call the Floating Square Root Subroutine.

F SIN

0003

Call the Floating Sine

FCOS

0004

Call the Floating Cosine Subroutine.

FATN

0005

Call the Floating Arctangent Subroutine.

FEXP

0006

Call the Floating Exponential Subroutine.

FSQU
'

Subroutine.

FLOG

0007

FNEG

0010

Call the

FIN

0011

Call the Floating Input Subroutine.

0012

Call the Floating Output Subroutine.

Fle

0013

Call the Fix Subroutine.

FLOT

0014

Call the Float

FOUT

‘

Call the Floating Logarithm Subroutine.

Floating Negate Subroutine.

8-13

éubroutine.

Table 8-2 (Cent) Floating-Point Pseudo-Instructions
Mnemonic

Octalf

FNOP

0015

operation. Available to the user.

FNOP

0016

operation. Available to the user.

FNOP

0017‘

operation. Available to the user.

0020

Floating Increment and Skip if Zero:

0177

Increment the content of the core memory lo-

FISZ

Operation

cation designated by bits 5-11 of the FISZ'
pseudo-instruction and skip the next sequential pseudo—instruction if the content of this
locaton becomes zero. An FISZ pseudo-instruction may only reference page zero locations between 0020 and 0177 inclusive.
FJMP
to

0200

Floating Jump:

0777

as a

Performs the

same

function

standard I MP memory reference instruc-

tion except that it is not possible to FJMP to
a

location

page zero directly.

FADD

lnnn

Call the Floating Add Subroutine. Use nnn to
calculate the effective address of the operand.

FSUB

2nnn

Call the
nnn

Floating Subtract Subroutine. Use
calculate the effective address of the

operand.
FMPY

3nnn

FDIV

4mm

Call the Floating Multiply Subroutine. Use
nnn to calculate the effective address of the

operand.
Call the Floating Divide Subroutine. Use nnn
to
calculate the effective address of the

operand.

FGET

Snnn

Call the Floating Get Subroutine. Use nnn to
calculate the effective address of the operand.

FPUT

6nnn

Call the Floating Put Subroutine. Use nnn to
calculate the effective address of the operand.

FNOR

7000

Call the Floating Normalize Subroutine.

FNOP

7010

No operation.

FSKP

7020

Floating

‘

Skip
pseudo-instruction.
Skip:

8-14

the

sequential

Table 8-2 (Cont) Floating-Point Pseudo-Instructions
Mnemonic

Octal

FSNA

7040

Operation
Floating Skip on Non-Zero Accumulator:
Skip the. next sequential pseudo-instruction if
the content of the PAC is not zero.

Floating Skip on zero Accumulator: Skip the
next sequential pseudo-instruction if the con-_

7050

FSZA
'

‘

tent of the PAC is zero.

7100

FSPA
'

pseudo-instruction

Floating Skip on MinLis Accumulator: Skip
the next sequential pseudo-instruction if the

7110

Floating Skip on Positive Accumulator: Skip
if the
the next sequential
content of the- FAC1s positive.

'
-

FSMA

content of the PAC is less than

zero.

‘
‘

Change Floating Data Field: Obtain the 0p_i e’rand" for all subsequent indirectly addressed,
floating-point, memory reference pseudo-in-

7on1

FCDF

‘

struCtions from memory field 11.
'

7lnl

Unused.

70n2

Restore the normal order of all FDIV and
FSUB
until the next FSWl
instruction is executed.

‘

'FSWO
"

or 71112“

operations

pseudo-

FSWl

;

-70n3

‘ReVerse‘ the order of all FDIV and FSUB”
operations until eitherthe next FSWO pseud0~
instruction ”is executed, the interpreter is re-

or=71n3

entered by the next effective J MS I 7 instruc-

tion, -or the Floating Input Subroutine is entered;

FHLT

70n4

.or 7.1‘n4

Floating Halt: Halt and display the content
of the floating PC in
the hardware accumu-

lator.

70n5

Unused.

to 70n7

71n5
to 7ln7

~Unused.
.

FJMS
-

7200
to—7777

Floating Jump to Subroutine: Performs the
function

standard JMS memory
reference instruction, except that it is not
location on page zero'
possible to FJMS to

same

directly.
8—15

Table 8-2 lists all of the floating-point pseudo—instructions, their

-mnemonics and the operations they perform. The memory refer-

pseudo-instructions (octal codes lnnn to 6nnn inclusive) are
interpreted as calls to floating—point subroutines which require arence

guments. All of these functi as are also available to the user in
single-instruction mode. The FJ MP and FJ MS pseudo—instructions

perform operations which are not possible in single-instruction
mode, but they «are essentially equivalent to the standard PDP—8
‘JMP and JMS instructions. Note, however, that no directly addressed FJMP or FJMS pseudo-instruction may reference a loca—

tion on page 0. This restriction allows octal codes 0000 to 0177
and 7000 to 7177 to be interpreted as extended pseudo-instructions.

The

‘

pseudo-instructions corresponding to octal codes 0001 to

0017 generate‘calls to subroutines which are available to the user
in single-instruction mode. 0000 is the FEXT

(leave interpreter)

operation, and 0020 to 0177 are FISZ operations (see Table 8-2).
Aside from the FNOR

operation, the pseudo-instructions corresponding to octal codes 7000 to 7177 are not available in singleinstruction mode. The octal codes for some of these pseudo-instructions may have either a 1 or a 0 in bit position 5; this is
because, the interpreter does not decode bit 5 of the designated
pseudo-instructions. Several» possible octal codes do not have as‘signed pseudo—instructions (e.g. 71n1). These codes are unused,
and should not be supplied as input to the interpreter.

#206
xcc
TLs
ans 1 7
FIN

FMPY TEN
FPUT STORE
FEXT
HLT
4
.2408

TEN,
.

STORE,
‘

OOOO
O

IINITIALIZE KEYBOARD
/AND.TELEPRINTER
IENIER INIERPRETER
IGET NUMBER FROM TELETYPE
IMULTIPLY BY’lfigfl
ISTORE RESULT
/LEAVE INTERPRETER
IAND HALT
IEXPONENT
IHIGH-ORDER MANIISSA
lLow-ORDER MANIISSA

l5-WORD_FLOATING-POINT
ISIORAGE LOCATION

O
O

Figure 8-4

Using the 23-bit F_PP in Interpretive Mode
’

8-16

Using the 27- bit FPP, the constant TEN would appear as

follows:
rav.‘
'

29145
0000

9600

)EXPoVENt
'

MI 614- ORDER
/LON- ORDER

MANTI SSA

The program example of Figure 8—4 performs the same opera—

:tions as the example of Figure 8-3, however this program has been
coded to execute in interpretive mode: Note that this program rethan the equivalent single-instruction mode
quires lesshoweverstorage
the execution time
version,
interpretive mode
required
will be
core

considerably longer.

Floating Input and Output
The FIN pseudo-instruction calls the Floating Input Subroutine
'to accept one decimal floating-point number from the Teletype
keyboard, convert this input to a binary floating-point number,
normalize and truncate the number if necessary, and-load the num~

The input routine is normally called in intérpretive mode using the FIN command.’The input routine may
her into the PAC."

also be“ called by an effective I MS to the start of the routine (see‘
Table 8—1 for exact memory location). Input is terminated when

the routine recognizes any typed character which could not be' part
of the input. For example, the conversion of “12.0.” would be
‘

“

”

.The characters + and
terminated upon reCeipt of the second
will not be recognized as input terminators and should not be
——

used

following numbers, all terminated by carriage
returns, are examples of legal input. They are all equivalent.
as

such. The

726.7
.7267E+@5
+7267E-1

Floating Input Subroutine echoes each _character as itis
typed, including the terminator. Upon completion of any floating
input operation. core memory location 0052 of the memory field
The

Programs using floating input should begin with a KCC instruction to
keyboard and a TLS instruction to initialize the
printer.
initialize the Teletype

8-17

in which the FPP resides contains zero if the input was invalid,
and a non—zero value if the input was valid. Core location 0053 in

this field contains the ASCII code for the terminating character
last received. Core location 0054 of this field may be set

by the
user. A value of zero loaded into this location causes the input
routine to echo only a carriage return whenever a carriage return
is typed as an input terminator. Any non-zero value loaded into
location 0054' causes the input~ routine to echo a carriage return
and a line feed whenever a carriage return is received. Location
0054 is originally loaded with 77778.
.

‘

Using the 23-bit FPP’s, if the example illustrated in Figure

is started at location 0200 and the

Teletype, the program will halt at
locations A and B containing:
A;

8-5

types “0X1.0Y” at the
location 0210 with storage

user

3399
3363

3303
.

63.1

2390
Gaga

while’ location 0053 will contain 03313, the ASCII code for the
second terminator.

Using the 27—bit FPP, storage locations A and B containz‘
A;

.3'

IZERO

GQMGQN

Q .. b

IONE

and register 0053 will contain 0331 ——the second terminator.
NOTE
Since the input routine calls the floating—point interpreter,
after input, FSWITCH is set to 0 (FSWO) even if it was
set to 1 when input was called

Switch).
8—18

(see the section, Floating

input routine recognizes RUBOUT as a special character
which is not echoed. If a RUBOUT is typed during input. all characters received since the last input terminator are ignored. For
example; typing:
The

276(RUBOUT) IT
to the

input routine has the same effect as typing:

Input will be terminated with the binary floating-point‘eguivalent
if decimal 1 in the PAC. The current input element must be re'

typed from the beginning.

*200

KCC

TLS'
JMS

FIN
FPUT A
FIN
FPUT B

FEXI
HLI

‘/INITIALIZE KEYBOARD
lAND TELEPRINTER
/ENTER INTERPRETER
/INPUT A NUMBER
/STORE IN LOCATION A
/ACCEPT ANOTHER NUMBER
/STORE¢IN LOCATION B
fEXIT INTERPRETER
/AND HALT

g
B
0
a
G
G

Figure 8-5

Floating Input Routine

The FOUT pseudo—instruction calls the Floating Output Subroutine to print the contents of. the FAC on the Teletype.2 The
'FPP maintains four

core

memory locations on page 0 of the

memory field in which it resides. These locations may be set by
the user to determine-the format for all floating-point output.

Loading any non-zero value into locatiOn 0056 causes all output
to be printed in FORTRAN. Fa.b format, where a is the content
2

Programs using floating output
initialize the Teletype printer.

should begin
8—19

with a TLS instruction to

of core location 0057 and b is the content'of location 0060. As
with FORTRAN, field overflow causes the field to be filled with

asterisks, while field underflow causes the output to be right justi—
’

fied.
If location 0056 contains zero, the FF? loads 0016;; into location 005 7 and 00068 (0007 8 fOr the 27-bit FPP) into

location 0060

then prints all output in FORTRAN E14.6 format (E14.7 for 27-bit

FPP)

In either case, each element of output will be followed by a
carriage return and line feed if the content of location 0055 is not

Loading a zero into location 0055 suppresses the terminating
carriage return and line feed.
Upon loading, the FPP initializes these four core locations for
E14.6 format (E14.7 in 27-bit FPP), with each element terminated by a carriage return and line feed.
zero.

NOTE

.The output routine destroys the contents of the. FAC.
If the number to be typed is: needed for further calculation, it should be saved prior to calling the output

routine.

OVERFLOW AND

UNDERFLOW

Under the 27-bit FPP only, overflow and underflow on
are

input

treated like exponent overflow and underflow (see section on

Error traps). Typing a number like:
109006E+ 61

willresult in exponent overflow, and the error trap will be taken
if the user has set it. The capacity of the input routine is approx-

imately 10'38 < X <1038. If more than 8 significant digits are
input, the result will be truncated.
Use of F ISZ and Auto-Indexing
Core memory locations 0010 through 0017 may be used for
auto-indexing in interpretive mode. If one of these locations is
referenced indirectly in interpretive mode, the contents of the loca-

tion will be incremented

by three before it is used for effective
‘

address generation.
8-20

The Floating ISZ

pretive mode.

(FISZ) operation is only availablevin inter-

A FISZ

pseudo-instruction-must be directly ad-

dressed, and may only reference a page 0 location greater than
0017. A hardware 182 is performed on the referenced page 0
location, and the next pseudo—instruction is skipped if the content
of the location becomes zero. If the content of the referencedloca—

tion does not become zero, the next sequential pseudo-instruction
is executed.
The program example of Figure 8-61uses auto-indexing in inter-

pretive mode to pickup 20 r(octal-) floating-point numbers from
a
»

buffer in core and calculate the sine of each. The sines of the

numbers are stored in a separate buffer area and are also printed
on

the Teletype. After each' iteration,

FISZ is performed on a

counter and the program loops back until the counter becomes
zero, at which time the program exits the interpreter and halts.

(Assume that the floating—point package and the user’s program
are in the same data field.)

‘

/INPUT

BURI=4$$
BUF2=600

BEGINS

BUFFER

[OUTPUT

*16

BEGINS AT 0400

BUFFER

0600

‘

R16:

/IN'PUT

R17,

IOUTPUT BUFFER

R29:

/DATA

BUFFER

ELEMENT

POINTER
POINTER
COUNTER

1:200
KCC

TLS
JMS

LOOP:

IINI TIALIZE
7

IENTER

FGET
FPUT

R16

FGET

R16

R~17_

INDXR

lINI TIALIZE

THE THREE
lAUTO’INDEX REGISTERS
lGET NUMBER FROM INPUT BUFFER
/TAKE ITS SI-NE
IPUT RESULT IN OUTPUT BUFFER
/PRINT RESULT
IDCNE ALL NUMBERS?

FSIN
FPUT

FOUT

FI SZ

R23

FJMP

LOOP
_

FEXT'
INDXR:

IND:

LOOP

HES:

BACK

EXIT
IAND HALT

HLT
BUF1'3
BUF2-3
-20
_

TELEPRINTER

INTERPRETER

IINITIAL
/INITIAL

VALUE

UPLUE

R16
R17

xrmrm.

VALUE

Rae

Figure 8-6

Use of FISZ and Auto-Indexing

8—21

User Subroutines
‘

Users who require special floating-point functions may code the
functions as assembly language subroutines and call them through
‘the interpreter in the same manner as the extended functions are
called.

Up to three such subroutines may be inserted in place of
the three FNOPs having octal codes 0015, 0016 and 0017. This
is accomplished by assigning a mnemonic to the user function,
equating the assigned mnemonic to the octal code of the FNOP to
be deleted, and inserting the entry address of the user subroutines};
into

location 7246 (for octal code 0015), 7247 (for octal
code 0016) or 7250 (for octal code 0017) of the memory field
core

in which the FPP and user subroutine reside.
User subroutines called

through the interpreter can themselves
call the interpreter and use all interpreter functions except calling
another user function or extended function through op code 0.
The extended functions could be called, however, using singleinstruction mode. All user subreutines must be in the same memory

floating-point package. They may enter the interpreter and change the floating data field. They can even change
field

the

the hardware data field, but when one user subroutine returns to the

interpreter, the floating data field, hardware data field, and floating instruction field are all restored to the value they had prior to
calling the user subroutine.
The program example of Figure 8-7 contains a user subroutine

called through the interpreter. This subroutine has been assigned
the mnemonic FUSR and octal code 0015. If the FPP is loaded
into the same memory field, the program will accept floating-point

Teletype, add all such input elements greater
than 0.5 to a running sum, and print the sine of the cumulative
total after-each input element is received.

numbers from the

8-22

AND CODE

FUSR=UUIS
FIELD 8

/ASSIGN

K60

/INITIALIZE KEYBOARD
/AND TELEPRINTER
IINSERT USER.SUBROUTINE ENTRY
-/ADDRESS IN INTERPRETER TABLE
/ENTER INTERPRETER
/ACCEPT NUMBER FROM KEYBOARD
/GALL USER SUBROUTINE
/PRINT RESULT (SINE 0F SUM)
IACCEPT NEW INPUT"

NNENONIO

*203.

START,

TLS_
TAD KUSR

DCA

JMS

INTABL
7

FIN

FUSR
FOUT
FJMP .«3

USUB-

KUSR,

INTABL, 7246‘
0
JMS

USUB,

I 7
FPUT TEM
FCDF 10

IENTRY ADDRESS FOR'USER
ISUBROUIINE To BE ENTERED
/IN INTERPRETER TABLE
/POINTER TO INTERPRETER
/TABLE ENTRY CORRESPONDING
/TO OOTAL CODE Ozzs
/ENTER USER.SUBROUIINE
/ENTER THE INTERPRETER
/STORE INPUT
/cRANGE To FLOATING
/DATA FIELD 1
/LOAD D.5 INTO FAC
/SUBTRACT INPUT
/IS INPUT GREATER TRAN D.5?.
/NO: DON'T ADD TO sum
lYES: GET CUMULATIVE sum
/ADD IN LATEST INPUT
/STORE HEW sum
/LOAD sum INTO FAc AND
/LEAVE INTERPRETER
/TAKE SINE 0F sum
/RETURN To NAINLINE
/POINTER TO SINE ROUTINE
/POINTER To FIELD l CONSTANT
-

FGET
FSUB
FsmA
FJMP
FGET
FADD
FPUT

I COMPR
TEN
.+4

SUN
TEN
sum
FGEI sum

FEXT
JMS

PSIN,
COMPR,

JMP
5ODO
PTs

sum,

TEN,

*23@

wasas
ENG

I
I

PSIN
USUB

IFLOAIING-POINT'SIORAGE

/FLOATING-POINT TEMPORARY
IELD

IFIELD 1

CONSTANT

2.5

980

Figure 8-7

Coding a User Subroutine (23—bit FPP)
8-23

If using the 27-bit FPP, the field 1 constant is stored as:
FIELD

*EOO

xcovsmm

2034

PTS;

.s-

Floating Skips
The interpreter maintains a core memory location designated
as the floating program counter
(floating PC) which is originally
set to the address of the location following the JMS I 7 which
called the interpreter, and thereafter updated by the interpreter
whenever a floating—point pseudo-instruction is executed. The floating PC may be incremented by the floating skip pseudo—instructions, which are functionally equivalent to normal PDP-S skip
microinstructions except that floating skips reflect the status of the
PAC rather than the hardware accumulator. Floating skip pseudoinstructions may be microprogrammed in the same manner as
7
minrnincfnlr’finnc All
grcup
Vt)»;
flQating Skips aSSUIflC that
the content of‘the FAC is normalized. There is no provision for
clearing the FAC when skipping on condition.
nnnrofp

utw

1.4

111154 UAALU»; uvsAVAA-J-

example of Figure 8-8 uses floating skip pseudo-instructions to accept floating—point numbers from the Teletype until it
The

receives ‘a number with an absolute value greater than 10.0.
‘

$290

INPUT,

ch
TLs
JMS 1 7
FIN

IINITIALIZE KEYBOARD
IAND TELEPRINTER
:

FSNA
FJMP .-2
'.
FPUT TEN
~FSPA

FNEG
FSUB TEN
FSPA FSNA
FJMP INPUT
FGET TEM

FEXT
HLT

TEN,

IENTER INTERPRETER
IACCEPI INPUT FROM KEYBOARD
/IS THE INPUT NUMBER 3.0?
/YES: GET NEw INPUT
/Na: STORE TEMPORARILY
/Is INPUT POSITIVE?
-/NO: TAKE ABSOLUTE VALUE
/YES: SUBTRACT Ia.a
/Is INPUT GREATER THAN 10.0?
/NO: GET NEw INPUT
/YES: U5E_TNTs INPUT
/LEAVE INTERPRETER
/HALT wITH INPUT IN FAC
/CONSTANT = 13.0

24am
OOOO

TEN,

0;@;6

Figure 8-8

Use of Floating Skips (23-bit FPP)

8-24

Using the 27-bit FPP, the constant ten is stored as follows:

TEN.

/CONsTANT

2045

10.0

0006
“6020

Floating Data Field
The floating CDF (FCDF)

pseudo-instruction used
1s

Change

the data field from which the operand of an indirectly addressed,
floating-point, memory reference pseudo-instruction is obtained.
The program example shownIn

FCDF

pseudo—instructiOn.
T.FIELD D

*EOD.

TLs
CDF

FPP

/U$ER

PROGRAM

ISET

FCDF

FPUT

FGET

TEN

K56

FIELD

IFIELD 0 THEN
/SET DATA FIELD

AND

FROM
1

/SET

OUTPUT

t/GET

NUMBER

FROM

FIELD

PRINT IT

/LEAVE

INTERPRETER

xmoamr'..

740D

/POINTER

42240030
TEN:
56
K56:
PFSPTI..F3PTI

INTERPRETER

/ENTER

HU"

#400

FIELD

INTO FIELD

PRINTER
FIELD = O

DATA

IIVSTRUCTION

/AND

'FEXT

INTO

ILOAD

FOUT

F8PTI.

IINITIALIZE

CIF IO
JMS I K7400
FGET I PF8PT1

KTAOO.

Figure 8—9 illustrates the use of the

‘

TO INTERPRETER

xcov5TANT

10.0

/POINTER-To

OUTPUTREGISTER

IPOINTER

FORMAT

SPEC
_

xcovsrANTs

ISET

OUTPUT

FORMAT

/F8.1

Figure 8—9

Use of the “FCDF Pseudo-Instruction (23—bit FPP)

Using the 27—bit FPP, the constant ten is stored as:
TEN:

2fl45

ICCNSTANT

3006

lfl-Q

0066

If the FPP is loaded into memory field 1, this program will enter
the interpreter from memory field 0, load the floating output regis8-25

ter with a

format specification stored in field 0; and then print a

floating-point number which is also stored in field 0.
Floating Switch
The floating switch instructions (FSWO and FSWI) regulate the
operation of the FDIV and FSUB pseudo-instractions. Using
floating switch 0, FSUB and FDIV are interpreted normally. Following any occurrence of an FSWI pseudo-instruCtion in interpretive mode, the order of all FDIV and FSUB operations is
reversed. That is, every FDIV operation will divide the content
of the PAC into the floating-point argument supplied (rather than
vice versa) and every FSUB operation will subtract the content
of the PAC from the floating-point operand. The result of these
operations will be stored in the PAC in either case. After each
occurrence of an FSWl pseudo-instruction, the order of FDIV
and FSUB operations will continue to be reversed until either:
1

The.

‘

.l uv

tar-fun Inq‘xln

navf

oar-n

“VAL

ovsiuvntuu

u H u

An thofnunfinn

nae“
PM; uuu— ILIDLL uwuuu

Io.

nvnnnfnr‘

unvvutuu

The interpreter isrre-entered by the nexteffective JMS I 7

instruction.
3.

A floating input operation is performed in either mode.

The floating switch is normally set to 0.
Certain mathematical calculations may be facilitated by using

pseudo-instruction to reverse the order of FDIV and
FSUB operations. For example, Figures 8-10 and 8-11 contain two

the FSWl

program segments, both of which calculate the value of the

ex-

pression:

g—x

130+

X2+B1+
0

X“ + B2 +

‘7

A3
x2 + Ba

Both program segments execute in interpretive mode, however the
uses the FSWl

pseudo~instruction to reverse
the order of certain FDIV and FSUB operations, while the second
first program segment

program-segment, does not.
8-26

#203

JMS I

[ENTER INTERPRETER
IGEI x
ISQUARE IT
ISTORE TEMPORARILY

rear x

rseu
‘FPUI XSQR
FADD 35
st1
FDIV A5
FADD 32

FADD
FDIV
FADD
FADD
FDIV
.FADD
FMPY
FSUB
FPUT
FEXT
HLI

/USE INVERSE DIV AND
lFORM A3/(X12+BS)
/FORM BZ+A5/(Xt2+85)

suB
—

/(x12+32+A3/(X¢2+Bs))

xson

'“/A2/<Xt2+32+A3/(Xt2+33>)

Bl
XSQR
Al
Be
x
PIOVZ.
ANS
‘

/Xt2+Bl+A2/(X12+BZ+A5/(X12+B3)J
_

1P1/2-XtBQ+Ai/<X12+31+A2/(...>>>
»

‘

IEXII INTERPREIER
-

Figure 8—10

‘

/FORM X12+B3

Use of the FSWl Pseudo-Instruction

Floating Halt
The floating halt pseudo-instruction is used mainly for debug—
ging floating-point code. When a floating halt is detected in‘ interpretive mode, the interpreter will halt with the address of the next
floating-point instruction displayed in the hardware accUmulator.
"The user can continue execution by pressing the CONT switch on
the programmer’s console. Normally, the floating halt pseudoinstructions would be removed or replaced by FNOPs once the
program has been debugged.
‘

SINGLE INSTRUCTION MODE VERSUS

INTERPRETIVE MODE

The relative advantages and disadvantages of each of the two

IfiOdes of FPP operation are Summarized in Table 8-3. For most
applications, interpretive mode is more convenient than single—
instruction mode and more economical in terms of core storage

requirements. However, the user who is especially speed-conscious
may employ single-instruction mode to eliminate interpreter overhead and reduce execution time.
8-27

#200
n

Jfia

iENTER
IGET X

FGET

FSOU

ISOUARE

INTERPRETER
IT

FPUT

XSOR

/STORE Xf2

FADD

/FORM X? 2+B'3

rsmna .TEMPORARI u

FPUT 5TEM
FGET A3
FDIV TEM

’

IGET

IDIVIDE BY Kt2+33

FADD

XSQR

/X'2+BZ+A3/(X?2+BS)

FPUT

TEM

ISTORE

FGET

FDIV
FADD

TEM
Bl~

TEMPORARILY

FADD

KSQR

IX?2+8l+A2/(Xf2+B2+A3/(X72+B3J>

FPUT

TEM

ISTORE

FGET

FDIV

TEM

FADD

FMPY

FNEG
FADD
FPUT

TEMPORARILY

/X(Bfl+Al/(X'2+Bl+A2/(..-)))w
AND ADD RATHER
/THAN INVERSE SUBTRACT

'AVEGATE

PIOV2
ANS

FEXT
HLT
TEM:

IEXTRA TEMPORARY
INEEDED
INOT

Figure 8-11

LOCATION
FLOATING SWITCH

USED

Program Segment of Figure 8-10 Without
FSWl Pseudo-Instruction

As an example of the'trade-offs to be considered when Choosing

between interpretive mode and single—instruction mode, the inter-

pretive modeprogram segment of Figures 8-10 and 8-1 1 is recoded
in single—instruction mode and presented in Figure 8-12 (pointers
to FPP routines are given in Table 8-1). The single instruction
mode version requires 18 (decimal) additional storage locations,
but it is approximately 800 microseconds faster than the interpretive mode version.
'

8-28

Table 8-3 Single-Instruction Mode Versus Interpretive Mode

Single-Instruction

Interpretive Mode

Full instruction set.

FISZ, FJ MP, FJ MS
and floating skips not

Instruction Set:

available.
‘

Execution Time:

Generally shorter.
(50 microseconds .or

Generally longer.

20-25% shorter for
the EAE FPP.)

Core Requirements:

Memory Field
Limitations:

Generally lower (re-

Generally up to 33%
higher (requires 2 or 3
words per operation).

quires 1 or 2 words
per Operation).

Instructions must re-

All of extended mem-

side in the field which

ory is available for
program and data

contains the FPP.

storage.
Other Advantages:

User programs may

overlay the unavailable floating-point.
.

functions and the

interpreter.
”ERROR TRAPS
The following error conditions are detected by the FPP:
1.

Attempt

to 'FIX

number

whose

greater than 2047.
2.

absolute

value

Attempt to divide by zero.
Illegal function argument ( log( X) where XIS not a positive number).

Exponent Overflow (27-bit FPP only).
Exponent Underflow (27—bit FPP only).

Exponent overflow and underflow errors are detected only by
the 27-bit FPP. These

conditions occur whenever a calculation

results in a number whose exponent has an absolute value greater
»

than 12810. The result of such a calculation is meaningless. In the
23- bit FPP’s, underflow or overflow occurs when a calculation
results1n a number whose exponent has an absolute value greater

than 61510.
8-29

JMS I
x
JMS I
JMS I
XSQR
JMS I
35
JMS I
A3
JMS 1
32
JMS I
XSQR
JMS 1

FGETP

IGET X

FSQP
FPUTP

/SQUARE IT
ISTORE X12

FADDP

IFORM xv2+35

rnrvzp
FADDP

/INVERSE DIVIDE To GET
/A3/(X12+B5)
IFORM BZ+A3/(X12+BSJ

FADDP

/Xr2+32+A3/ch2+Bs>

FDIVIP

/A2/CX12+BZ+A5/(X12+BS))

FADDP

/ADD Bl

FADDP

IADD

FDIVlP

/Al/(X12+BI+A2/(...J)

FADDP

lADD Be

FMPYP

/X(BB+A1((X12+BI+A2/(...)))

FSUBIP.

/CALL

A2
‘

JMS I
81
ans 1
xses
Jms I
At
JMS I
Be
JMS I
x
Jms I
PIOVZ
JMS 1
ANS
HLT
7526
7564
7322
vane
64:2
ssoa
s4ea

sz"

‘

FGETP,
FSQP,
FPUTP,
FADDP,
FDIVIP,
FMPYP,
FSUBIP,

INVERSEvSUBIRACT To

GET

/PI/2-X(B@+Al/(...))
FPUTP

Figure 8-12

lPOINTERS
/FLOATING
/FLOATING
IFLOATING
IFLOATING
/FLOATING

TO FPP ROUTINES:
GET

SQUARE
PUT
ADD
INVERSE DIVIDE

lFLOATING MULTIPLY
,IFLOATING INVERSE SUBTRACT

Program Segment of Figure 8-10 in Single
Instruction Mode

Error 3 leaves the FAC unchanged, while errors 1 and 2 set the

FAC to zero. Error 4 in the 27-bit FPP sets the FAC to a. very
large number. Error 5 in the 27-bit FPP sets the FAC to 0. The
contents of the hardware accumulator and multiplier quotient regis—

unpredictable when any of these errors occurs. The user
may set the interpreter to jump to a Specific location in his pro—
gram upon detection of one of these errors. To do this using the

ter are

8—30

‘

~,
“

23-bit FPP, .coreKIOCations 75.74, 7575 and 7576 shcufld be, loaded
with the address (in the same memory field) to which control
should be transferred upon detection of error 1, 2, or 3, respectively.
Using the 27-bit FPP, core locations 7574, 7575, 7576, 7573,
and 7577 should be loaded .with the address for transfer of control
upon detection of error 1, 2, 3, 4,, or 5 respectively. Normally,
these locations would, be set to point to user error message routines,
and the user program would abort upon detection of one of these
‘

errors.

EXTENDED FUNCTION ALGORITHMS
=

The algorithms to approximate the extended functions use either

.Chebyshev optimized Taylor series expansions or continued frac—
tion polynomials to calculate the value of a— function over some
small range. These algorithms provide fairly. uniform accuracy
over the specified range and, as opposed to converging series, they
‘

consist of

definite number of computations. Various extended

function algorithms .are deseribed individually in the following

paragraphs.

SIN(X)

--The argument" (the PAC in radian measure) is first reduced to
the interval:

3.". <X<71.
2
2

using 'the proper identities, and the quadrantjof theroriginal argu:ment is determined. SIN(X) is then calculated as a function of
the modified argument Y ,_____ .G(F) as follows:
‘
-

‘

Quadrant

Y_=

‘Using the Identity:

'
'

0‘

1—-F

'—F

‘

;

F'——1

SIN(Y)
«

SIN(Y)

SIN(Y)’ = SIN(qr—Y)

SIN(Y)

SIN(-f(Y-1r) )

SIN(Y)-_=SIN(—-(2-n--—Y) )

where F is the fractional portion of (1r/2)X.

Using either of the 23-bit FPP s, SIN(Y) is then calculated
8-.31

‘

over the range

(—7/2, 1/2) using the Chebychev optimized Tay-

lor series expansion:
SIN

A1Y+A3Y3+A5Y5+A7Y7

where :
A1: 1.570949
A3: ——.64592098
A5: .07948766
-.004362476
A7
=

Using the 27-bit FPP, SIN(Y) is calculated:
SIN

~725—Y

A1 Y+A3Y3+A5Y5+A7Y7+A9Y9

where 2

A1: 1.57079633
A3 2 —.6459637l1
A5: .079689679
A7 = -.00467376557
A9: .00015148419

COS(X)
The function COS(X) is calculated using the SIN function on
the basis of the following identity:

COS(X) =SIN( (1r/2) +X)
'

EXP(X)

algorithm for the calculation of EXP(X)

The

fraction polynomial to calculate e‘ as:
1

6":2°‘2
.If:

n z:

X108}
-’

=2‘

Integer part of Xloggc
part of Xlogze

r = Fractional

e1: 2an

then:
A

where:

‘

lrl<l
862

uses a

continued

Under the 23-bit FPP, the algorithm calculates 2r as follows:
.

l‘ :

1 +

—._._—————_

Y + A1

B1 + Y2
r

where:

y =

logze

“‘22

Under the 27-bit FPP, 2r

1n2
2

1442695

—_‘— .34657359

12.015017.

—601.80427

60.090191

is calculated using the values:
r1112
__

2
‘

logze -= 1.442695

“2’2

A0:

3465735903
1201501675
_

A1 = —601.804267
Bl:

600901907

ARCTANGENT (X)
The algorithm for the calculation of TAN—l (X) also Uses a continued

fraction

polynomial to
‘

calculate

the

TAN-1(X) for

0<X<1.
L

The argument is reduced to therange O<X<l

using the identi-

ties:

‘If:

x<0,

TAN—1px): ——TAN—1(X)

0<X<1, TAN—1(X)=TAN~1(X)

1<X,

TAN-1(X) -== (ar/Z)—-TAN-1(1/X)
8-33

Once X has been reduced, TAN“(X) is calculated as:
TAN“ (X): x

A‘

3., +

X2 + B1 +
..

.
.

X1 4- B"., 4-

‘

\
.

where (under the 23-bit FPP):

A,.1

X” +. B.

B"
_.17465544
3.7092563
A1
B1 = 6. 762139
~—7. 10676
AB
B2: 3.3163354
.26476862
A3 =
B3 = 1.44863154
—

‘

.
.

—

while under the 27-bit FPP:
'

~
.

.174655439
B” =
A, =
3.709256%
2:.
6.7621392
B1
-—7. 10676005
A;
3. 31633543
B2 =
.26476862
A-i =
1.44863154
33
=

LOG(X)

*Log(X) is calculated by.using a Chebyshev optimized Taylor
series to approximate the log-_.of the mantissa of X.
F : Mantissa of X

then

log, (X)

Exponent of X
.2

[I + log._,(F)] logp (2)

The logg (F) .is approximated as follows :
’

log2 (F) =CIZ +c3z3 + (3525

Under

-—

the 23- bit FPP:
Z

=(F— V75)/(F+ W5)

c1=28853913
c3 = .9614706
-

V75

.59897865
=

.7071068‘

‘logeZ :7 .6931472 7

8-34

1/2

while under the 27-bit FPP:

Z',

C1
C3
=
C5
V75
log(. 2

:
=

(F— 1/75)/(F+ V75)
2.88539129
.961470632

59897865

.707106781

.69314718

The ranges of the extended functions are as follows:
SIN, COS

—-2047<X<2046

EXP

——141.5<X<1415,

ATAN

all X

LOG

all X>O

FIX, and FLOAT routines

The SQUARE, SQUARE ROOT,
are
1n Table 8- 1.

explained

Execution Time for EAE Floating-Point Operations
Instruction times

are

for single—instruction mode calls. Add

”sec. for interpretive mode, or 71 pSCC. for interpretive mode with
'

indirect addressing.

‘

Typical Time

Operation
F ADD

160 p.860.
'

FSUB

180 psec.

200 “sec.

FMPY
p

FDIV

160 usec. or 190 psec.
I

FGET

55 usec.

FPUT
’

FSUB (with FSWl)

FDIV (with FSWl)

35 ,rsec.

215 ,usec.
210 ”sec. or 240 “sec.

8-35

Execution Time for EAE Extended Functions

Average Execution
'

Function

Range

~10“ < X < 0

SIN

2.30 msec.

0 < X < 10615
COS

2.25 msec.

—10“15 < X < 0

2.45 msec.

0 § X < 10615
ATAN

Time

240 msec.

—10615 < X < —1

2.78 msec.

-1 S X < 0

2.33 msec.

0 g X <1

2.27 msec.

1 < X < 10615

2.73 msec.

—1415 < X < 1415

2.33 msec.

EXP

LOG

SQROOT
SQUARE

O < X < 10615

2.43 msec.

—10615 < X < 10615

1.43 msec.

“10300 < X < 10300

200 ”sec.

Execution Time for Non-EAE Floating-Point Operations
Instruction times

single-instruction mode calls on the
PDP—S/E. All 55 psec. for interpretive mode, or 72.8 usec. for
interpretive mode with indirect addressing.
are

for

Instruction

Typical Time

FADD

(Operands have same

180 ,tsec. plus normalization

order of magnitude.)

time.

(Operands differ by 3
orders of magnitude.)

270

FADD

time.

(Operands difler by 6
orders of magnitude.)

360 psec. plus normalization
time.

(Operands differ by 12
orders of magnitude.)

530 ,usec. plus normalization
time.

FADD time plus 25 “sec.

FSUB
FMPY

“sec. plus normalization

(Positive

operands.)

FMPY (At least one

990 ,usec. average.
1025 ”sec. average.

negative operand.)
8-36

FDIV

1077 am. or 1113 #sec.

(Both operands
.

positive.)
(At least one

FDIV

111,8 ,Lsec. or 1153 ”see.

negative operand.)
21+41.6N ,usec. where N

FNOR
FG-ET
-

shifts are required.

57.6 ,usec.

‘

FPUT

39.6 ,usec.

FSUB- with st1

Same as FSUB.

FDIV with. st1

Add 54.0 ,isec. t5 FDIV time.

FSQU

Same as FMPY.

FSQR

Accuracy of Extended Functions
Function
SIN

1965 ,usec. average.

Approximate
Range

Accuracy (no. of significant digits)

‘1r/2<X< 7/2

—50<X<50

COS

23-Bit

27—Bit

6 digits

7 digits

6% digits

-—200<X<200

5 digits

6 digits

-—7r/2<X< 111/2

6 digits

6 digits
61/2 digits

—30<X<30

—100<X<100

‘

5 digits

‘

—125<X<125

6 digits

~89<X<88

61/2 digits

EXP
‘

—-50<X<100

5 digits

—25<X<35

7 digits

—1<X<1

6 digits

7 digits

LOG

TAN-1

0<X<1000

6 digits

—1<x<1

6 digits

all X

6 digits

8-37..

7 digits
7

7 digits

Conditions determining function accuracy:
1.

All functions

digits (7 digits under
27-bit FPP) over their primary range. Primary ranges.
are as

are

accurate to six

follows:
SIN, COS
EXP

“7r/’2<X<= 17/2
-1<X<‘=I

LOG

0<X<=1

TAN—1

—1 <X<=1

The SIN and COS functions are accurate to six digits

digits under 27-bit FPP) for all X, but the answer loses

significance as X diverges from zero.
3.

Accuracy decreases as X becomes very large, or very
close to critical points.

CORE STORAGE MAPS
User programs may overlay portions of the FPP which are not

used, such as 1/0 routines and extended functions. The following
storage maps list

core'

allocations for the Extended Arithmetic

Element FPP, the non-EAE package, and the 27-bit FPP. Page 0

storage is the same for all packages.
EXTENDED ARITHMETIC ELEMENT
FLOATING—POINT PACKAGE

Core location

Contents

7400-7577
7000—7177

Interpretive dispatch routines.
Argument fetch, FPUT, FGET, FNOR
extended functions calling sequence.
FADD, FSUB, FNEG and part of FDIV.

6600—6777

FMPY and FDIV.

6400-6577

Inverse

7200-7377
‘

and

Floating Subtract and Divide, FSQR

and FHLT.

5600-6377

FIN and FOUT.

5400-5577

FIX, FLOT and constants for extended functions.
FATN and FLOG.

5200—5377
5000-5177

FSIN, FCOS, FEXP and utility routines.
8-38

NON—EAE
Core location
7400-75 77

FLQATING—POINT PACKAGE
Contents

Interpretive dispatch routines, «FSQU, FJMP

and FJ MS

7200-7377

Argument fetch, FPUT, FGET, FNOR, ex.tended functions calling sequence and part
of FDIV.

FADD, FSUB, FNEG, part of FDIV;

7000—7177”
6600-6777

FMPY and part of FDIV.

6400-65 77

Inverse

Subtract

Floating

and

Divide, and

FSQR.
5600-6377

FIN and

5400—5577

FOUT.

FIX, FLOT and constants for extended functions.

5200-5 377

FATN and FLOG.

5000-5177

FSIN, FCOS, FEXP and utility routines

l-Part of FMPY and FDIV, and.

4600-4777

routines.

interpreter

27-BIT FLOATING-POINT PACKAGE
‘

Core location

COntents

Interpretive Dispatch Routines; Floating Square,

7400-7577

J MP, J MS.

7200-7377.

FPUT, FGET; Normalize; Extended Functions
Call Sequence; Floating Halt

7000-71 77

Floating Add, Subtract, Negate.
Floating Multiply, Divide.
Inverse Floating Subtract and Divide; Square

6600-6777
6400-6577

Root.
1

5600-6377
'

5400—5577

Floating Input and

output Routines.

FIX; FLOAT; Constants for Extended Func'

tions.

5200—5377

Arctangent; Log.

5000—5177

SIN; COS; Exponential; Utility Routines.

4600-4777

Parts of
_

Floating Multiply, Divide; Interprew
ter Routines; Argument Fetch.
8-39

PAGE ZERO STORAGE

Core location

Contents

.7.

MAP (an FPPs)

0007

Pointed to interpreter.

0040—0042

0053

Floating-point temporary storage.
Interpreter constant storage.
Floating accumulator (FAC).
Floating-point operand storage.
Valid input switch.
Last input terminator register.

0054

Line feed after carriage return switch.

0055
0056

CR/LF after output switch.
E/ F output format Switch.

0057

F format output field width specification.

nnan

F format decimal digit specification

0043

0044—0046
0047—005 1

0052

uuvv

0061

Internal
.

0062

SUMMARY OF

pointer for interpreter.
Reserved for future expansion.

FLOATING-POINT INSTRUCTIONS

Memory Reference Instructions
Code

Instr
FJMP

0000

FADD

1000

FSUB

2000
3000

FMPY

'FDIV

4000

FGET

5000

FPUT

6000

FJ MS

7000

8—40

Expanded Instructions
Instr

Code

FISZ=

FEXT=

FSQU=
FSQR=

FSIN=

FCOS=

FATN=

FEXP:

FLOG=

FNEG=

F IN:

(Square)
(Square Root)

FOUT=

1.2

FF IX:

PLOT:

FNOP=

'FNOR=

7000

FCDF:

7001

(Available to User)
(Available To User)
-

FSWO=

7002-

FSW1 :

7003

FHLT=

(Available To User)

(Bits 6-8 New Fltg.
Data Field)

7004
'

FNOP=

7005

FNOP:

7006

FNOP:

7007

FSMA=

71 10

FSZA=

7050

FSPA:

7100

FSNA:
FNOP=
FSKP=

7040
70107020

3—41

Chapter 1
.

Answers to selected exercises on page 1-10
2.

10010

12.

1100100

14.

111

110

16.

1110101

18.

101
1
110
001
011
110
110 000 000

10.

111

20.

110

10.

_12-

3641
4087‘
63
4095

b.2.

111

010

14.
'

16.

111

011

101

110

111

Answers to selected exercises on page 1-14
2.

6.
8.

6
575
40
30

10.
12.

7777

14.

16.

255
2372

10.

110 100.

7664

b.2.

111 011

110

1 000

101

1 001 000 001
'

14.

111 110 101
100 101 011 010

16.

1 010 100 011

2500
6005
7777

_12.

100 010 100

111

‘

1104

10.
12.

6.'3
_

d.2.
4.

512
482

8.
10.

4095

12.

174

2431

Answers to selected exercises on page 1-26
2.

6.
b.

110
10 111 000
1 100

10.

10 001

12.

1 010 010 101

One’s Complement

Two’s Complement

000 100 000

101 000 100 001
000 000 000 000

101
111

‘

1—
1—-\
P

F”

wwpwgew

111 111 111
111 011 011 011
011 111 111 111

011

110 011 001
000 000 000 000

100

‘111 011 011

101

100

100 000 000 000
011 110 011 010
000 000 000 001

101 000 101

11 001

110 110 010

1 010 011

110 101

111 010 001

100

g. 2.
4.

1331

110

3623

h. 2.

205
1105

667
2767

204

433,254
172,166

25
112

Chapter 2
Answers to selected exercises
1. The locations listed in parts b,'f, g, h, and i must be addressed indirectly. All others may be addressed directly.
2.

Group 2

(SZL)
(AND)
(CLA CMA)
(NOP)
(JMS)
(SMA CLA)

MRI

b.099“?

Group 1
Group 1
MRI
Group 2

AND 0

182 Y

Parts at, c, and 8 contain digits which can not be represented with
binary numbers. Part b has too many digits to be represented by
lZ—bits. Part d is- a legal instruction if a leading zero is assumed.
.

The logical AND of the AC with the contents of
location 0 replaces the accumulator.
Increment the contents of location 100

the

current page and skip the next instruction if the
contents become 0 after the incrementation.
e.

DCA I Y
.

Deposit and clear the accumulator indirectly into
the location whose address is contained in location 100.

g. TAD 30

Two’s complement add the contents of location 30
to the accumulator.

1. .

JMP Y

Transfer program control
current page of memory.

CLA CMA CML

can?!»

SZA
SPA SNA
CLA SPA SNA SZL

A—2

location 73

the

After Execution
Location
Content (octal)
AC
0000
205
1537
206
2241
207
4000'

Program:
7200

1205,,

1206
3207

‘

7402

’

1537

2241
0000

7360

7710

illegal

One instruction may not be used to rotate once and"
rotate twice at the same time. On the PDP~8 and
PDP-8/ S it is also illegal to combine an increment
and a rotate microinstruction, thus part dis legal on the
PDP-S/ I and PDP-8/ L but it is illegal on the PDP-S
and PDP-S/S.

‘

One instructibn may not include

g.‘ illegal

'
‘

"i.

illegal
"

groups
One instruction may not
Group 1 and Group 2.

members of both skip

combine microinstructions from
-

8. SZA
SKP
SNL
Instruction to, be skipped

9.-— Any testing of the accumulator is .done before the OSR instruction

is executed.
10.

a.-

,
1

Location
200
202

203'

405
406‘
407

TAD 210
TAD 211.
DCA 212
HLT
0002
0010
0000

1210
1211
.3212
7402
'

Content

CLA
TAD“ 550
DCA 552
TAD 551
DCA 550
TAD 552
DCA- 551
"HLT

A-3

0002.
0010
0000

402
403
404

.7200

Location
400
401

Octal

Content
CLA

201

204
.210
211
212

Octal
7200 -_
1350
3352

1351
.

3350

1352
3351
7402

‘

Chapter 3
Answers to selected exercises

/ SUBROUTINE TO SUBTRACT TWO NUMBERS

*fim
LUV

CLA CLL
TAD K1200
JMS SUB

START;

1500
HLT

*300

0
CIA

SUB,

TAD I SUB
ISZ SUB
‘

JMP I SUB
1200

K1200,
$

‘23}:
/ LOAD LOCATIONS 2000 TO 2777
*200
CLA CLL
START,
TAD K2000
DCA LOCPTR
DCA COUNT
TAD COUNT

DEPOSIT,

DCA I-LOCPT-R
ISZ COUNT

Tsz LOCPTR

TAD LOCPTR
TAD M3000
SZA CLA
IMP DEPOSIT
HLT
0
2000
0
—-3000

COUNT,
K2000,
LOCPTR,
M3000,
$
‘

/TRIPLE PRECISION ADD
‘
.

*2“)

1mm.

‘

CLA CLL
TAD AL
TAD BL
DCA ANSL
RAL
TAD AM
TAD BM
DCA ANSM

TADAH’

TAD: :BH
DCA ANSH
HLT
1211
0314

AH,
AM,
AL,
BH,
BM,
BL,

7125
0114
4157

0176
0

ANSH,

0
ANSM,
0
ANSL,
$
/DOUBLE PRECISION RESULT
-

*200

CLA CLL
TAD A
CIA
DCA MINUSA
TAD"‘B

START,
"

SZL

ISZ CH
NOP
CLL

ISZ MINUSA
IMP .—6
'DCA CL

HLT
0

MINUSA,
A,
B,
CL, ,
CH,
$

0011
1234

/ DOUBLE PRECISION MULTIPLE OF 2
*200
CLA CLL
START,
DCA NH
TAD EXP
'

‘

ROTATE,

CIA
DCA MINUSE
TAD N

DCA N
TAD NH
RAL

DCA NH

'A—S

CLL
ISZ MINUSE
IMP ROTATE
HLT
-

N,
NH,
EXP,
MINUSE,
$

1234
0

‘

/How MANY NEGATIVES?
*200
CLA CLL
START,
DCA NEGS
TAD K2777
DCA 10
TAD M1000
*DCA COUNT
TAD I 10
TEST,
SPA CLA
ISZ NEGS
ISZ- COUNT
JMP TEST
TAD NEGS
HLT
»

‘

NEGS,
K2777,
M1000,

2777
—-1000
0

COUNT,
$

/20 SECOND DELAY
*200
TAD CONST
START,
‘DCA COUNT
‘

TAD. CONsrl
-

DCA COUNT1
ISZ COUNT1
JMP .-1
Isz COUNT
JMP .-3_
HLT
'

6030

CONST,
COUNT,
CONSTl

#41000 DECIMAL

0
,

COUNT1,

/ 52‘ DECIMAL

A-6

/ 20 OR 40
*200

START,

SECOND DELAY
CLA CLL
TAD M2

HLT

OSR

DCA TWICE
TAD CONST
DCA COUNT
TAD CONSTl
DCA COUNTl
ISZ COUNTl’
J MP .-—1
ISZ COUNT
JMP .—3
ISZ TWICE

DELAY,

J MP

DELAY

HLT
5703

CONST,
COUNT,
CONST1,
COUNTl,
M2,
TWICE,
$

44
0

—2
0

Chapter 4
'

Answers to selected exercises
3.

/ SET LOCATIONS TO SWITCH REGISTER
/ VALUE
.

LOC.

*200

CONT.

0200 7300

1214
0202 3215
0203 1213
0204 3216
0205 7404
0206 3615
0207 2215
0210 2216
0211 5205
0212 7402
0213 7770
0214 2000

0215 0000
0216 0000

CLA CLL
TAD K2000

0201

‘

M10,
K2000,
POINT,
COUNT,.
$

DCA POINT
TAD M10
DCA COUNT’
OSR
DCA I POINT
ISZ POINT
ISZ COUNT
JMP .—4
HLT
7770
2000
0
0

A-7

/ ADD TWO NUMBERS AND DISPLAY SUM
Loc.

CONT.

71nn

v—vv

*200
(‘IA

.avv

V“;

0201 7402

("11'

3211

HLT
OSR
DCA A

7402

HLT

7404

OSR'
TAD A
HLT
JMP .—10
0

0202 7404

0203
0204
0205
0206
0207
0210
0211

1211
'

7402

5.200
0000

(flquflnr6
Answers to selected exercises
1.

/ SUBROUTINE ALARM AND CALLING FOR IT
*

200

START,

ALARM,

CLA CLL
TLS
JMS ALARM
HLT
0
TAD M5
DCA RINGS
TAD KBELL
JMS TYPE
ISZ RINGS
JMP .—3
JMP I ALARM
_

TYPE,

TSF

JMP .—1
TLS
CLA CLL
JMP I TYPE

M5,

—5

RINGS

KBELL,

207

/ ASCII FOR THE BELL

A-8

/TAB SPACE THE TELEPRINTER
*200

CLA CLL

START,

TLS
HLT
OSR
JMS TAB

'/ ACCEPT NUMBER OF
[SPACES FROM SR

JMP .—3

'/READY. TO TAB MORE

TAB,

CIA
DCA NUMTAB
TAD ‘KSPACE
.

JMS TYPE
ISZ NUMTAB
’JMP

.—3‘

JMP I TAB
0
TSF

TYPE,

JMP .—1

TLS
CLA CLL
-JMP 1 TYPE
0

NUMTAB,

KSPACE,

240

/TEST ANSWER SHEET
*200
.

START,

CLA CLL

TLS

HEADING,

TAD HEAD1
DCA POINTR
TAD AMOUNTDCA COUNT
JMS CRLF
.V

TAD 1

POINTR

JMS TYPE
ISZ POINTR-

ISZ COUNT
JMP .—4
JMS CRLF

NUMBRS,

TAD K260
DCA INTS
ISZ INTS
TAD INTS
JMS NUMTYP
TAD INTS
TAD M271
SZA CLA
JMP .—6

A-9

TEN.

TAD- K260

IAC
JMS TYPE
TAD K260

JMS NUMTYP
HLT

HEADI

HEAD

POINTR.
HEAD.

0
-'

324

/H
/I
/s
/T

3:
31 1
323,

317

322

331

240

/SPACE
/T

324
305
323
324

AMOUNT,
COUNT,
K260,
INTS,
M271,
K215,
K212,
,

K256,

TYPE,

—14
0

/E
/s
/T

/# OF HEADING CHARACTERS

260
0
~271

215
212
256

/NEGATIVE OF ASCII FOR A 9
/ ASCII FOR CR

/ASCII FOR LF
/ ASCII FOR PERIOD

0
TSF
J MP .—1

TLS
CLA CLL
J MP I TYPE

CRLF,

TAD K215
J MS TYPE
TAD K212
J MS TYPE

IMP I CRLF

NUMTYP,

O
J MS TYPE

TAD K256
J MS TYPE
J MS CRLF
J MP I NUMTYP

A-10

/TWO DIGIT OCTAL SQUARE CONVERSATIONAIA

/ PROGRAM

*200
CLA CLL

START,

TLS
J MS CRL'F'

/GET FIRST DIGIT

J MS LISN.
.TAD M260

~RAL
RTL

CLL
’

DCA NUMBER
JMS LISN
TAD M260
TAD NUMBER
DCA NUMBER~

MULT,

TAD NUMBER

CIA
DCA TALLY
TAD NUMBER
ISZ TALLY
JMP .——2
DCA NUMSQR
TAD MESAGl
DCA POINTR
TAD M10
DCA ENDCHK
J MS MESAGE
TAD M4
,

TYPSQU,

‘

TYPANS,

UNPACK,

DCA DI'GCTR
DCA STQRE
TAD NUMSQR
CLL RAL
TAD STORE
RAL
RTL
DCA STORE
TAD STORE
AND K7
TAD K260
JMS TYPE

TYPOCT,

ISZ DIGCTR
JMP UNPACK
TAD MESAGZ
DCA POINTR
TAD M7
DCA ENDCHK
JMS MESAGE
‘JMS CRLF,

JMP START+2

A—ll

'/GET SECOND DIGIT
'/NUMBER IS NOW IN AC

TYPE,

,TSF
JMP .——1
TLS
CLA

CRLF.

JMP I TYPE
0

LISN,

TAD K215
M
1' Y P
TAD K212
JMS TYPE
JMP I CRLF
0

KSF
JMP

.—

KRB

MESAGE,

TLS
JMP I LISN
0

TAD I POINTR

NUMBER,
M260,
TALLY,
NUMSQR,
MESAG]

POINTR,
M10,
ENDCHK,
STORE,
M4,
DIGCTR,
K7,
M7,
K260,
K212,
K215,
MESAGZ,
STARTI,

JMS TYPE
ISZ POINTR
ISZ ENDCHK
JMP .—4
JMP I MESAGE
0

-—260

0
0

STARTl
0

-—10
0
0
—4
7

—7

260

212
215
STARTZ
323

321

325

/U
/A
/R
/E
/D
/=

301

322
305
304

275

A-12

‘

STARTZ,

240

/SPACE

‘317

/O
/C
/T
/A
/L

303
324
301
314
256

’

/PERIQD

A-13

ASCIIl- Character Set

Octal

6-Bit
Octal

8-Bit

Character

6-Bit
Octal

8—Bit

301

241

302

”

242

C
D

243

244

303

304

$
%

305

306

307
310
31 1
3 12

06
07'
10

245

45‘

246

247

250251
252
253
254

50
51

/
':

255
256'
257
272

55
56
57

273

‘

&
’

(
)

313

3 1-4

3 15

16*

316
317
320

321

;

322

274

s
T
U

323

3 24

325

326‘

27.

327
330

@
[

275
276
'277
300

3 31

]

Z
O

332
260

T(/\)2

60
61
62
63
64
65

<-(— 2

2,
3
4

261
'

262

263
264

265
266
267

4270

27 1

5
6

Octal

H
_

Character

>
?

77
33
34

35
36
37

Leader/Trailer

335
336
337
200

LINE FEED

212'

Carriage RETURN
SPACE

215
240
377
000

RUBOUT
'Blank

BELL

TAB

21 l

FORM

214

207

71
0

1 An
2

333

53
54

334

abbreviation forwArnerican Standard Code for Information Interchange.
The character in parentheses is printed on some console terminals.

B—l

.CHARACTEKCQDES
C

8-bit
ASCII.
Code

"DEC 026
Card

EEC 029
6-bit

‘

Code

Card
Code

Code

S
2 5,
_

a 3‘,

Remarks

a a

c»

o a:

240‘
241
242
243
244
245
246
247

250
251
252
253
254
255
256
257

260
261
262
263
264

265
266
267

270
271
272

41
42

43
44
45
46
47

51
52
53
54

8—5

blank
12—8—7
0-8-5
0—8—6
11—8—3
0—8—7
11—8—7
8—6

12—8—5
11—8—5
11—8—4

0—8—4
12—8—4‘"
11—8—4

blank
11—8—2
8—7
8—3
11—8—3
0—8—4
12

5
'

&
'

opening parenthesis
closing parenthesis

(
)

asterisk

plus

12—6—6

0—8—3

56
57

128—3

12—8-3

0—1

0
1

62
63
64
65
66

2
3

5
6
7

2
3
4
5

8
9
8—2
11—8—6

8
9
11—8—2
0—8—2

8
9

12—8—4
8-6
0—8—6
0—8—7

1.2—8—6

8—3
11-8—6
12—8—2

8—4
12—1
12—2
12—3
12—4
12-5
12—6
12—7

8—4
12-1
12—2
12—3
12—4
12-5
12—6
12—7

273
274
275
276
277

300
301
302

00
01
02

303

304
305
306
307

04
05

74-

75
76
77

06,
07

quotation marks
number sign‘w’
dollarsign
percent
ampersand
apostrophe or acute accent

0—8—3

exclamation point

space (non-printing)

comma

minus sign or hyphen

—

period or decimal point

3
4

slash

5
7

colon

;

semicolon

;
_

=
1

at sign

A
B

0
D
E
F
G

less than
equals
greater than
question mark

CHARACTER CODES
c

[8-bit
ASCII
Code

6-bit
Code

2 5

DEC 026
Card
Code.

DEC 029
Card
Code

8 3

o I;

310
311
312
313
314
315
316
317

12—8
12—9
1 1 —1
1 1~2
1_ 1 —3
11—4

10
1 1

12
13
14

15
16
17

1 1 —7

323
324
325
326
327

23
24
25
26
27

11-8
11—9
0-2
0~3
0—4
0—5
0—6

330
331
332
333
334
335
336
337

30
31
32
33
34
35
36
37

H
l
J

1 1-1

K
L
M
N

1 1 -2
1 1 ~3
.

11~4

11-5
1 1—6

1 1—6

‘21
22

12—8
1 2~9
'

1 1—5

320
321
322

0—7
0~8
0—9

12—8—2‘5’

11—84(6)
0—8—2

12—8—7”)
0—8—5‘31

Remarks

E :3;

‘

11—7
11—8
11—9

P
Q

0—2
0—3
0-4
0-5
0—6

8
T

‘

0-7
0—8
0—9
11—8—5
8—7
12—8—5
8—5
8—2‘3)

U
V
W
X
Y
‘

[

opening bracket, SHIFT/K

backslash. SHIFT/List

]

closing bracket, SHIET/ M

‘

circumflex”)
underline‘W

Footnotes:
(1) On some DEC 026 Keyboards this characterIs graphically

represented 330
(2)
‘

(3)

(4)
(5)

On most DEC Teletypes circumflex is replaced by up—arrow (t).
Acard containing this code in column 1 with all remaining
columns blank is an end—of—file card.
On most DEC Teletypes underline is replaced by backarrow (<~).
On some 029 keyboards this character is graphically represented
as a cent

(6)
(7)
(8)
(9)

(10)

sign (G).

On some 029 keyboards this character is graphically represented
as logical NOT (—).
On some 029 keyboards this character is graphically represented
as vertical bar ( I).
On some LP8 line printers the Character diamond (0)is printed
instead of backslash.
On some LP8 line printers, the character heart (0)Is printed
instead of underline
The number sIgn on some terminalsIs replaced by pound sign(£ ).

B-3-

apartial list of flowchart symbols which can be used

The following is
~

to'diagram the logical flow of a program. The symbols, may be made
sufficiently large to include the pertinent informatiOn.
The direction of flow in a program is represented by lines drawn between symbols. These
lines indicate the order in which the operations are to be performed. Normal direction

REPRESENTATION
OF FLOW
LEFT

TO RIGHT

oa'

RIGHT

TO LEFT
-

of flow is from left to right and top to bottom". When the flow direction is not from left
to‘ right or top to bottom, arrowheads are
placed on the reverse direction flowlines.
Arrowheads may also be used on normal flow
lines for increased clarity.
.

TOP

‘

T0
aorrou

on
-

BOTTOM
TO
TOP

TERMINAL

The. oval symbol represents a terminal point
in a program. It can be used to indicate a
start, stop, or interrupt of program flow. "The
appropriate word is. included within the

symbol.

PROCESSING

The rectangular symbol represents a processing function. The process which the symbol
is used to represent could be an instruction or
a-

grOup of instructions to carry out a given

task. A brief description of the task to be performed is included within the symbol.

DECISION

A diamond is used to indicate a point in a
program where a choice must be made to determine the flow of the program from that
point. A test condition is included within the

symbol and the possibleresults of the test are
.used to label the respective flows from the
symbol.

’

symbolis used to represent an operagroup of operations not detailed in
the flowchart. It is usually detailed in another
flowchart. A subroutine is often represented
This

PREDEFINED
PROCESS

tion

in this manner.

CONNECTOR

The circular symbol

represents an entry from

an exit to another part of the
program
flowchart. A number or a letter is enclosed
to label the corresponding exits and entries.
This symbol does not represent a

.
.

program

Operation
ANNOTATION

F“—

An addition of

descriptive comments or ex-

planatory notes for clarification is included
within this symbol.

INPUT/ OUTPUT

This

symbol is used in a flowchart to repre-

sent the input or output of information. This

[:7

symbol may be used for all input/ output
functions, or symbols for specific types of input or output (such as those which follow)
may be used.

This

symbol may be used to represent the

W Cl

line keyboards, switch settings, etc.

PUNCHED

The input Or outpUt of information in which

TAPE

MAGNETIC
TAPE

manual input of information by means of on-

the medium is

punched tape may be repre-

sented by this symbol.

This symbol is used in a flowchart to repre»
sent magnetic tape

input or output.

Page

«1

D-l
D-2
D-3

PDP-S/ E Memory Reference Instructions
Loading Constants into the Accumulator

....................

I
2

Group
Group
Operate Microinstructions
3
Group
Operate Microinstructions
Programmed Data Transfer Instructions
KM8—E Memory Extension

5
-6
7
8
-9

pucoedeucouovueo

Operate MiCroinstructions

D—4

............................

D-5
D-S

......................

KES-E Extended Arithmetic Element
Teletype Keyboard/ Reader

D-S
D—6

..........................

D-7

..........................................

Teletype Teleprinter/Punch
Readers
2 PPS-E Paper Tape Punch

D-7

..........
‘

I PR8-E Paper Tape
'

.............................. ............

3 PC8- E Reader/Punch
4 TC08-P DECtape Control
5

TC58 DECmagtape System

D-7
D-7

............................................

D-8

..............

D—8

...............

D-8

..........................................

6 RKOS- P Control and RKOl Disk Drive and Control
7 DF32—D Disk File and Control

D -9

8 RF08 Disk File
9 ~TM8-E/F Control

D- 10

D—9

....................................

...........................................................

...................

'._

D- 10

.....................

0 LE-S Line Printer

.....
_

D-ll

1 CRS-E Card Reader and Control or
..............

......... .....................

VCS-E'CRT Display Control

4 VWOl

...............

.................

Writing Tablet
5 DC02-F 8- Channel Multiple Teletype Control
-26 BBOS- P General Purpose Interface Unit
27 Universal Digit Controller
8 DR8- BA 12- Channel Buffered Digital I/O
9 MP8- E Memory Parity
O Synchronous Modern Interface
1 Multicycle Data Break Locations

.....................................................

...... ......

.......

...........

~. ..........

..................................................

....................................

..................................

2 KM8—E Time-Share

..............

3 DKS-EP Programmable Real Time Clock
4 DK8- EA Line Frequency Clock ..~1
5 DK8- EC Crystal Clock .:

..................

...............................

................ ................................
.

-36 KP8-E Power Fail Detect
-37 DP8-EP Redundancy Check Option
D-38 DRS-E Interprocessor Buffer
D-39 ADOlA 12-Bit Analog-to-Digital Converter
D-4O AASO Digital-to-Analog Converter
D-4l ADS-EA A/ D Converter
D-42 AAOSA/AA07 Digital- to-Analog Converter
and Control
D-43 AFC-8 Low-Level Analog Multiplexer
D-44 AF04A Guarded Scanning Integrated Digital
Voltmeter (IDVM')

..............

;

..........

............................

........................................

................

..............................

..............................................

............................................................

........................

D-3

............................

..........................................

-23

“D21

............................

Optical Mark Card Reader and Control
-22 XYS-E Incremental Plotter Control

uuuueuueuc

D-2

..............

..............................................

D-l

D-ll
D-ll
D—12
D-12
D-12
D-13
D-l3
D-l3
D-14

‘D-l4.
D-lS

D-15

D-lS
D-15

D-l6
D-l6
D-l6
D-l6
D-l'l
D-l7
D-17

mm
mm
D-18

Table

Dfl PDP-B/ E Memory Reference Instructions
(Refer to Chapter 3)

Execution Times

Mnemonic Octal
Code

Indicators

AND Y

IR:0,F,E

Symbol

Direct
Address

Indirect
Address

Auto}ndexed

2.6

3.8

4.0

IR 2 LEE

'Logical _AND
between Y and
AC

TAD Y

Operation

2.6

3.8

4.0

Two’s comple-

ment Add Y to

AC
‘

rsz

IR: 2,F,E

2.6

3.3

DCA Y

IR=3,F,E

2.6

3.8

4.0

Increment Y
and skip if zero

4.0

Y
and clear AC

Deposit

JMS

IR=4,F,E

2.6

3.8

4.0

subroutine at Y

IMP

IR : 5,F

1.2

2.4

2.6

Jump to Y

Jump

Table D-2 Loading Constants Into The Accumulator

Mnemonic

Decimal
Constant

Octal
Code

NLDOOO =
NL-DOOI ==
NLOOOZ ::

7300
7301.
7305

NL0002 :
NL0003 :
NL0004 :
NL0006 :
NLOIOO:
NLZOOO :

7326

7325
7307
7327
7203

Instructions Combined

CLA CLL
CLA CLL IAC
CLA CLL IAC

RAL

(or)

NL3777 :
141.4000:
NLS777 :

NL6000 :
NL7775 :
NL7776 :
NL7777 :

6
64
1024

7330
7352
7333

~1024
—~—3
——-1

7350

~O
~1025

RTL
CLA CLL CML IAC RAL
CLA CLL IAC
RTL
CLA CLL CML IAC RTL
CLA IAC BSW
CLA CLL CML RTR
CLA CLL CMA RAR
CLA CLL CML RAR
CLA CLL CMA RTR
CLA CLL CML IAC RTL
CLA CLL CMA RTL
CLA CLL CMA RAL
CLA CLL CMA
'

7332

2047

-—2

CLA CLL CML

7346
7344
_

7340

D~2

Table D-3

Octal

Mnemonic
Symbol

NOP

Group 1 OperateMicroinstructions

Code

7000

Sequence

Operation.
No

operation Causes
program delay.

——

IAC

7001

fig

RAL

7004

1.2

Increment AC. The content of
.the AC1s incremented by one in
two’s complement arithmetic.
Rotate AC and L left. The con-

tent of the AC

and the L
rotated left one place.

RTL
'

7006
7010

7012.

the ”left.
successive

Rotate

Equivalent to two
RAL operations.
Rotate AC and L right. The con-

two'

places

tent of the AC and L are rotated

RTR

are

RAR

its

right one place.
Rotate two places to the right.
Equivalent to two successive

RAR

BSW

7002‘

CML

7020

CMA

7040

Byte swap.
Complement L.
Complement AC. The content of
.

CIA

7041

operations.

2, 3

the AC is set to the one’s com—
plement of its current content.

Complement

and increment acto form two’s

cumulator. Used
CLL

7100

CLL RAL

7104

1, 4

CLL RTL

7106

1, 4

CLL RAR

7110

CLL RTR

7112

STL

7120

1, 4
1, 4
1, 2

complement.
Clear L.
: Shift
positive number one left.
Clear link, rotate two left.
Shift positive number one right.
Clear link, rotate two right.
Set link. The L is set to contain
a

CLA

7200

CLA IAC

7201

binary 1.

Clear AC. To be used alone or in
CPR 1 combinations.

Set AC 2 1.

GLK

7204

1, 3
1, 4

CLA CLL

7300

Clear AC and L.

STA

7240

Set AC =.—1 Each bit of the AC
is set to contain a 1.

Get link. Transfer L into AC11.

D—3

Table D4 Group 2 Operate Microinstructions

Mnemonic
Symbol

Octal
Code

Sequence

Operation

HLT

7402

Stops the program after
completion of the cycle in proHalt.

If this instruction is combined with others in the CPR 2
group the other operations are
completed before the end of the
cess.

cycle.
OSR

7404

OR with switch register. The OR
function is performed between
the content of the SR and the
content of the AC, with the result left in the AC.

SKP

7410

SNL

7420

SZL

7430

7440

7450

7460

Skip, unconditional. The next instruction is skipped.
Skip if L i 0.
Skip if L 0.
Skip if AC : 0.
Skip if AC 75 0.
0, or L 9f 1, or
Skip if AC

SZA
SNA
SZA

SNL

both.
V

SNA SZL

7470

7500

SMA

O.
Skip if AC ;£ 0 and L
Skip on minus AC. If the content
of the AC is a negative number,

‘

the next instruction is skipped.

SPA

7510»

SMA SNL

7520

Skip on positive AC. If the content of the AC is a positive number, including zero, the next instruction is skipped.
1, or
Skip if AC < 0, or L
=

both.
'

SZL

7530

SMA SZA

7540

SNA

7550

CLA

7600

LAS

7604

SPA
SPA

CLA

7640

SNA CLA

7650

SMA CLA

7700

SPA

7710

SZA

CLA

HH‘H VNNW

Skip if AC 2 0 and'if L
Skip if AC < 0.
Skip if AC > 0.

Clear AC. To be used alone or in
CPR 2 combinations.
Load AC with SR.
Skip if AC : 0, then clear AC.

Skip if AC 75 0, then clear AC.
Skip if AC < 0, then clear AC.
Skip if AC 2 0, then clear AC.

'D—4

TaHeD-SGroanOperateMicminstnctions
Octal
Code

Mnemonic

Symbol
NOP

Operation

7401
7421
7501
7521

..MQL
MQA
SWP

No Operation

Load Multiplier Quotient
Multiplier Quotient OR into Accumulator
Swap Accumulator and Multiplier

Quotient
7601

CLA

Clear Accumulator

CAM

7621

Clear Accumulator and Multiplier

7701

Clear Accumulator, Load Multiplier
Quotient into Accumulator (CLA

Quotient (CLA MQL)

‘

ACL

MQA)
Load'Multiplier Quotient into Accumula;
tor, Clear Multiplier Quotient
'

,
‘

7721

CLA SWP

-...

.Ta'ble D-6 Brogrammed Data Transfer Instructions

Mnemonic

Octal
Code

Symbol
~

ION
IOF

6001
6002
6000
6003

SKON

SRQ
GTF
RTF
'SGT
CAF

6004
6005
6006

6007

Operation
Interrupt Turn On
Interrupt Turn Ofl
Skip if Interrupt On, IOF
Skip if Interrupt Request
Get Flags
Restore Flag, ION
Skip if “Greater Than” Flag is Set
Clear All Flags
.

Table D-7 KM8~E Memory Extension

Mnemonic

SYmbol

Octal
Code

6004
6005
62N1
62N2

GTF
RTF
CDF
CIF
CD1

62N3

Operation
Get Flags
Restore Flags, ION
Change to Data Field N (N20 to 7)
Change to Instruction Field N (N20 to 7)
Change Data Field, Change Instruction Field
-

(CDF CIF)
RDF
RIF
RIB
RMF

6214
-

6224

6234
6244

Read Data Field
Read Instruction Field
Read Interrupt Buffer
Restore Memory Field

D-S

Table D-8 KE8-E Extended Arithmetic Element
.

Mhnmnnun

nnfn‘

Symbol

Code

ileJ lenvlllv

VUG‘JL

Operation

MODE CHANGING INSTRUCTIONS
SWAB
SWBA
SKB

7431
7447
7471

Switch from Mode A to B
Switch from Mode B to A
Skip if Mode B

STANDARD INSTRUCTIONS
7621
7501

CAM

MQA
ACL

MQL

7701
7421

SWP

7521

o—>

VAC, 0» MQ

MQ “OR”ed with AC~> AC
MQ—>AC (MQA CLA)
AC-> MQ, 0—> AC
AC—> MQ, MQ—+ AC

MODE A INSTRUCTIONS
SCA
SCA CLA
SCL
MUY
DVI
NMI
SHL
ASR
LSR

7441
7641

7403
7405
7407
741 1
7413

7415
7417

Step Counter “OR” with AC
Step Counter to AC
Step Counter Load from Memory
Multiply
Divide
Normalize
Shift Left
Arithmetic Shift Right
Logical Shift Right

MODE B INSTRUCTIONS

7403
7405
7407
741 1
7413
7415
7417

ACS
MUY
DVI
NMI
SHL
ASR
LSR

AC to Step Count

Multiply

’

Divide
Normalize
Shift Left
Arithmetic Shift Right

Logical Shift Right

DOUBLE PRECISION INSTRUCTIONS
'

DAD

DST
DPIC

DCM

DPSZ

7443
7445
7573
7575
7451

Double Precision Add
Double Precision Store
Double Precision Increment
Double Precision Complement
Double Precision, Skip if Zero

D-6

Table D-9 Teletype Keyboard! Reader
Mnemonic

Octal
Code

Symbol

Operation
'

KCF
KSF
KCC

6030
6031
6032

KRS
KIE
KRB

6034
6035
6036

Clear Keyboard Flag
_.

Skip on Keyboard Flag

Clear Keyboard Flag, and AC, Advance
Reader
ReadrKeyboard Bufier Static
Set/ Clear Interrupt Enable
Read Keyboard Buffer, Clear Flag

\Table D-10 Teletype Teleprinter/ Punch

Mnernonio
Symbol

Octal
Code

TFL
TSF
TCF
TPC
TSK
TLS

6040
6041
6042
6044
6045
6046

Operation
Set Teleprinter Flag
Skip on Teleprinter Flag

Clear Teleprinter Flag
Load Teleprinter and Print
.Skip on Printer or Keyboard Flag
Load Teleprinter Sequence

Table D-ll PR8-E

Paper Tape Readers
‘

Mnemonic

Symbol
RPE

RSF
RRB
RFC

RCC

Octal
Code

Operation

6010
6011
6012
6014
6016

Set Reader/ Punch Interrupt Enable
Skip on Reader Flag
Read Reader Buffer
Reader Fetch Character
Read Buffer and Fetch New Character
.

(RRB, RFC)

PCE

6020

Clear Reader/ Punch Interrupt Enable

Table D-12 PP8-E Paper Tape Punch

Mnemonic

Symbol

Octal
Code

Operation

RPE

6010

Set Reader/ Punch Interrupt Enable

PCE
PSF
RCF
PPC
PLS

6020

Clear Reader/ Punch Interrupt Enable

6021
6022
6024

Skip on Punch Flag
Clear Punch Flag
Load Punch Bufier and Punch Character

6026

Load Punch Buffer Sequence

D—7

Table D-13 PCS-E Reader/ Punch

Octal
Code

Mnemonic

Symbol
RPE
RSF
RRB
RFC

6010
6011

Operation
Set Reader/ Punch Interrupt Enable
Skip on Reader Flag
Read Reader Buffer
Reader Fetch Character
Read Buffer and Fetch New Character
Clear Reader/ Punch Interrupt Enable
Skip on Punch Flag
Clear Punch Flag
Load Punch Buffer and Punch Character
Load Punch Butter Sequence

‘

6012
6014

’

6016

RFC, RRB
PCE

6020

PSF
PCF
PPC'
PLS

6021
6022
6024
6026

Table D-14 TC08-P DECtape Control
Mnemonic

Octal

Symbol

Code

Operation

(as)

DTRA
DTCA
DTXA
DTLA
DTSF
DTRB
DTXB

6761
6762
6764
6766
6771
6772‘
6774

Read Status Register A
Clear Status Register A
Load Status Register A

2.6
2.6

‘

Time

Clear and Load Status Register A

Skip on Flag
Read Status

Load Status Register B

Address Locations:

2.6
3.6
2.6
2.6
2.6

7754 2 Word Count
7755 = Current Address

Table D-15' TC58 DECmagtape System

Mnemonic

Symbol

Octal
Code

MTSF
MTCR
MTTR
MTAF

6701
6711
6721
6712

MTRC

6724
6714
6716
6704

MTCM
MTLC
none

MTRS

MTGO,
none-

6706
6722
6702

Operation
.

Skip on Error Flag or Magnetic Tape Flag
Skip on Tape Control Ready
Skip on Tape Transport Ready
Clear Registers, Error Flag and Magnetic
Tape Flag
Inclusive OR Contents of Command Register

fInclusive OR Contents of AC
Load Command Register

Inclusive OR Contents of Status Register
Read Status Register
Mag Tape “GO”
Clear AC
7

D48

Table D-16 RK08-P Control and RK01 Disk Drive and Control
'

Mnemonic

Symbol

Octal
Code

DLDA

6731

DLDC

6732
6733
6734
6735
6736
6737
6741
6742
6743
6745
6747
6751
6752
6753
6755
6757-

Time

(as)

Operation
—“

Load Disk Address

2.6

(Maintenance Only)

DLDR
DRDA
DLDW
DRDC
DCHPDRDS
DCLS
DMNT
DSKD
DSKE
DCLA
DRWC
DLWC

“

_DLCA

DRCA

Load Command Register
Load Disk Address and Read

26
2.6
2.6
2.6
3.6
4.6

Read Disk Address
Load Disk Address and Write
Read Disk Command Register
Load Disk Address and Check Parity
Read Disk Status Register
'

2.6

Clear Status Register
Load Maintenance Register

Skip on Disk 'Done
Skip on Disk Error

‘

Clear All
Read Word Count Register
Load Word Count Register
Load Current Address Register
Read Current Address Register
-

Table D-17 DF32-D

2.6
3.6
3.6
4.6
2.6
3.6
3.6
3.6
4.6

Disk File and Control
‘

Mnemonic

Octal
Code

Symbol
DCMA
DMAR
DMAW
DCEA
DSAC
DEAL
DEAC
DFSE
DFSC
DMAC

6601
6603
6605
6611
6612:
6615
6616

6621
6622
6626

Time

Operation

—

(#3)

Clear Disk Address Register
Load Disk Address Register and Read
Load Disk Address Register and Write
Clear Disk Extended Address
Skip on Address Confirmed Flag
'Load Disk Extended Address
Read Disk Extended Address
Skip on Zero Error Fla'g
Skip on Data Completion Flag
Read Disk Memory Address Register
,

Address Locations:

’

7750 = Word Count
7751 2 Memory

2.6
3.6
3.6
2.6
2.6

3.6
3.6
2.6
2.6

3.6

_Address

D-9

Table D-18 RFGS Disk File

Symbol

Octal
Code

DCIM

6611

DIML

6615

DIMA.

6616

DFSE

6621
6623
6641
6643

Mnemonic
‘

Operation

Disk Interrupt Enable and Core
Memory Address Extension Register
Load Interrupt Enable and Memory Ad-

‘

Clear

‘

dress Extension Register

DISK
DCXA
DXAL
DXAC
DMMT

’

6645
6646

Load Interrupt and Extended Memory Address
Skip on Disc Error
Skip Error or Completion Flag
Clear High Order Address Register
Clear and Load High Order Address Reg\
ister
AC
& Load DAR into AC
Clear
'

Initiate Maintenance Register

Table D-l9 TM8-E/ F Control

Mnemonic
Symbol

Octal
Code

LWCR

6701

CWCR
LCAR
CCAR
LCMR
LFGR
LDBR
RWCR
CLT
RCAR
RMSR
RCMR

6702
6703
6704
6705
6706
6707
6711
6712
6713
6714
6715

.RFSR

6716

RDBR
SKEF

6717

‘6721

SKCB

6722

SKJD
SKTR
CLF

6723

6724

.6725

Operation
Load Word Count Register
Clear Word Count Register
Load Current Address Register
Clear Current Address Register
Load Command Register
Load Function Register
Load Data Bufier Register
Read Word Count Register
Clear Transport
Read Current Address Register
Read Main Status Register

Read Command Register
Read Function Register & Status
Read Data Bufier
Skip if Error Flag
Skip if Not Busing
Skip if Job Done
Skip if Tape Ready
Clear Controller and Master

D-10

Table»D'-20 LE-S Line Printer

MnemOnic

Octal

Symbol

Code

PSKF
PCLF
PSKE

6661

PSTB

Operation
Skip on Character Flag
Clear the Character Flag
Skip on Error
Load Printer Buffer, Print on Full Bufler or
’

6662
'

6663
6664

Control Character
Set Program Interrupt Flag
Clear Line Printer Flag, Load
and Print
Clear Program Interrupt Flag
.

PSIE

PCLF, PSTB
PCIE

6665
6666
A

6667

Character,
.

Table D-21 CRS-E Card Reader and Control or-CM8;E
Optical Mark Card Reader and Control

Mnemonic

Octal

Symbol

Code

RCSF
RCRA
RCRB
RCNO

6631
6632
6634
6635
6636

Operation
'

RCRC
RCNI
RCSD
RCSE
RCRD
RCSI
RCTF

Skip on Data Ready
Read Alphanumeric
Read Binary
'

Read Conditions Out to Card Reader
Read Compressed
Read Condition 'In .From Card Reader
Skip on Card—Done Flag
Select Card Reader and Skip if Ready
Clear Card Done Flag

6637
6671
6672

6674

Skip If Interrupt Being Generated
Clear Transition Flags

6675

6677

Table D-22 XY8-E Incremental Plotter Control
Mnemonic

Octal

Symbol

Code

PLCE

6500
6501

‘

PLSF
PLCF
PLPU
PLLR
PLPD

6502
6503
6504

Clear Interrupt Enable
Skip on Plotter Flag
Clear Plotter Flag
Pen Up
Load Direction Register, Set Flag

6505

Pen Down

PLCF, PLLR 6506
’

PLSE

Operation

6507

Clear Flag, Load Direction Register, Set Flag
Set Interrupt Enable

D—11

Table 13-23 VC8-E CRT
Mnemonic

Octal

Symbol

Code

DILC

6050

DICD
DISD
DILX
DILY
DIXY

6051
6052
6053

DILE

6056
6057

‘

6054
6055

DIRE

Display'Contl-ol
Operation

Clears Enables, Flags and Delays
Clears Done Flag

Skip on Done Flag
Load X Register
Load Y Register

Clear Done Flag; Intensify; Set Done Flag
Transfers AC to Enable Register
Transfers
to AC

Display Enable/ Status Register

Table D-24 VW01 writing Tablet

Mnemonic

Octal

Symbol

Code

WTSC
WTRX

605 4
6052
6072
6074
6064

WTRS

WTSE
WTMN

Operation
Set Tablet Controls
Read X
Read Status
Select Tablet
Clear Set XY
_

Table D-25 DC02-F Iii-Channel Multiple Teletype Control

Mnemonic

Symbol

Octal
Code

Operation

MTPF

61 13

Read Transmitter Flag

MINT

6115
6117
6123
6125
6127
6111
6112

Set Interrupt Flip-Flop
Select Specified Station
Read Receiver Flag‘Status

61 14

Receive Operation
Combined MKRS & MICCC
Skip on Transmitter Flag
Clear Transmitter Flag

MTON

MTKF
MINS

MTRS
MKSF
MKCC
MKRS
NONE

MTSF

6116
6121

MTCF

6122

MTPC
NONE

6124

6126

Skip on Interrupt Request

Read Station Status
Skip on Key Board Flag
Clear Receive Flag

Transmit Operation
Combined MTCF & MTPC

D—12

Table D~26 BB08-P General Purpose Interface Unit

Mnemonic

Octal

Symbol

Code

GTSF

6361
6362

GCTF
GRSF
GCRF

GRDB

(#5)

Operation

2.6

Skip on Transmit Flag
Clear Transmit Flag
(User-Assigned)
Skip on Receive Flag
Clear Receive Flag

6564
6371
6372
6374

Time

2.6
2.6
2.6
2.6
2.6

Read Device Buffer

Table D-27 Universal Digit Controller (UDC)

Mnemonic

Symbol

Octal
Code

UDSS

UDSC
UDRA
,UDLS

UDSF
UDLA
UDEI

Time

Operation

(,uS)

6351

Skip on Sean Not Busy

2.6

6353
6356
6357
6361
6363

Start Interrupt Scan
Read Address and Generic Type

3.6
3.6

Load Previous Status

4.6

Skip own: UDC Flag and Clear Flag

2.6
3.6
2.6
3.6
3.6
4.6

Load Address

6364

UDDI

6365
6366
6367

UDRD

UDLD

Enable UDC Interrupt Flag
Disable UDC Interrupt Flag
Clear AC and Read Data
Load Data and Clear AC.

Table D-28 DR8-EA 12-Channel Buffered Digital 1/ O
Mnemonic

Octal

Symbol

Code

DBDI

65x0

Disable Interrupt

65x1
65x2
65x3
65x4
65x5
65x6

Enable Interrupt
Skip on Done Flag

DBEI
a

DBSK
DBCI
DBRI
DBCO
DBSO
DBRO

Operation

65x7

Clear Selective Input Register

Transfer Input to AC
Clear Selective Output Register
Set Selective Output Register
Transfer Output to AC

D—13

«Table D-29 MPBE-Memory Parity
Mnemonic

Symbol

Octal
Code

‘SMP, CMP

6100
6101
6103
6104
6105

CEP

6106

SPO

6107

DPI

SMP
EPI
CMP

Operation
Disable Memory Parity Error Interrupt
Skip on No Memory Parity Error
Enable Memory Parity Error Interrupt

Clear Memory Parity Error Flag

Skip- on No Memory Parity Error,
Memory Parity Error Flag
Check for Even Parity
Skip on Memory Parity Option

Clear

Table D-30 Synchronous Modem Interface
Mnemonic

Symbol
SGTT
SGRR

‘SSCD
SCSD
SSRO
SCSI
SRTA
SLCC
SSRG
SSCA
SRSZ

Octal
Code
6405
6404

6400
6406
6402
6401

6407

SLFL
SSBE
SRCD

6412
6410
6411
6414
6415
6413
6416
6417

SSTO

6403

SRSl

Operation
Transmit Go
Receive Go
Skip if Character Detected
Clear Sync Detect
Skip if Receive Word Count Overflow
Clear Synchronous Interface
Read Transfer Address Register
Load Control
Skip if Ring Flag
Skip if Carrier/AGC Flag
Read Status 2
Read Status 1
Load Field
Skip on Bus Error
Read Character Detected (if ACO:0)
.

Maintenance Instruction (if ACO=1)
Skip if Transmit Word Count Overflows

Break Address Locations:

For additional interfaces:

7720
.

7721

Test Characters
7722
7723
7724
Receive Word Count
7725
Receive Current Address
7726 Not Used
7727 Transmit Word Count
7730 Transmit Current Address

Device Codes
42,‘ 43
44, 45

46, 47

D-l4

Break Locations
_

7700-7710
7660-7670
7640-7650

Table D-31 Multieycle Data Break Locations

Date Break
Device

Assigned
Locations

Channel
‘

‘

DPS-EA/ BB
DP8-EA/ EB

7640-7650
7660-7670
7700-7710
7720-7730

4
‘

.
.

DPS-EA/EB.

‘

7750,7751

7752,7753
7754,7755

‘

DP8- EA/EB
DF32- D

(Reservedfor Industry Standard

Magnetic

TC08-P

Table D,-32 KM8-E Time-Share
Mnemonic

Octal

Symbol

Code

Operation
‘

6204'

CINT
SINT
CUF
SUF

Clear User Interrupt

6254
6264
6274

Skip on User Interrupt
Clear User Flag
Set User Flag

Table D-33 DK8-EP

Mnemonic

Octal

Programmable Real Time Clock
--

Symbol

Code,

CLZE
CLSK
CLOE
CLAB
CLEN

6130

Operation
,

6131
6132
6133
6134

6135
6136
6137

CLBA

CLCA

AC to Clock Buffer

Load Clock Enable Register

CLSA

Clear Clock Enable Register per AC
'Skip on Clock Interrupt
Set Clock Enable Register per AC

Clock Status to AC
Clock Butler to AC
Clock Counter to AC

Table D-34 DK8-EA Line Frequency Clock
A

‘Mnemonic

Octal

Symbol

Code

CLEI

6131
6132
6133

CLDI
CLSK

‘

Operation
Enable Interrupt
Disable Interrupt

Skip on Clock Flag and Clear Flag
D-15

Tape)

Table D-35 DK8-EC Crystal Clock
Mnemonic

’

Symbol

CLEI
CLDI

Octal
Code

Operation

61 3 1
6132
6133

CLSK

‘

Enable Interrupt
Disable Interrupt

Skip on Clock Flag and Clear Flag

Table D-36 KP8-E Power Fail Detect

Octal
Code

Mnemonic

Symbol

’

Operation

SPL

6102

Skip on Power Low

Table D-37 DPS-EP Redundancy Check Option
Mnemonic

Octal
Code

Symbol

Operation
Test VRC and Skip

6110
6111
6112

RCTV
RCRL
RCRH
RCCV
RCGB
'RCLC
RCCB

Read BCC Low
Read BCC High
Compute VRC
Generate BCC
Load Control
Clear BCC Accumulation

61 13

61 14
61 15
6116

Table D-38 DR8-E Interprocessor Bulfer

Symbol

Octal
Code

DBRF

65x1

DBRD

65x2

Mnemonic

‘

Operation
Skip if the receive set to a 1
'

incoming data into the AC, clear lie-ceive flag

Read

DBTF

65x3

Skip if the transmit flag is set to a 1

‘DBTD

65x4

Load the AC into the transmit buffer, transmit and set the transmit flag

‘

65x5

Enable the Interrupt Request line

DBDI

65x6

Disable the Interrupt Request Line

DBCD

65x7

Clear done flag

DBEI

D—16

Table D-39 ADOIA, lZ-Bit Analog-to-Dig‘ital Conferter

.Mnemonic

Symbol

Octal
Code

Time

Operation

‘

(,us)

‘

ADSF
ADRB

6531
6532

Skip on A/ D Done ‘Flag‘

‘

Read A/ D Buffer
ConVert Analog Input
Select Multiplexer Channel and Gain
Read A/ D Buffer, Clear Flag, and
Start Conversion
Select Channel and— Gain and
Read A/ D Bufier
-

6534

ADCV
ADSC
ADRC

6535

6536

2.6
2.6
2.6
3.6

6537

ADSR

Table D -40 AA50 Digital-to-Analog Converter

"j'

3.6
4.6

‘

Mnemonic
Symbol

Octal
Code

DAC 1

6551
6552
'6553
6554
6555
6556

DAC 2
DAC3
DAC4
DAC 5
DAC6

‘

Operation
_

Select DAC 1
Select DAC 2
Select DAC 3
Select DAC 4
Select DAC 5
Select DAC 6

3.6
'

Symbol

Code

Operation

ADCL
ADLM

6530

Clear Flags
Load Multiplexer
Start Conversion
Read A/ D Buffer
Skip on A/ D Done Flag

6532
6533
6534
65 35
6536
6537

ADRS

Skip on Timing Error
Load Enable Register
Read Status Register
D17

‘

2.6
2.6

Octal

ADRB
ADSK
ADSE
ADLE

(us)

Mnemonic

ADST

Time

2,6

Table 13-41 ADS-EA A/D Converter

6531

‘

3.6
3.6

Table D-42 AA05A/AA07 Digital-to-Analog Converter
(‘nnhnl
and
uuu

v Vllll VI.

7‘

Mnemonic

Octal

Symbol

Code

Time
"

Operation

DACL
DALD

6551
6552

DALI

DAUP

(us)

Clear DAC Address
Load DAC Address

6562
6564

2.
'

Load DAC Input Register
Update All Channels

2.6
2.6
2.6

Table D-43 AFC-8 Low-Level Analog Multiplexer
'

Octal
Code

Mnemonic

Symbol
ADSG
ADSA

Time

ADRB
ADSG
ADSA

Table D-44 AF04A Guarded Scanning
Voltmeter (IDVM)
Mnemonic

Symbol
VCNV
VSEL
VINX
VSDR

VRD
VBA

VSCC

2.6
2.6

2.6
2.6
2.6
2.6

Integrated Digital
Time

Octal
Code

Operation

6541
6542
6544
6561
6562
6564'
6571

(as)

Set Multiplexer Gain
Set Multiplexer Address
Skip on A/ D Flag
Read A/ D Converter Buffer
Set~Multiplexer Gain
Set Multiplexer Address

6542
6544
6531
6534‘
6542
6544

ADSF

Operation

(II-S)

Select Channel and Convert
Select Range and Gate
Index Channel and Convert

2.6

Skip on Data Ready

2.6
2.6
2.6
2.6

Read Data and Clear Flag
Byte Advance
Sample Current Channel

.D—18

2.6
2.6

READ-IN MODE (RIM) LOADER
The RIM Loader is used to load programs punched on RIM

format paper tape into core memory. -It is stored in core memory
locations 7756-7776 (218 locations), and started at location 7756.
There are two versions of the RIM Loader, permitting either the

high- or the lowfspeed reader to be used as an input device. The
locations and corresponding instructions for both versions are
listed below. Figure E-1 provides instructions for loading the
RIM Loader.

RIM Loader Programs

Table E-l

INSTRUCTION
’

Location
_

7756
7757
7760
7761
7762
7763
7764

Low-Speed Reader
6032

6036
7106
’

7006
7510
5357
7006
6031
5367
6034

7770
7771
7772

7420

7773

3776

7775

7767

7774

6014
6011
‘5357
6016
7106
7006

6031
5357

7765
7766

High-Speed Reader

3376
5356

7510
5374
7006
6011
5367
6016

7420
3776
3376
5357

Note: Location 7776 is used for temporary storage.

lE—l

IN ITIALIZE

SET ROTARY
SELECTOR SWITCH
TO MD

SET SWITCHES 6-8
To DESIRED
FIELD*

INSTRIXZTION

*EEETAPE USERS SHGJLD.
LOAD RIM INTO FIELD 0

SET SWITCHES 9-11
T0 DESIRED
DATA HELD I

PRESS
EXTD ADDR LOAD

sE'T SR
TO 7755

I
PRESS
ADOR LOAD

SET SFI=
FIRST INSTRUCTION

LIFT DEF

SET SR=
NEXT INSTRUCTICN

LIFT DEP

RIM IS LOADED

Figure E-l

Loading the RIM Loader
E-Z

BINARY (BIN) LOADER

‘

The BIN Loader is used to load programs punched on BIN
format paper tape into core memory. It is stored in core memory

locations 7625-7752 and 7777 (1278 locations), and started at
used to load a RIM
location 7777. The RIM Loader is

usually

format tape of the BIN Loader.

When the BIN Loader is used to load a binary tape,

must be
'

caution.

exercised to ensure that the tape is started with binary

(code 200) under the read station. If the tape is
started before this code, the contents of core memory may be lost.
Figures E-2 and E—3 provide instructions for loading the BIN
Loader by means of the RIM Loader and using the BIN Loader

leader code

to store a binary program.

E-s

LOAD RIM

SET ROTARY
SELECTOR SWITCH
TO MD

RIM

I
SET SWITCHES
9-” TO FIELD IN
WHICH BIN IS
TO BE LDMED

PRESS
EXT 0 ADM LOAD

SET SR¥7756

PRESS ADDR LOAD

HIGH'SPEED

LOW- SPEED

READER

wan HSR on

PUT am
m

LOADEFI

nsnit

mess
CLEAR MD

com

PRESS HALT

SET SWITO'ES
6~8 TO FIELD
BIN WAS LOADED
INTO

SET SR-7TTT

PRESS
ADDR LOAD

PRESS EXAM

YES
BIN LOADER
IS LOADED

Loading the BIN Loader

Figure E—2
'

13—4

LOAD BIN

SET ROTARY
SELECTOR SWITCH
TO AC

l
SET SWITCHES
6-8 To FIELD IN
WHICH BIN IS
LOADED

SET SWITCHES 9-“
T0 FIELD IN WHICH
PROGRAM IS TO BE
LOADED

PRESS
EXTD m LOAD

ISET
SR TO
.777?!
[
PRESS
ADDR LOAD

HIGH-SPEED READER

?
TURN HSR ON

SET SR=3777

PUT TAPE IN HS

PRESS CONT

OBJECT TAPE
IS LOADED

Figure E-3

Using the BIN Loader to Load a Binary Tape
E-5

app MP
”miscellaneoustable's

Powers of Two

11
3

NH
16

128

255

NHO

‘UY-hw
O NJO‘

L0
05
025

0125,

0.062- 5
0031

0015 625
0007 812 5
0003 906 25
0001 953

512

125

p. .‘ 024

0000 976 562

2 048

0000 488 281

0000 244

192

0000

384

0000 061

035

156

578

140 625

122 070 312

32 768

0000 030 517

'65 536

0000 015 258 789 062

131 ‘072

0000 007 629 394 531

262

125

144

0000 003 814 697 265

524 288

0000 001

048 576

0000 000 953 674 316 406

097

625

907 348 632 812

000' 476 837

152

0000

194 304

0000 000 238 418 579

,388 608

0000 000

158 203
101

125
562 5

119 209 289 ,550 781

16 777 .216

0000 000 059 604 644 775 390 625

554 432

0000 000 029 802 322 387

108 864

0000 000 014 901

695 '312

728

0000 000 007 450 580 596 923 828

268 435 456

0000 000 003 725 290 298 461

536 870 912

0000 000 001 862 645

134 217

161 193 847 656 25
125

914 062 5

149 230 957 031

Octal-Decimal Conversion
The following table gives the multiples of the powers of 8 To
convert a number from octal to decimal using the table, add the
decimal number opposite the digit value fOr each digit position.
To convert 402773 to decimal, the
following numbers are obtained

from the table and added.
Position

Table entry

Digit

16384
0
128
56
7

\iqmoel

FNu-PUI

1657510 = 402778
'
'

This process is reversed to convert a number from decimal to
octal. Subtract out the largest table entry which allows a positive

remainder, then take the column number (position coefficient)
of the table entry as the Nth digit of the result, where N is the

(digit position) of the table entry. Continue this
process, operating on the remainder from each step in the next
step, until all digits of the result have been found. For example,
to convert 2336510 to an equivalent octal number:
row

number

*20480

~256O

—320

5X84

5x83

5x 82

Ox 81

5
—5
0

/5X80
555053/
_

F—2

2336510

‘

'
’

Octal-Decimal Conversion Table
Position Coefficients

(Multipliers)

'
'

Octal

Digit
’

Position/
811

(80)

2nd'(81)

'16

3rd

(82)

128

‘192

256

320

~384

448

4th

(83)

512

1,024

1,536

2,048

2,560

3,072

3,584

5th

(84)

4,096

8,192 12,288

16,384

20,480

24,576

28,672

6th

(85)

lst

32,768 65,536 98,304 131,072 163,840 196,608 229,376

Octal-Decimal Fraction Conversion Table
Octal
.000

.001

Decimal
.

Octal

000000

.001953

100

Decimal

Octal

Oclal

Decimal

125000

.200

250000

.300

.126053

.201

.251333

.301

.370053

.3760m

.101

Decimal
.

375000

.002

.003006

.102

124000

.202

.‘253006

.302

.003

.005650

.103

.130050

.203

.255550

.303

.360659

.004

.007612-

.104

.132612

.204

.2571112

.3304

.362612
.304765

‘

“

,.005

.000705

.105

.134765

.205

.250765

-305

.006

.011713

.106

.136716

.206

.261716

.306

..007

1013671

-107

«5338611

.207

.263671

.3072.

.010

.015025

.110

.140625

.210

.205625

.310

.300025

.011

.017573

,111

.142576

.211’

.207573

.311

.302576

.012

.010531

.112

.144531

.212

.209531

.312

.301531

.013

.021404

.113

.146434

.213

.271464

.313

.306464

.‘114

.146437

:214

.273437

.314

.300437

.115

.150390

215

.275300

.315

.400330

’

.014

.023437

.015

.025390-

,531’37i3

...3101671

‘

.016

.027343'“

.116‘

.152343

216

.277343

.‘316

.402343

.017

.020296

v.117

.154296

217

.270206

.317

.404236

.020

.031250

.120

.156250

.2207

3231250

121

. 1511203

.221

263203

.321

.035156

.122

.-160156

.222

.265156

.322

.410156

.037109

.123

.237109

.323

.412100

021

.022

033203

.400250

.320

406203

.023
.

024

.025

‘

030062

.041015

124

.125

.026

.042960

.027

.030

.046675

.130

.031

.046626

.131

:032

.050731

.033

.052734

.034

.054607

.035
.

036

.037

044321

056640

.056593
.

060546

.040

.002500

.041

.004453

.042

.006406

.043

.003359

.044

.070312

.045

.072265

.046

126

.127

1:12

.162109
.

.223

164062

.160015'
.

224

225'

167066

226

230

.296875

.330

.421675

.231

.235623

.331

.423620

175761

.232

.300731

.332

177734

.233

.302734

.155546

.237

234

.425761

333

427734

304637

.334

.429667

.235

.306640

.335

.431640

.236

.300593

.336

310546

.337

.435546
.437500

433593

187500

.240

.312500‘

.340

.160453

.241

.314453

.341

.439453

191406

.242

.316406

.342

.441406

103359

.243

.316359

343

.443359

.144

.153312

.244

.320312

.344

.445312

145

.107205

.245

1322265

.345

.447265

.074210

.146

.109213

.246

.324216

.346

.449210

.047

.0761“

.147

.201171

.247,

.326171

.347

.451171

.050

.075125

.150

.203125

.250

3211125

.350

.453125

0600711

.151

205078

.251

.330076

.351

.455018

207031

.252

.332031

.352

206964

.253

333904

.353
.354

.457031
.4509“
.460937

.051
.052
.053

.062031
.

0639114

143

4173611

171675

163553

.142

.173625

.181640

141

.416015‘
.419021

.135

140

414002

.325

.1796”

5291015

.327

134

324

.326

.137

202966

133

1:16

200062

.294021

.227

.103021

152

15:1

.054

.065037

.154

.210937

.254

.335937

.055

.087650

.155

.212390

.255

.056

.035643

.057

001756

.060

.093750

.061

.095703

062

.097656

063

.003609

.064
.005
.060

.007

.070
.071
.072
.

07:1

.074

101562

156

214643

.157

.216796

160

.218150

3371190

355

3391143

.356

257

.341796

.357

.466706

260

343750

.360

.460750

220703

261

345703

361

162

.222656

262

347656

362

472656

163

224609

263

.345609

363

474609

164

226562

264

.351562

.364

.265

.353515

.365

105466

.166

.230463

.266

107421

.167

232421

.267

.355466
.357421

.366

109375

234375

270

.350375

111326

.171

.236326

.361326

.371

.172

.235261

.271
.272

.303251

27:1

365234
367167
:369140

.113201
.

115234

.117107
.

119140

121003

.077

123046

17o

173

.174
.

470703

161

.226515

.462690
.464043

256

.165

103515

.076

.075

240734

.242107

.274

175

244140

.275

176

246093

177

.246046

F—4

276
277

.367

.476562
.476515
.400466
.462421

372

.454375
.466326
.4662111

373

374

.4921"

.375

.434140

370

371053

376

373046

311

490234

456053
.4930“

Scales of Notation

2K in Deéimal
2'

0.001
0.002
0.003

0.004
0.005
0.006
0.007
0.008
0.009

0.01
0.02
0.03

1.00695 55500 56719
29
1.01395 94797
1.02101 21257 07193
6067
1.02811 38266
1.03526 49238 41377
1.04246 57608 41121
1.04971 66836 23067
1.05701 80405 61380
1.06437 01824 53360

0.1

1.07177 34625 36293
1.14869 83549 97035
1.23114 44133 44916
1 31950 79107 72894
1 41421 35623 73095

3874 62581
72557 11335
16050 79633
64359 01078
17485‘ 09503
75432 38973
38204‘ 23785
05803 98468
78234 97782

1.00069
1.00138
1.00208
1.00277
1.00347
1.00416
1.00486
1.00556
1.00625

0.04
0.05

0.06
0.07

0.08

0.09

10
10-

O
1

12
144
1 750
23 420

2
3
4

1.000 000 000 000 000
0.063 146 314 631 463
0.005 075 341 217 270
0.000 406 111 564 570
0.000 032 155 613 530

5
303 240
0.000 002 476
0.000 0
3 64.1 100
6
206
46 11
200
7
0.000 000 015
575 360 400
8
0.000 000 001
7346545000900000000

0.9

000 00
146 31

243 66

651 77

221

704

2 657

132 610 706
157 364 055
327 745 152
257 143 561
104560276

64
37
75
06

151571 65665 10398
162450 47927 12471
174110 11265 92248
186606 59830 73615

m Octal.

10-1

:tl‘l

0.2
0.3
0.4
0.5
0.6
0.7
0.8

101

10-~

112 402 762 000
351 035 564 000
432 451 210 000
411 634 520 000
142 0.36 440 000

10
11
12
13

0.000 000 000 006 676 337 66

14_

0.000
0.000
0.000
0.000

500 000

15
16
17
18

0.000 000 000 000
0.000 000 000 000
0.000 000 000 000
0.000 000 000 000

34 327 724 461
434 157 115 760
5 432 127 413 542
67 405 553 164 731

200 000
400 000
000 000

nlogz 10.

nlong

nlogz 10

nlong

1
2

0.30102 99957
0.60205 99913
0.90308 99870
1.20411 99827
1.50514 99783

3.32192 80949
6.64385 61898
9.96578 42847
13.28771 23795
16.60964 04744

1.80617 99740
2 10720 99696

4
5

000 022 01
000 001 63
000 000 14
000 000 01.

log10 2, n log2 10 1n Deumal

000 000 000 537 657 77
000 000 000 043 136 32
000 000 000 003 411 35
000 000 000 000 264 11

7
8

240823 99653
2.70926 99610
3.01029 99566

9
10

19.93156
23 25349
26 57542
29.89735
33.21928

85693
66642
47591
28540

09489

Addltlon and Multlpllcatlon Tables
Addition

Multiplication

Binary Scale
0 + 0
0
0+1=1+0=1
1 + 1 = 10

0 X 0

0X1=1x0=0
1 x 1

Octal Scale
0

102

030405

2040610121416

203040506071011

3061114172225

3‘

~10

Mathematlcal Constants in Octal Scale
3.11037

552421.

0.24276

301556.

e-'

v11:

1.61337

611067.

v‘;

In 1r

1.11205

loam =
V10 =

1r"

2.55750

521305.

027425

530551.

ln7 =

1.51411

230704.‘

10327 =

404435.

'0810 e = 0.33626

754251.

151544

163223.

logze =

1.34252

3-12305

407267.

log: 10 = 3.24464

F—5

0.44742 147707.

—

0.43127 233602.
0.62573 030645.

1.32404

746320.

166245.

ln2

0.54271

027760.

741136.

In 10 _=

2.23273

067355.

W9“

digital equipment

‘

users

computer
society

OBJECTIVES

Society

(DECUS) was
Digital Equipment Computer Users
established in March of 1961 to advance the effective use of
Digital Equipment Corporation’s computers and peripheral equip-‘
ment. It is a voluntary, non—profit users
group supported by
whose
to:
are
DIGITAL,
objectives
0

advance the art of computation through mutual education

and interchange of ideas and information,-

establish standards and provide channels to facilitate

the free

exchange of computer programs among members, and
0

provide feedback to the manufacturer on equipment and programming needs.
A

The Society sponsors technical symposia; twice a year

(Spring

and Fall) in the U. 5., once a year in Europe, Canada; and Australia. It maintains

Program Library, publishes a library catalog,

proceedings of symposia, and a periodic newsletter: DECUSCOPE.
A DECUS—Europe organization was. formed in 1970 to assist
in the servicing of European members.
,

G-l

DECUS PROGRAM LIBRARY

The DECUS Program Library is one of the major activities of the
group. It is maintained and operated separately from the
DIGITAL library andcontains programs contributed by users. Prousers

grams are available for all DIGITAL computer lines. The Library
contains many types of programs, such as executive routines, editors,

debuggers, special functions, games, maintenance and various
other classes of programs. Library Catalogs are issued which list
all programs available from DECUS.

There are submission standards which programs must meet be—
fore they are accepted into the Library. Review procedures deter-

mine whether the program remains in the
or is removed.

Library, is changed,

Forms and information for submitting programs to the Library
may be obtained from a DECUS office.

Programs are available to all memberson a request basis. Requests for programs should be made on DECUS Library Request
forms and directed to the DECUS
cases,

Program Library.

In most

nominal service charge will be associated with a program.

Information on program charges is published in the Library Cata—

log. European members may forward requests through the European DECUS oflice in Geneva.
As of December 1972, the

Library contained approximately

1,600 programs.

DECUSCOPE
DECUSCOPE is the Society’s technical nersletter, published
since April, 1962. The aim of this informal news “scope” is to
facilitate the interchange of information. Society members are

ideas, programming notes, letters, and appli«
cation notes for publication. DECUSCOPE is mailed as issued to
all members. Circulation reached 16,500 in 1972.

invited to submit

All submissions'to DECUSCOPE should be sent to‘the Editor,

DECUSCOPE, at the DECUS Ofice in Maynard orGeneva.
G—2

ACTIVITIES

»
.

Two nation-wide symposia are held each

‘

year—one in the spring

and the otherin the fall. Seminars are also held annuallyin Europe,
Canada, and Australia. The proceedings and papers presented at
the symposia and seminars are
published shortly after each meeting and are sent automatically to meeting attendees and, for a
nominal charge, upon request to mothers
DECUS sponsored the first workshop meeting of the Joint Users
Group of the Association for Computing Machinery in April,
1966, and has actively participated in workshops held each year
since. The purpose of the Joint Users Groupmeetings is to establish means for intercommunication among user groups.
DECUS is also

member of- the Joint User

JUG. This

Group Library
catalog; contains lists

Catalog Project sponsored by
of programs available from several major user groups. Members
of the participating user groups will be eligible to requestprogram
documentation from other groups through their Program Inter—
change Chairman, i.ve., for DECUS members, the DECUS Executive Director. Specific details on this interchange program are
available from the DECUS office.

DECUS encourages subgrouping of users with common interests. Local User, Groups. (LUG’s) have formed in many areas for
_

the purpose of providing closer communication between users in

specific field or area. Special Interest Groups (SIG’s) have
formed for the purpose of providing closer communication between USers of a specific product or application area. They provide
valuable grOup feedback to the manufacturer on specific program—
‘ming and hardWare needs. Current Special, Interest" Groups are:
PDP—6/ 10 Mainframe Group, Education SIG, Physics» SIG,
Graphics SIG, PS/ 8 SIG, PDP-8 Newspapers Users Group,
LUDEC—Laboratory Users of Digital Equipment Computers, and
a

Users SIG

BATCH
MEMBERSHIP

Membership in DECUS is voluntary and does not require the
payment of dues. Members are invited to take an active interest

Society by contributing to the program library, to DE—
CUSCOPE, and by participating in its meetings and symposia.
There are two types of membership in DECUS: Installation Mem—
bership and Individual Membership.
in the

G-3

MEMBERSHIP—DECEMBER 1972

Installation Delegates—~—7,0SZ

Individual Members—8,243

Installation Membership
An

-...

D!"
nroanivation
Vflnir‘l'l lane hut-plan:
mg
t”Minus.“
.......

m...

11nd

n!-

119.3

A”

n.-

U1.

(«I

nnm

UUIII‘

pater manufactured by Digital Equipment Corporation is auto—
matically eligible for installation membership in DECUS. Mem—

bership status is acquired by submitting a written application'to
the Executive Director or European Secretary for approval by
the DECUS Board.
An organization may appoint one delegate for each DEC com-

puter owned. The delegate should be one who is immediately
emerned with the operation of the computer he represents and
who is willing to take an active part in DECUS activities. He is

entitled to vote

all DECUS

policies and during the election

of officers.
‘

’

Mal“ Membership

There are two classes for individual membership:
1.

desiring membership in DECUS who are em—
ployed at an installation but are not appointed delegates.

Individuals who have a direct interest in DECUS 01‘ DEC

‘

Individuals

computers but are not‘employees of a DECUS installation
member.
An individual member is not entitled to vote on DECUS policies

during elections. Written application indicating desire to join
must be submitted to the DECUS oflice for approval by the
DECUS Board. There is no limit to the number of individual members that may join from either an installation or a non-installation.
or

EXECUTIVE BOARD, POLICIES AND ADMINISTRATION

Society’s policies are formulated by an Executive Board
elected by vote of Installation Member delegates.
The Administrative office is located at Digital Equipment Corporation, Maynard, 'Massachusetts, 01754, and all correspondence
should be directed to: the attention of the DECUS Executive
.»=
Director.
”The EuropeanRegio‘nal Administrative office is located at- Digital Equipment ‘S.A.,r 8‘13? Route'de l’Aire,_CH-1-211- Geneva 26,
The

‘

'
'

Switzerland.

'
‘

~
.

G—4

DECUS PROGRAM LIBRARY CATALOG
.,

The DECUS Program Library, Catalog

contains detailed infor—

mation on programs available from the DECUS Library. Catalogs
are

divided according to computer line. A member must request

the catalog of. his interest. PeriOdic updatesare sent automatically.

necessary to find an item in a.‘
table or array (i.e. the index).

Absolute address: A binary number
that is permanently assigned as

Arithmetic

Binary and octal operations, 3-11
Division, 3-13
Double precision, 3-14
Multiplication, 3-13
Overflow, 3-11

the address of a core storage location.
Absolute value, 8-6
Access time: The time required to
locate an off-line storage location.

Powers of two, 3-16

Accessing data: The process of locating the oif- line storage location

Programming operations, 3-10
Subtraction, 3-13

with which data is to be trans”ferred.

Accessing data

Arithmetic unit: The component of
a computer where arithmetic and?

blocks, 7- 15

logical operations are performed,
1-32, 2-5.
Array: A. set or list of elements,
usually variables or data.
ASCII: An abbreviation for Ameri-

Accumulator. A 12-bit register in
which the result of an operation
is formed; abbreviation: AC, 2-6
-

Accuracy of extended functions, 8-37
Address: A label, name, or number
which designates a locat1on where

can Standard Code for Information Interchange, 4-16

information is stored._

Assemble.

Address modification, 3-19

symbolic
*

program

Address load switch, 4-3

Addressing,

Algorithm: A prescribed set of welldefined rules or processes for the
solution of a problem in a finite
number of steps.

Alphabetic data, 1-34
Alphanumeric: Pertaining to a char-'
acter set that contains both letters
and numerals, and usually other
characters.
AND group microinstructions, 2—25
AND instruction, '2-9

AND logical operation, 1-29
Answers to exercises, A-l

Appending symbols
symbol table, 5—46

extended

A variable or constant which
is given in the call of a subroutine as information to it.
A variable upon whose value
the value of a function de-

pends.
3.

The

knOWn

translate

from

binary
by substituting binary
program

reference

factor

eration codes and; absolute or relocatable addresses for symbolic
addresses.
Assembler. A program which translates symbolic op—codes into machine language and assigns mem—
ory locations for variables and
constants.

of the
0010'
from
through 0017 is addressed indirectly, the content of that location
is incremented by one rewritten
in that same location, and used as
the effective address of the current instruction.

When

indexing.
Aato—
absolute
10cations

one

Auto—indexing, 8-20
Automatic processes, 1-3

Argument:
1.

operation codes for symbolic op-

2-13

Direct, 2:16
Indirect, 2-16'

‘

Auxillary storage: Storage that sup
plements core memory such as
disk or DECtape.
B

Background program, 6-24
Base address: A given address from

Index—1

’

which an absolute address is derived by combination with a relative address, synonymous with address constant.
BIN Format, 4—17

instructions and data necessary to
set up and call a given routine.
Careers in programming, 1-2
Central processing unit: The unit of
a computing
system.that includes
the circuits controlling the interpretation and execution of instructions—the computer proper,
excluding I/O and other peripheral devices.

BIN Loader, 4—18, 5—8
Self-Starting BIN, 5-9
to the number
a
radix
with
of two.
system

Binary: Pertaining

Binary
Addition, 1-18
Counting, 1-7
Division, 1~25
Format, see Binary format.
Loader, see BIN Loader.
Multiplication, 1-23
Notation, 8—6

Channels,
DECtape, 7-2
Mark-track, 7 -2
Timing, 7—2
Character: A single letter, numeral,
or symbol used to represent 1nformation.

Number system, 1—6
Subtraction, 1-20

(complement and increment
AC), 2-27
CLA (clear the AC), 2-19, 2-20 to
CIA

Binary code: A code that makes use
of exactly two distinct characters,
0 and 1. Same as object code,

2-22

Clear: To erase the contents of a
storage location by replacing the
contents, normally with zeros or

2-2
Biz: A

binary digit. In the PDP-8

each
of
12
bits.
posed

computers,

word

com—

spaces; to set to zero.

Clear switch, 4-3

Block: A set of consecutive machine
words, characters or digits handled as a unit, particularly with
reference to I/O.

CLL (clear the link), 2—19, 2—20

Chilewomplement
-2

CML (complement the
2-20

Bootstrap: A technique or device
designed to bring a program into
the computer from an input de—

ful to the computer, or to an assembler, compiler or other lan-

Bootstrap loaders, see BIN Loader,
Hardware bootstrap, etc.
Branch: A point in a routine where

guage processor.

Coding a program, 3-6

one of two or more choices is
made under control of the rou-

Combined

Calling sequence: “A specified set of

microinstruction

mne-

monics, 2-27
Combining
Microinstructions, 2-23
Skip microinstructions, 2-25

tine.

link), 2-19,

Coding: To write instructions for a
computer using symbols meaning-

Vice.

Branching program, 3-30
Breakpoint, 5-56, 5—68, 5-73
Buffer: A storage area.
Bug: A mistake in the design or implementation of a program resulting in erroneous results.
Byte: A group of binary digits usu—
ally operated upon as a unit.

the AC), 2-19,

Command: A user order to a computer
system,
usually
given

through a Teletype keyboard.
Command summaries;
Editor, 5-40
DDT, 5-65
ODT, 5—83
Comments, inserting, 3-22

Compatibility: The ability of an instruction or source language to be
used on more than one computer.

Compile: To produce. a binary-coded
program. from a program written

Index—2

Count: The successive increase or
decrease of a cumulative total: of
the number of times an event oc-

(insource (symbolic) language, by
selecting appropriate subroutines

’

from a subroutine library, as di—
rected by the instructions or other
symbols of the source program.
The linkage is supplied for com—
bining the subroutines into a workable program, and the subroutine
and linkage are translated into binary code.

curs.

operation (see Loop).
Counting, in binary, 1-7
Current address register, 7—4

Compiler: A program which trans-

Current Location Counter: A counter
kept by an assembler to determine the address assigned to an

and
formulas
lates statements
written in a source language into
a machine language program, e.g.
a FORTRAN Compiler. Usually
generates more than one machine
instruction for each statement.

Current page or page zero bit, 2 1'5

Complement: (One’s) To replace all

Cycle time. The length of time it

instruction

constant

being

Cycle stealing, 6-41
D

Data: A general term used to denote
any or all facts, numbers, letters
and symbols. It co'nnotes'basic el—
ements of information which can
be processed or produced by a-

.have been met.
Conditional skip: Depending upon
whether a condition within the
program is met, control may transfer to another point in the program. See Operate group
tions.

computer.
Data blocks, DECtape, 7-1
Data break: A facility which. permits I/O transfers to occur on. a.
cycle-stealing basis Without dis—
turbing program execution, 6-‘1
Single cycle, 6-41
3—cycle, 642

instruc-

Data channels, DECtape, 7-2
Data field, 4-5, 4-20

Data formats, 1‘-34
.DCA (deposit and clear AC), 2410
,

Continue switch, 4-4

Control unit, 1-32, 2-5, 2—6
Conversion
Decimal to binary, 1-‘9
Decimal to octal, 1-12
Of fractions, 1-12, 1-17, F1-4
Octal to decimal, 1-12

DDT, see Dynamic Debugging
Technique
Debag: To detect, locate and correct
m1stakes in a program.

vConvert:
1. To change numerical data from
one radix to another.
2. To transfer 'data from one recorded format to another.
_

Core memory: The main high-speed
storage of a computer in which
binary data is represented by the
switching polarity of magnetic
cores, 2-7

as-

takes the computer to reference
one word of memory.

0 bits with 1 bits and vice Versa.
(Two’s) To form the one’s coma
plement andadd 1.

Console: Usually the external front
side of a device where controls
and indicators are available for
manual operation of the device.
See Computer console or Teletype
console.

sembled.

Computer console, 4-1 to 4-6
Computer fundamentals, 1-1
Conditional assembly: Assembly of
certain parts of a symbolic program only if certain conditions

’

Counter: A register or storage location (variable) used to represent
the number of occurrences of an

Debugging programs, 5-42
DECdisk systems, 4-23
DECtape
Bookstrap, TCOl, 7-31
Control flag, 7-16
Control words, 7-11
Controls, 4-22

C7OP3Y program (DTC—8), 7-15,
-2

Current address register, 7-4
Data blocks, 7-1
Data channels, 7-2

Index—3

Data word format, 74

ifiesthe location of an instruction
operand, 2-16
Directory device: A device (such as
a disk)
which is partitioned by
software into several distinct files.
A directory of these files is mainon the device to locate the

DELETE program, '7-29
DEWAIT program, 7-21
Error codes, 7-7
Error conditions, 7-16
Error flag, 7-6
ESCAPE program, ‘7-30
Flag, use of, 7-13
Format, 7-1

‘ngrmatting

program

}.iai ned
es.

(TOG-8), 7-

Division in binary and octal, 1-23

Division, programming, 3-14
-

Functions, 7-8

Double precision: Pertaining to the
use of two computer words to
represent one number. In the

GETSYS program, 7-30
IDTAPE subroutine, 7-17
INDEX program, 7-28

Interrupt handling, 7-16
IOT instructions, 7-10
Library directory, 7-27
Library system, 7-26
Mark-track channels, 7-2Programming, 7-11

PDP-8,

double

precision result

is stored in 24 bits.

1-35, 3-14

Downtime: The time interval during
which

DTC-8,

READ program, 7-22
SEARCH program, 7-21
Skeleton library, 7-27
Software, 7-20
Standard format, 7-3

device is inoperative.

see

DECtape COPY Pro~

gram

DTP, see DECtape flag
Dummy: Used, as an adjective to indicate an artificial address, instruction, or record of information
inserted solely to fulfill prescribed
conditions, as in a “dummy” vari~

Subroutine, TD8~E, 7-32
Status registers, 7-6
Subroutines, TC08, 7-20
System, 4-21
Timing channels, 7-2
Transport unit, 4-22

able.

Dump: To copy the contents of all
or

UPDATE program, 7-29

part of core memory, usually
storage medium.

onto an external

DECUS, G—l

DWAIT subroutine, 7-21

Defer state, 2-7

Dynamic Debugging Technique
(DDT), 5—43

Delays, programming, 3-29
DELETE program, 7-26
Delimiter: A character that separates, terminates and organizes elements of a statement or program.

Deposit switch, 4-4
Device flags: One-bit registers which
record the current status, of ade:
vice.
Device selection code. 6-2

Command summary, 5-65
Defining symbols, 5-46
Errors, 5-49

Example program debugged, 5-63
Execution commands, 5-55
Internal symbol table, 5-67
Loading, 5-43
Output, 5-50_
Program modification, 5-50
Storage requirements, 5-48

DF32D DECdisk, 4-23

Digit: A character used to represent
‘one of the non-negative integers
smaller than the radix, e.g., in binary notation, either 0 or 1.
Digits, significant, 1-8
Digital Computer: A device that operates on discrete data, performing sequences of arithmetic and
logical operations on this data.
Direct address: An address that spec-

Editor, see Symbolic Editor

Effective address: The address actually used in the execution of a
computer instruction.

Eight’s complement arithmetic, 1-22
Elementary programming techniques, 3-1
Address modification, 3-19
Arithmetic overflow, 3-11

Index—4

Fetch state, 2- 7‘

Auto indexing, 3-27

Coding

3-6

Field:
1. One or more characters treated
as a unit.

program,
Double precision arithmetic, 3-14
Flowcharting, 3-3
Inserting comments and headings,

3-22
Location assignment, 3-6
Looping a program, 3-24
Multiplication and division, 3-13
Powers of two, 3-16
Program branching, 3- 30
Program delays, 3-29

2. A specified area of a record
used for a single type of data.
3. A division of memory on a
PDP- 8
computer referring to
a 4K section of core.

File: A collection of related records
treated as a unit.
File structured device: A device such
.as disk or DECtape which contains records organized into files
and accessible through file names
found in a directory file. See directory device.

operations,

Programming arithmetic
3- 10

Programming phases, 3-2

Subtraction, 3-13
Symbolic addresses, 3-7
Symbolic programming
tions, 3-8
Writing subroutines, 3—16
End—zone, 7-2
'

conven-

Alphanumeric characters
used to identify ‘a particular.file.
F ilename extension: A short appendage to the filename used to iden—

F ilename:

Error traps, 8—29.

Equivalents, decimal-octal-binary, ,1‘

ESCAPE
.

program,

Fixed point: The position of the
radix point of a number system is

instruction

Execute: To carry out an
program

according to a predetermined convention.
Flag: A variable or register used to
constant

Exclusive 0R, 1- 30
a

puter.

the com-

record the status of a program or
device. In the latter case, also

Execute state, 2-7

called a device flag.
Flip-flop: A device with two stable

Exercises,
Answers to, A—1-

operations,
1
Ariitzlémetic

states.

1-26,1-27,

Floating point: A number system in
which the position of the radix
point is indicated by one part of
the number (the exponent) and
another part represents the significant digits (the mantissa).
Floating-point packages, 8-1

tech“

Elementary programming
niques, 3-1
Input/Output programming, 6-1
Number systems, 1—5

Programming fundamentals, 2- 28
System operation, 4-24
Exponent, in floating-point numbers,
1- 35, 8- 4_
Overflow, 8-29
Underflow, 8-29
Extended address load switch, 4-4
Extended arithmetic element, 4-23
Extended function algorithms, 8—31

External storage: A separate facility
or device on
which data usable
by the computer is stored (such
paper tape, DECtape or disk).

FAC, 8-7

Core storage maps, 8-38
Floating data field, 8- 12, 8-22, 8- 25
Floating halt, 8—27
Input and output, 8- 17
Instruction field, 8-22
Instruction summary, 8-40
Notation, 8—4
Operand, 8- 26
Pseudo-instructions, 8- 13—8- 15
Subroutines, 8- 8, 8- 10, 8- 11
Using, 8-7
/

Extended memory, 4-20

run

tify the type of data in the file;
e.g. BIN signifying a binary program.

7-26

Examine switch, 4— 4

Flowchart: A graphical
tion of the operations

representa-

required to

carry out a data processing opera—
tion

Flowcharting, 3-3
Index—5

Foreground program, 6-24
Format: The arrangement of data.
Also a FORTRAN statement.

Half duplex: Describes a communications channel capable nf transmission and/or reception, but not
both simultaneously.
Hardware: Physical equipment, e.g.,
mechanical, electrical or electronic
,

Format routines, 6-8

Fractions,
Binary and octal, 1-15
Converting binary and octal to dec‘
imal, 1-17
Full duplex: Describes a communications channel capable of simultaneous and independent transmission and reception.
Function subprogram: A subprogram
which returns a single value result,
usually in the accumulator.

devices.

‘

Hardware bootstrap loader, 5-10
Head: A component that reads, records or erases data on storage
device.

Headings, inserting, 3-22
High-speed reader/punch, 4-18
HLT (halt), 2-22
I

IAC (increment the AC), 2-20

GETSYS program, 7-26

IDTAPE Subroutine, 7-17

Greater Than Flag (GTF), 4-24

Group I (operate) microinstructions, 2-18
CLA (clear AC), 2-19 to 2-22
CLL (clear link), 2-19, 2-20
(complement AC), 2-19, 2-

Inclusive OR, 1-30

Incrementing a tally, 2-11
INDEX program, 7-26
‘

Format, 2-19

Indirect address: An address in a
computer instruction which indicates a location where the address
of the referenced operand is to be
found, 2-15, 2-16

IAC

Initialize: To set counters, switches,

Czth

CML (complement link), 2-19, 2-20

(increment AC), 2-10
Legal combinations, E-2
NOP (no operation), 2-20
RAL (rotate AC, and L left), 2-20
RAR (rotate AC and L right), 219, 2—20
RTL (rotate AC and L twice left),
(rotate AC and L twice
right), 2-19, 2-20
2 (skip) microinstructions, 2-

RTR

Grgup
1

and addresses to zero or other
starting values at the beginning of,
or

at prescribed

Input and output units, 2-5
Input unit, general organization of
PDP-8, 1-32, 1-33
Input/output transfer instructions,
see

IOT instructions.

Ins3e5‘ting

Format 2-21
HLT (halt), 2-22

Illegal combinations, 2—23
OSR (inclusive OR of AC with
switch register), 2-22
SKP (unconditional Skip), 2-22
SMA (skip on minus AC), 2-21,
2-22

SNA (skip on non-zero AC), 2-21,
2-22
SNL (skip on non-zero link), 2-22
SPA (skip on positive AC), 2-21;
2-22
SZA (skip on zero AC), 2-21, 2-22
SZL (skip on zero link), 2-22
H

Halt switch, 4-4

points in, a com-

puter routine.

comments and headings, 3-

Instruction: A command which
causes the computer or system to
'

perform an operation. Usually one
line of a source program.

Instruction field, ,4-5, 4-20
Instructiou register (IR), 2-7
Internal storage: The storage facilities forming an integral physical
part of the computer and directly
controlled by the computer. Also
called main memory and core
memory, 1-32

Interpreter:

A program that trans-

lates and executes source language
statements at run-time, 8-11

Interpretive mode, 8-12, 8-20, 8-27
Index—6

“

‘

Least significant digit: The right-most
digit of a number.

Interrupt program debugging, 5-81
Interrupt programming, 6-22
Interrupt request flag, 6422

Library routines:

A collection of
standard routines which can be

Interrupt service routine, 6-25
I/0: Abbreviation for
IOT instructions,
Format of, 6- 2

incorporated into larger programs.
Library syStem, DECtape, 7-26
LING-8 Computer system, 1-3.
Line feed: The Teletype operation
which advances the paper by one

input/output

DECtape, 7-10'
Usage, 6-3
IR (instruction register), 2—7
Iteration: Repetition of a group of
‘

line.
Line’ number: In source languages
such as FOCAL, BASIC, and
FORTRAN, a number which begins a line of the source program
for purposes of identification. A

instructions.
I

Job: A unit of. code which solves a

numeric label.

problem, i.e. a program and all its

Link.

related subroutines and data.
IMP (jump), 2-10

JMS (jump to subroutine) 2-12
Jump: A departure from the normal
sequence of executing instructlons
in a computer.

-2.

symbol.

K
K: An

abbreviation for the prefix

kilo, i.e. 1000 in decimal notation.
Keyboard/printer devices, 49

pointer

to the next
An address
element of a list, or the next
block number of a file.
,

Linkage: The code that connects two
separately coded routines.

List:

Label: One or more characters used

to identify a source language statement or line.

Language, assembly: The machineoriented programming language
used by an assembly system, e.g.
PAL III, MACRO-S/and SABR.
Language, computer: A systematic
meahs of communicating instruc-

3.
..

in which programs are written and

Leader: The blank section of tape at
the beginning of the tape.

Load. To

place

data into

internal

Location assignment, 3-6

guage such as PAL III or FOCAL

computer.

symbol which defines it-

Location: A place in storage or memory where a .unit of data or an instructlon may be stored, 3-6

Language, Source: A computer lan-

LAS (load AC from switch register),
2-27

Literal. A
self

Loaders, 5— 1

tation.

which require extensive translatlon
in order to be executed by the

the
termi-

Pushd'own

storage.

tions and information to the com-

Language, machine: Information. that
can be directly processed by the
computer, expressed in binary no~

A set of items,
To print out a listing
line printer or
nal.
See
list.

console

-pu.ter

Least significant binary digit, 148

A one- bit
register in the
PDP- 8, 2-6
An address pointer generated
automatically by the PALS
or MACRO-8 Assembler to indirectly address an~ off-page

Logic operations, 1-29
Loop: A sequence of instructions
that is' executed repeatedly until a
terminal condition prevails.
Looping a program, 3-24
M

(memory address register), 2—8
Machine language programming: In
MA

this text, synonymous with assembly language programming. This

Index—7

term is also usedto mean the ac-

JMS (Jump to subroutine), 2- 12
TAD (two’s complement add), 2-9

Macro instruction: An instruction .in
a source language that is equ1valent to a specified sequence of
machine instructions.

Memory unit, 1-33, 2- 5, 2- 7
Microinstructions, see also Programming fundamentals.
Combined mnemonics, 2-23, 2-27
Skip, 2—25
Mnemonic coding, 2'3
Mode,
Interpretive, 8-27
Single instruction, 8-27
Modes of operation,
Command, 5-12

tual binary machme instructions.

Major state generator, 2-7
Mantissa of
1-35

floating-point number,

High-order, 8-6
Low-order, 86
Manual Input: The entry of data by
hand into a device at the time of
.

pI’OCCSSng.
Manual operation: The processing of
data'1n a system by direct manual

techniques

Text, 5-12
Monitor: The master control program that observes, supervises,
controls or verifies the operation

of a system.

Manual program loading, 4-6, 4-7

significant digit: the left-most
digit, 1—8, 8-5
Multiplication in binary and octal,

Mask: A bit pattern which selects
those bits from a word of data
which are to be used in some subsequent operation, 1-29

Most

Mass storage. Pertaining to a device

Multiplier quotient register, 4-5, 4-24
Multiprocessing: Utilization of sev-

such

disk

DECtape which

stores large amounts of data readily accessible to the central pro-

cessing unit.
rectangular array of elements. Any table can be consid~

Matrix: A

ered a matrix.

non-zero

1-23

eral

processors

execution of two or more pro—
grams in core at the same time.
Execution cycles between pro—

Memory:
1.

computers

logically or functionally divide
jobs or processes, and to execute
them simultaneously.
Multiprogramming: Pertains to the

MB' (memory buffer register), 2-7
’

The alterable storage in a com-

grams.

puter.
2.

Pertaining to a device in which
data can be stored and from
which it can be retrieved.

Memory address register (MA), 2-8,
Memory bufier register (MB), 2-7
Memory, extended, 4-20

Memory field, 420
Memory

protection: A method of

preventing the

contents of some

part of main memory from being

destroyed or altered.
Memory reference instructions, see
also Programming fundamentals.
AND (Boolean AND), 2- 9
DCA (deposit and clear AC), 2-10
Format, 2— l4
ISZ (increment

and skip if zero),

2-10
List of, Dl-2
JMP (jump), 2-10

Negative numbers and subtraction,
1-20

Nesting:
1. Including a program loop in—
Side loop
2. Algebraic
nesting, such as
(A+B*(C+D)), where exer
cution proceeds from the innermost to the outermost level.

N0P: An instruction that specifically
does nothing (control proceeds to
the next instruction 1n sequence),
2-20
_

Normal mode, 7-2,7—13

Normalize: To adiust the exponent

and mantissa of a floating-point
number so that the mantissa ap—
pears in a. prescribed format, 8—5

Numeric translation routines, 6—11
Index——8

OR: (Inclusive) A logical operation
such that the result is true if either
or both operands are true, and
false if both operands are false.
(Exclusive) A logical operation
such that the result is true if either
operand is true, and false if both
Operands are either true or false.
When" neither case is specifically
indicated, Inclusive OR is assumed.
See also Inclusive OR and Exclusive OR.
Group of microinstructions (SMA,
SZA, SNL), 2-25
Logical operation, 1-30

Numbers,
Double precision, 1-35
Floating-point, 1-35
Representation in the PDP-8, 1-34,
Number systems,
Definition of basic concepts, 1-5
Primer, 1—5
0

Object Program: The binary coded
program which is the output after
translation of a source language

program.
Octal: Pertaining to the numbersys—
tem with a radix of eight.

of execution for combined
microinstructions, 2—26
Origin: The absolute address of the
beginning of a section of code, 3-6
OSR (inclusive OR switch register
and AC), 2-22
Output: Information transferred from

Order

Octal coding, 2-2
'Octal Debugging Technique (ODT),
5-68‘
Commands, 5-71
Errors, 5—81
Features, 5-68
Loading and starting, 5-70
Operation, 5-69
Programming notes, 5-82
Punching binary tapes, 5-77
Storage requirements, 5-69
Using, 5-69

the internal storage of a computer
to output devices or external storage.

Overflow: A condition that occurs
when a mathematical operation
yields a result whose magnitude is
larger than‘the program is capable
of handling, 8-20

Octal numbers, 1-11
Addition, 1-19
Division, 1-26
Multiplication, 1-24

Page: A 128nword section of PDP-8

Multiplication table, 1-25
Subtraction, 1-22
Octal to decimal conversion, 1-12
Off-line: Pertaining to equipment or
devices not under direct control
of the computer, or processes per—
formed on such devices.

Orr-line: Pertaining to equipment or
devices under direct control of the
computer and to programs which
respond directly and immediately
to user commands, e.g., DDT.

Opera-rid:
1. A quantity which is alfected,
manipulated or operated upon.
2. The address, or symbolic name,
portion of an assembly language instruction.

Operate microinstructions, 2-18
Operation code, 6-2
Operation specification bits, 6-3
Operator: The symbol or code which
indicates an action (or operation)
to be performed, e.g. + or TAD.

core

memory

dress which is

Panel lock, 4-3

beginning at an ada multiple of 200.

Paper tape formats, 4-14 to 4—17
Paper tape loaders, 4-18
Parity error, 7-6, 7-7
Pass: One complete cycle during
which a body of data is processed.
An assembler usually requires two
passes during which a source prois translated into binary
gram
,

code.
Patch: To

modify

routine

rough or expedient way.
Peripheral equipment: In a data pro-

cessing system, any unit of equipment

distinct

from

the

central

processing unit which may provide

the system with outside storage or
communication, 4-18.

Phases of programming, 3-2
Pointer

address: .Address of a core

memory

Index—9

location

containing the

actual

(effective) address of de-

sired data, 2-15
Position coefficient, as used in number systems, 1-6
Powers of two, 3-16

Priority interrupt: An interrupt which
is given preference over other in-

terrupts withm the system, 6-30
Procedure. The course of action
taken for the solution of a problem. See also Algorithm.

Program: _The complete sequence 'of
Instructions and routines necessary
to solve a problem.

Program counter (PC), 2-6
Program interrupt, 6-1, 6—22 to 6-30
Programmed transfer, 6-1 to 6-4
Programmer’s console, 4-1 to 4-6

Programming fundamentals, 2-1
(AC), 2-6

Accumulator

AND group microinstructions (SPA,
SNA, SZL), 2-25
AND instruction, 2-9
Arithmetic unit, 2-5
Binary coding, 2-2
CLA (clear the AC), 2—19, 2-22
CLL (clear the link), 2—19, 2—20
(complement the AC), 2-19,

CZIVIig
(pomplement the link), 2-19,
CM216
2

Combining microinstructions, 2-23
skip microinstructions,

ngtsbining

Control unit, 2-5, 2-6
Core memory, 2-5, 2-7
Current page or page zero bit, 2-15
DCA (deposit and clear AC), 2-10

Exercises, 2-28
Group. 1 microinstructions, 2-18
Group 2 microinstructions, 2-21
HLT (halt), 2-22
IAC (increment the AC), 2—20
Incrementing a tally, 2 11
Illegal combinations of microinstructions, 2-23
Indirect addressing, 2- 15
Input and output units, 2- 5
Instruction register (IR), 2-7
Interrupts, 6- 22 to 6 30
182 (increment and skip if zero),
2- 10
IMP (Jump), 2 10
--12
JMS (Jump to subroutine),2
Link, 2-6

Major state generator, 2-7
address register (MA), 2-

Ngengosry

Memory buffer register (MB), 2- 7
Memory page, 2—13

Memory

reference

lflStI‘UCthfl

Memory unit, 2-5, 2-7
Microprogramming, 2-23
Mnemonic coding, 2—3
NOP (no operation), 2-20
Octal coding, 2—2
OR group microinstructions (SMA,
SZA, SNL), 2-25
Order of execution of combined
microinstructions, 2-26
OSR (exclusive OR switch register_
with AC), 2-22
PDP-8 organization and structure,
2-4, 2-5
Pointer addresses, 2-16
Program counter (PC), 2-6
RAL (rotate AC and link left),
2-20
RAR (rotate AC and link right),

2-19, 2-20

(rotate AC and link twice

RTL

left),

2-20

(rotate AC and link twice
right), 2-19, 2—20
Rules for combining microinstructions, 2~28
RTR

SKP

(unconditional skip), 2—22

SMA (skip

minus

2-22

AC), 2-21,

SNA (skip on, non-zero AC), 2-21,
2- 22
SNL (skip on non-zero link), 2- 22
SPA (skip on positive AC), 2- 21,
2- 22
SZA (skip on zero AC), 2-21, 2 22
SZL (skip on zero link), 2- 22

TAD (two’s complement add),2

Pseudo-operation: An instruction to
"the assembler; an operatiOn code
that is not part of the computer’s
hardware command repertoire.
Pusha'own list: A list that is constructed and maintained so that
the next item to be retrieved is
the item most recently stored in
the list.

Q
Queue: A waiting list. In time-sharing, the monitor maintains a
queue of user programs waiting
for processing time.

Index—10

RK8 DECdisk, 4-23

R
7

Routine: A set of instructions arranged in proper sequence to
cause the computer to perform a
desired task. A program or sub-

Radix.‘ The base of a; number system; the number of digit symbols
required by a number system.

RAL (rotate AC and link left), 240

program.

Random access: A storage device in
which the .paddressability Of data
is eifectively independent of the
location of the data. Synonymous
with direct access.
‘

RAR (rotate AC and link right),
2- 19, 2- 20
from
Read: To transfer

R808 DECdisk, 4-23
RTL

(rotate

and

link twice

left), 2-20
(rotate AC and link twice
right), 2-19, 2—20

.RTR

Rules for combining microinstructions, 2- 28

information

Run: A single, continuous execution
of a program.

input device to core memOry.

Read-in mode, see RIM.

Run light, 4- 5

READ subroutine, 7-22

Ready status, 6-3
Real-time: Pertaining to computation
performed while the. related physical process is ..tak1ng place so

S‘

Search mask, 5- 75
'

Segment:

results of the
can be used in
that

cal process.

computation
guiding the physi~

That part of a long program
which may be resident in core
at any one time.
2.' To divide a program into two

segments or to store
routine on an external storage device to be
brought into core as needed.
or more

Recursive subroutine: A subroutine
capable of calling itself.

part of

access: Pertaining to the sequential or consecutive transmis-

sion of data to or from core, as
with paper tape: contrast with
random access.

Relative address: The number that
specifies the difference between
the actual address and a base address.
1
-

tme whose instructions are written

that they can be located and

executed in different parts of core
memory.
Response time: Time between initiating an operation from a-remote
terminal and obtaining the result.

Includes transmission time to and
from the computer, processing
time and access time for files em-

ployed.
R.estart

To resume execution of a

program.

Restrictions using breakpoints, 5-5 8
‘

Reversing DECtapes, 7'- 14
RIM format, 4-17

RIM LOADER, 4—18, 5-2-

Sign-magnitude notation, 8-7

Simulate: To represent the function
of a device, system or program
with another device, system or
'

‘

program.
_

Single-cycle data break, 6-40
Single instruction mode, 8-8, 8—27
Single-step: Operation of a computer
m, such a manner that only one
instruction is executed. each time
the computer is started.
Single step switch, 4-4
..

RF08 DECdisk controller,

bits

to the
movement of
left or right frequently performed
in the accumulator.

Shift. A

Reiocatable: Used to describe a rouso

Serial

data,

‘

Record: A collection of related items
of data treated as a unit.
“

subroutine, 7- 21

Skip chain, 6- 29
Skip microinstructions, see Group. 2
microinstructions.

23
‘

SKP (unconditional Skip), 2— 22

SMA (skip
2— 22

Index—1 1

minus

AC), 2-,21

SNA (skip
2-22

non-zero

ment, the action of either

AC), 2-21,

SNL

into core or storing it
tem device.

(skip on non-zero link), 2-22

Software:

collection of proroutines associated

grams

The
and

With

computer.

Switch:

6—30
See

Symbolic address: A set of characters used to specify a memory
location within a program, 3-7

Symbolic Editor: A PDP-8 system
library program which helps users
in the preparation and modification of source language programs
by adding, changing or deleting

positive AC), 2-21,

Starting address of a program, 3-6
display indicators, 4-6
Statement: An expression or instruction in source language.
Status display indicators, 4-5
Status registers, 7-6
STL (set the link), 2~27
Storage allocation: The assignment

'State

lines of text, 5-11

Commands, 5-24
Example of use, 5-35
Generating a tape, 5-30
Loading a tape, 5-31
Loading and operation, 5-28
Switch register options, 5-18
Summaries, 5-38, 5-40
Symbolic language,
Conventions, 3-8
Special characters, 3-9
System: A combination of software
and hardware which performs
specific processing operations.
SZA (skip on Zero AC), 2-21, 2-22
SZL (skip on Zero link), 2-22

of blocks of data and instructions
to specified blocks of storage.

Storage capacity: The

amount

data that can be contained in
storage device.

Storage device: A device in which
data can be entered, retained and

retrieved.

Store:_To enter data into a storage
devrce.

String: A connected sequence of entities such as characters in a com-

Table: A collection of data stored
for ease of reference, generally

mand string.

Subroutine, closed: A subroutine not
stored in the main part of a program, such

subroutine is norentered with a
JMS instruction and provision is
made to return control to the
main routine at the end of the

Subroutine, open: A subroutine that
must

used.

be relocated and inserted
routine at each place it is

matting

‘

Text routines, 6-8

TCOl

Bootstrap Loader, 7-31
TCOS DECtape Control Unit, 7-4
TDS-E DECtape System, 7-32
TDS-E DECtape Subroutine, 7—32
TU56 DECtape Transport, 7-4
Teletype
,

Subroutines, programming, 3-16
Subscript: A number or set of numhcrs used to specify a particular
item in

array.

complement add), 2-9
Tape formatting, see DECtape 'for-

subroutine.

TAD (two’s

mally called

into

—

ues are recorded.

Language,

Source program: A computer program written in a-source language.
on

_A device or programming

Symbol table: A table in which symbols and their corresponding val-

”source.

SPA (skip
2-22

the sys-

technique for making selections.

Sorting program, 3-31

language:

Switch register, 4-3'

Software priority interrupt system,

So'urce

tem-

porarily bringing a user program

‘

array.

Console, 4-11
Controls, 4-1}, 4-12
IOT instructions, 6-4 to 6-6

Keyboard, 4—12

Subtraction, pregammg, 343

Punter, 4-13
Punch, 4~14

Swapping: In a time-sharing environ—

Reader, 4~13

Induce-12.

yields a result whose magnitude is

storage: Storage locations reserved for immediate results.
Terminal: A peripheral device in a
system through which data can
enter or leave the computer.

Temporary

Timesharing: A method of allocating central processor time and
other computer resources to multiple users so that the computer, in
effect, processes a number of pro-

smaller than the program is capable of expressing.

UPDATE program, 7-26

Updating current line counter, 5—15
User: Programmer or operator of a
computer.

V
4,

A symbol whose value
of a prochanges during

Variable.

grams s1multaneously.

Time quantum: In time-sharing, a
unit of time allotted to each user
by the monitor.

execution

gram.

switches to enterfToggle.
data into the computer memory.

Translate: To convert .fromone language to another:

Weighting tables, 1-6, 1-8

use

Word count register, 74

Underflow:
when

occurs

floating point operation

(DDT), 5-58
.WRITE subroutine, 7-22
Word searches

A condition that

word count reg-

of data which may be stored in
one addressable location.

complement arithmetic, 1-20
U

see

“

Word: In the PDP-S, a 12-bit unit

Truncation: The reduction of precision by' dropping one or more
of the least significant digits; e. g.
3.141592 truncated to four decimal digits1s 3 141.
Two’s

ister.

Write: To transfer information from
memory .to a peripheral de-

core

v1ce or

Index— 1 3

to-aux1liary storage.“

‘

NOTES

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