Digital PDFs

XX-9B073-BE

September 1973

183 pages

Original

8.4MB

Document:	Introduction To Programming The PDP-11 Donald S. Lawrence, Jr.
Order Number:	XX-9B073-BE
Revision:	0
Pages:	183
Original Filename:	http://bitsavers.org/pdf/dec/pdp11/training/Lawrence_Introduction_To_Programming_The_PDP-11_197309.pdf

OCR Text

INTRODUCTION TO PROGRAMMING
the
PDP-ll

------_._---- ----------

•

The reproduction In part or whole Is strictly prohibited. For copy Information contact the Digital Educational Services Department in Maynard. Massachusetts 01754.

.-.;,

~--

~
., ?' -7
\ --(
t/ - - -.
/l

INTRODUCTION TO PROGRAMMING
the
PDP-II

Donald S. Lawrence, Jr.
DIGITAL EQUIPMENT CORPORATION

preliminary Printing, May, 1.973
(Chapters I-III)

!!Q!!

handbook is for information purposes and is subject
to chanqe Without notice

This

A•• oclated Document••

PDP-ll Proce••or Handbook
PDP-ll Peripheral. and Interfacinq Handbook
PDP-ll Paper Tape Software Proqramminq Handbook

Trademarks of Diqital Bquipment Corporation include.
DEC

PDP-ll

DBCtape

RSTS-ll

DIGITAL (logo)

RSX-ll

COMl'BX-ll

UNIBUS
11

~
. ./

PREPACE

The primary purpose of this book i8 to serve as an
introduct:ion to the PDP-11 family of comput.ers, althouqh it
will meet t.he needs of a general readership as well.

assumes little or no previoua computer experience on the pe":~

ot t.he reader, and thU8 contain. introductory information of
a qeneral nature and d18cu•• ion of fundamental concepts in

addition t.o supplying material pertinent. to t.he PDP-l1.
It. i8 intended t.o provide an underst.anding of computers
1n general and the PDP-ll family in particular, and t.o serve
as a prelude t.o more advanced document.at.ion •

•
Donald S. Lawrence, Jr.
May, 1973

ili

TABLE or COm'BI1T8
PAGB

CHAPI'BR

II.

INTRODUCTION TO COMPt1l'BRS

1-1

1.1 PERSPECTIVE

1-1

1.2 DBFINITION

1-2

1.3 CLASSIFICATION

1-6

1.4 APPLICATION

1-8

NUMBERS AND OTHER STUFF

2-1

2.1 INTRODUCTION

2-1

2.2 NUMBER SYSTEMS

2-2

2.2.1 B••ie Prineiple.
2.2.2 Decimal _umber By.tem
2.2.3 Binary Number 8Y8~"
2 • 2 • 4 octal NWDber Sylltem
2 • 3 CONVBRSJ:ONS

2-23

2.3.1 Introduetion
2.3. 2 Decimal to Binary
2.3.3 Binary to Decimal
2.3.4 Decimal to octal
2.3.5 Oetal to Decimal
2.3.6 Binary to oc:tal
2.3. 7 oetal to Binary
2.4 ARITHMETIC OPBRATIONS

2-35

2.4.1 Introduction
2.4.2 Addition
2.4.3 Direct Subtraction
2.4.4 Complementary Addition
2.5 LOGIC OPERATIONS

2-55

2.5.1 Introduetion
2.5.2 AND
2.5.3 Incluaive OR
2.5.4 Bxcluaive OR
2.6 BXBRCISBS

2-63

CHAPTER

PAGE

III.

THE PDP-11

3-1

3.1 SYSTEM ORGANIZATION

3-1

3.1..1 Introduction

3.1.2 The UNIBUS
3.1.3 Memory
3.1.4 Central Proces.or
3.1.5 Input-output Devices
3.2 ADDRESSING MODBS

3-19

3.2.1 Introduction
3.2.2 General Reqi8ter Addressinq Hodes
3.2.3 Prpgram Counter Addressinq Modes
3.2.4 !:xerei•••
3.3 INSTRUCTION SET

3-49

---------------------------------------------------------------(FORTHCOMING)
3.4 SYSTEM OPERATION

IV •

PUNDAMBNTALS OF PROGRAMMING

4.1 THB

PROGRAM

4.2 LANGUAGES

4.3 CONCEPTS AND TECHNIQUES
4.4 EXAMPLES

•

.~ .-

I:)

Chapter 1
INrRODUCTION TO COMPt11'ERS
1.1

PERSPECTIVE

The transition from manls first desire to count and measure
to your PDP-Il computer is indeed a great one, and the reader is
heartily encouraged to pursue the fascinating topic of computer
history.

For the purpose of this text, it is sufficient to say

that the development and continuing evolution of the computer has
brought about a dramatic change in our lives and promises even
greater change in the years to come.
During the last twenty years especially, there has been an
explosive proliferation of computing machines designed to meet a
vast range of applications.

Perhaps of even more importance than

the ever-improving developmental technology is the unceasing discovery of new ways in which we may use computers.

In fact, it

would seem that we have reached the point where :'he machine capa•

bility and the task are present, and it is only our lack of applicational insight that limits us.

1.2

DEFINITION

The majority of everyday users, as well as the novice, view
the computer as a Ilblack box."

That 1s to say, they know ~ is

done (the performance characteristics) but not ~ it is done (the
components and/or means of operation).

Though this knowledge is

superficial, it 1s 6ften sufficient; rarely 1s anyone required to

fully comprehend all the details of any computer system.

It is

in fact customary for the individual to accept a tlblack box" description of a computer or computer function at some level, and
then deepen his understanding when motivated by desire or necessity.

As indicated in the preface, this text assumes that you

~)resently regard

the computer itself as a .Iblack box, II and will

attempt to brlnq you beyond that level.

The computer may be defined a8 a machine, devised and used
by man because,(like other machines) it can perform certain tasks

better than man himself.

Technically, it is an electronic device capable of accepting
information, applyinq prescribed processes to that information.
and supplying the results of those processes.

Very basically

then, the computer canl
(1) accept INPtn' (information t.o be processed)·

(2)

P~OCESS the

(3)

produce Ot71'PUT (the results)

information (manipulate 1t in a prescribed way)

Based on this definition, we may represent the computer by
means of the followlnq block dlaqram (Fiqure 1-1).

....

INPtYr
UNIT

Figure 1-1

PROCESSING
UNIT

......

our PUT
UNIT

Simplified Compu~er Block Diagram

At this point, our definition and diaqram might well encor.~-

pass other devices, such as the electric adding machine and desk
calculator.

To differentiate, we will note that the computer

posseses two additional (and distinctive) characteristics I
(1) It is capable of manipulatinq a variety of symbols, and
is not restricted to numbers only.
(2)

It processes data.

It processes automatically, with only initial human in-

tervention reqUired.

The sequence of operationI' to be performed

(called the program) is first stored in the computer.
A deeper observation of computer operation will help illustrate these aspects of your Programmed Data Processor.

------_._--_._-------_.

Let us approach this more detailed representation by using
Y5!!:! as an example.

You are given the verbal directive, "Mentally

add the numbers fifty-four, eighty-seven, and thirteen. N

After an

individually dependent computational pawse, you orally respond,
r:i

liThe sum is one hundred and fifty-four."

Figure 1-2

Analoqous Computer BloC:1c Dla~ram

What has happened?

Your aural and vocal anatomy has served

as the means of INPOT and Ot1l'PtJT respectively, your brain has been
used to PROCESS the information.

For the present, the terms INPUT

and Ot7l'POT sufficiently describe the operations performed, but the
term PROCESS appears to be somewhat obscure.

Let us examine what

has taken place here a little more closely.
(1) You remembered the values given and called upon skills
o

previously learned and retained - therefore, MEMORY was· required
(2) The operations ~were ordered, With the values being man1pulated in a pre8cibed manner - thu8, Bome element of CONTROL was
present
(3) A mathematical calculation was performed - hence, an
ARITHMETIC function was involved

We may directly associate these features with units in the

basic computer block d1aqram (Fiqure 1-3) to complete this qeneral definition.

r - - - - - -- - -I
I
I

J,
INPtn'
UNIT

r---

I
I
I
I
I
"

UNI'l'
~

......

I
L_~

Fiqure 1-3

•

MEMORY
UNIT

I
I

---- -----,

CONTROL

......

Ot11'Pt11'

UNIT

ARITHMBTIC
UNIT

data f low
----- contro1 path

Basic Compueer Block Diaqram

INPt11' UNIT - UlXler the direction of the CONTROL UNIT, it
It

supplies the computer with all the information needed to accomplish a qiven task, the values to be operated upon (data) and the
operations to be performed upon those values (proqram)

MEMORY UNIT - Cont.ains information for the CONTROL UNIT (proqram) and the ARITBMJft'IC UNIT (dat.a) J holds intermediate and final

results

CONTROL UNIT - Direct.s the ent.ire proces8 by 8pecifylnq
to the ARITHMBTIC UNIT what operation. are to be performed, in
which order they are to be performed, and where t.o get/put the
data involved
ARITHMB'l'IC UNIT - Under the direction of the CONrROL UNIT,
it performs t.he actual operations, the aworkinq area"

Ot11'PUT UNIT - Under the direction of t.he CONl'ROL UNIT, it

records the results of computer operat.ions

1.3

CLASSIFICATION
A8

specified by the characteristic. in Table 1-1, every

comput.er may be basically cateqorized a. either ANALOO or DIGITAL
(there are hybrid computer. that have both analoq and digital
properties).

We have been discussing (and will cont.inue to dis-

cuss) only the digital computer, .for your PDP-ll belongs to that
class.

This comparison is made for reasons of completeness and

further definition.
Table 1-1 Comparison of Analog and Diqital Computers
•

DIGITAL

AN&LOG

(1) Variable electrical or
mechanical quantities
used to represent. dat.a

(1)

Discrete numerical values
U8ed t.o represent dat.a

(2' Varying and continuous level (2) Set and discontinuous level
of input yields set. and disof input. yields varylnq and
cont.lnuou8 level of out.put
continuous level of out.put.
(3)

Calculates
by means of a
..
measurinq proce8s

(4)

Example.

speedometer

(3)

Calculates by means of a
countinq process

(4)

Example.

odomet.er

We may also cateqorize compu~er. accord1nq to their ~.8iqn

·capability •• beinq eli:her SPECIAL PURPOSE or GBNBRAL PURPOSE,
terms that are very aelf-explanat.ory.

The special purpose ma-

chine 18 constructed to perform one task, or a closely related
qroup of tasks.

The 8in.,le sequence of operations it 1s to per-

form (its proqram) i8 "built in.·

If ever a proqram chanqe be-

comes necessary, a hardware modification (physical restructur1nq)

is required.

Conversely, the general purpose machine is desiqned

to be capable of performinq many varied tasks.

The many possible

operation aequenc•• (proqrams) are kept in the memory unit of the
qeneral purpose computer, and for this reason it i8 sometime8 Z·.~·
ferred to a8 a B.t.cred program machine.

To pe~form a qiven task,

the user. simply calle upon the appropriate proqram.

The change

from one task to another 1. accomplished by selectinq another
proqram already in the mentOry or enterinq it by means of the Input unit.

It should be apparent that previous di8cussion has been of
the general purpose comput.er, and we will conti~ue to confine our-

selves to thi8 type.

Your PDP-II i8 classed a8 both dig-ital and

general purpose.
Now that you have a basie urder8tandinq of what the computer

1s, let us qenerally discUS8 why and 1n what manner it 18 used.

1.4 APPLICATION
If given unlimited time to complete a task, factors such as
volume of data, complexity of calculation, and deqree of accuracy
become immaterial.

Por example, you alone could certainly process

the payroll of a larqe corporation or perform all the calculations
nece.sary to launch a mi•• ile.

The chance of you accurately doing

either in a matter of hours or minute., hoWever, is rather remote.

It i8 then speed of operation which i8 the ultimate consideration
in both cases.

This element of 8~, coupled with acc~acy and

reliability, is the underlying advantaqe of the computer, it is
the major reason for its existence and use.
We have said that the computer i8 \l8ed because it performs

certain taSKS "better" than man.

The interpretation of this term

18 dependent upon the -task, and may imply any combination of the
following features.

speed, accuracy, preci8ion, reliability, econ-

omy, efficiency, fea8ibility.
used?

Where then 18 the computer to be

Wherever 1ts attribute. enable the t.ask to be done "better. II
'1'0 l18t the Wide and ever-expanding range of specific appli-

cations would be an arduous chore (surely requiring the use of a

cOIDpU1:erl ).

If, by way of example. t.he result.s of a PDP-ll appli-

cat10nal survey were immediately available to be given here, the
variety of response would easily fill the remainder of the book.

And this would be for only one cOIIlput.er model of one corporationl
Keep1nq this in mind, we may denot.e four general areas of applica-

tion.

Busine8S - Computers used ift busines. applications are
usually involved wl~h record keep1nq. automatinq the many t.edious,
repetitious tasks a88ociat.ed with clas8ify1nq. proce88inq, and
ma1nta1n1nq information of all kinds.

A8 a rule, the business

computer 18 required to perform only a few simple calculations,
but it must be capable of hand11nq a qreat volume of data •

•
Simulation - Any qiveD task may be too danqeroUB, costly, or
intricate for man to attempt.
all.

It: may not be feasible for him at

In such situations, the computer is used to simulat.e all con-

dit.ions and interactlons, yieldlnq knowledge without. risk.

•

. .

Chapt.er 2
NUMBERS AND OTHER STUFP

2.1

INTRODUCTION

In writing a book Buch as this, it is very often desirable to

explain several thinqs simultaneously.

Thi8 is one of those timesl

The first chapter has defined the general purpose diqltal computer~

and shown that it manipulates data accordinq to a proqram of

instructions.

A loqical continuation could therefore be a detailed

look at .the PDP-1! in terms of orqanization and unit interaction.
On the other hand, since we have mentioned the program and indicated
its siqnificance, fundamentals of programming could just as reasonably follow.

Then too, a discus.ion of proqramminq languages and

data repre8entation might serve as a likely sequel.
In developing any of these topics, however, there 1s an inescapable involvement With number concepts.
be made

Numerical references must

in describing the PDP-l1 and its operation, proqram instruc-

t.ions and data are ultimately represented in numerical code.

This

chapter will then concern itself With those number concepts and op-

•

erations required for you to fully appreciate sUbsequent discussion
of proqramRdng the PDP-ll.
The subject of computer math. numbers and "other stuff," is
interruptive regardless of when it is introduced.

For this reason,

the reader may wish to IItQve past it for the present and make back-

ward references where necessary.

2.2

NUMBBR SYSTEMS

2.2.1 Basic Principles

ManOs earliest form of notation wae the tally mark, where

there existed a one-to-one correspondence between the marker and
the object to be counted.

The aggregate of the scratch marks,

pebbles, or notches was the "number" he wished to record.

This

principle of repetition proved cumbersome for even moderately
large numbers, however, and so there evolved various number systems to meet the increasing demands of civilization.

The number system is a standard means of representinq quantity.

It consists of a finite set of symbols, called numerals or

digits, and rules Which specify how the symbols are arranged to
form numbers.

The early number systeas offered an improvement

over recording each unit in that they combined unique quantities

of units int.o groups and assigned di8crete symbols to represent
those groups.

They featured the principle of addition, Where the

value of an entire number is determined by adding the values of
the individual symbols that comprise it, irrespective of position.
(MMMCCCXXXIII = 1__;+1

+1__ ~l~;+1 __+l'-'l-'l'+l9+1+1+l

= 3333).

Thouqh later development introduced subtractive (IV=4) and multi-

plicative CM::l.,Itf6f)

principles, these systems still served pri-

marily for qtUlnt1fic:atlon and record keepinq_ and had little provision for calculation (even the IIlOSt. mathematically adept of .
~omans

removed his sandals for MeVII times CLXXVIII).

The positional number _yet. .s which followed contain two
additional and di8tinctive feature. which qreatly simplify the
operations needed to manipulate number..
and the inclusion of the zero symbol.

the concept of position

Like their early counter-

parts, these 8ystems have discret.e symbols with unique values and
follow the principles of addition and multiplication.

The major

distinction i8 the principle of place value, which specifies that
the:.!_~,!_. not--oEJ.Y_.~ ~~~~! . ".!~!~~! .. ~"~~,.~h~_.~~.J..~J?_~,.~~l~~ .~ ..~,~Slpe

value for the position.

Thus the value associated with a symbol

is determined by both its absolute value and the value of its relative position Within the number (3333 = 3~~~+3~~+3~+3 = 3333).
The importance of the zero symbol ift a positional number
system is illustrated by the application of the count and carry
(or reqroupinq) principle.

For example, we count from zero to ten

in the familiar decimal system a8 follows.

Ii1
o

2
3
4
5
6
7
8

10
We cycle throuqh the diqits to nine, but do not create another

symbol in countinq an additional unit.

We instead carry the one

to the next (tens) place, and record it there.

To indicate that

there are no units, and to "hold" the units place, we record a
zero in that position.

The 'three number systems that we will di8CUS8 in some detail
are presented in Table 2-1.

These are all positional number systems

which demonstrate the principles we have previously mentioned, addition, multiplication, place value, count and carry.
Table 2-1 Digits and Bases of Selected Number S¥Stems
NUMBBR SYSTEM

DIGITS

BASE

Decimal

;,1,2,3,4,5,6,7,8,9

Binary

~,1

Octal

',1,2,3,4,5,6,7

The term base, introduced 1n the Table, is commonly used to

name or describe a number system.

The decimal system, for example,

is often referred to as the 'base ten system.

Por any positional

number system, the base (or radix) 1s the number of diqits it contains.

2.2.2

Decimal Number System
One of the few assumptions made in this text 1s that

you are familiar with the decimal number system.

It is the

mathematical language of the Itreal world; It a language that
you use on a daily basis.

You have memorized the rules and

operational procedures to the point that they are automatically applied, and performing any calculation is straightforward.

The purpose of this chapter is to have you become

equally well acquainted with the binary and octal systems.
We will briefly reintroduce the decimal syStem here in relation to our previous discussion of basic principles, and
later reference it to help illustrate those aspects it has
~

in common with the less familiar systems.
The decimal or base ten nwnber system is comprised of
the digits zero through nine <_,1,2,3,4,5,6,7,8,9).

It is a

positional number system, so that in progressing from right
to left within a decimal number, the value associated with
each position is an increasing power or multiple of the base.
This place value principle is presented in Table 2- 3 •

Table 2-2

/I
116

Powers of Ten

116

1.16

1J1xl16 =

116/1

1/1
2

1/1
3

1/1

1Jlx1~l/l

1,flf4/1

l/1x1J'xlJlxl/l

1/1,/11616

l/1xl..fx1~1J'xl/l

1[6[6,/1/1/1

1Pbc1J6xl.fxl~1J&lf6

1,1616/1,1616[6

1/1
5

1/1

1J1

Table 2-3

Positional Notation with Powers of Ten
Powers of Ten

lf4

191

l,Jlf6f6,J6J6f6 116J6,J6J6f6

116

1J6,f6J6f1

1.,/11416

Place Values

116

16
116

11616

114

As shown in Table 2-4, determininq the t.otal value' of
a decimal number is accomplished by applying the principles
of place value, multiplication, and addition.

multiplying

the discrete value of the digit by the value of the position

,..

in which it is placed, and then adding the resulting products.
Decimal Number as the Sum of Powers

Table 2-4

191 191 111 191 1f6
1.

1 $f
116 III
6

L..=

7xlll

7xl =

6xlf6

691

5x1J6$f =

5~f6

4xl,f6[616 =

4,161616

316,16/116

2xl[616,f4[6Jl1 =

216[6,[616_

= 6xl$f
2

= 5xlJ6
= 4xl_

4
3xll1
=
=
5

= 2x116
6

= lxll1
o

3xll1,I1~16

= lxl.f6J116,J6I1J1 = +1,16f6f6.f6J1J6
1,234,567

The count and carry principle, to be examined in more
detail when we later discuss arithmetic operations, can be
simply illustrated by counting or.addit10n.

As eVidenced by

the example below, presented earlier in discussing basic principles of positional number systems, we see that the terms
count and carry are qUite self-descriptive; count until the
base is equaled, and carry that indication to the next column.
~
1

2
3
4
5
6
7
8
9

carry. 1. 16
When performing addition, the procedure is as follows.
(1)

Add the digits in the column, (2) If the base is neither

equaled nor exceeded, record the sumJ (3) If the base is
equaled or exceeded, divide by the base, record the remainder,
and carry the quotient to the next column.
carries.

51459

16 8

Note the presence of the zero symbol in the sum,
indicating IIno hundreds ll and uno thousands," and also
"holding~

those places within the number.

2.2.3

Binary Number Syst&~
The binary or base two number system is comprised

of only two digits, zero and one, commonly referred to as
bits (!2!nary digi.!:.!).

As illustrated in Figure 2-1., this

system 1s' capable of representing but two conditions, and
thus lends itself to the decision-making process; ideally
practical for the computer.

Fiqure 2-1

Popular Binary Device in Action

Since physical and electrical entities have' but two
states (i.e., sWitch open/switch closed, current/no current),
internal components of a computer can be easily desiqned to
accommodate data in binary form.

Computers have been built

to operate with other number systems, but the increased number of digits along with the proportionally increased number
of possible conditions make these computers overly complex
in design and difficult to manufacture.

For this reason,

the PDP-ll and a majority of computers operate with the binary number system, considered the "language of the computer. II

Though the computer works internally with the binary
number system, this does not mean that all information input
must be so represented.
in binary form.

In fact, rarely is the data initially

If strictly numerical, it is generally octal

or decimal, but it is even more commonly expressed in one of
many alphanumeric computer languaqes.

As we will later dis-

cuss, there are several methods by which information in any
of these forms is converted to binary before it is processed
by the computer.
We noted earlier that the binary or base two number
system 1s comprised of the digits zero and one (~,l.).

the decimal system, it too is a positional number system.
Progressing from right to left within a binary number, the
value associated With each position is an increasing power
or multiple of the base.

This place value principle for the

binary system is presented in Table 2-6.

Table 2-5

2 =

2x2 =

2x2x2 =

2x2x2x2 =

2x2x2x2x2 =

2x2x2x2x2x2 =

2x2x2x2x2x2x2 =

128

2x2x2x2x2x2x2x2 =

256

2x2x2x2x2x2x2x2x2 =

51.2

2x2x2x2x2x2x2x2x2x2 =

1,1124

2x2x2x2x2x2x2x2x2x2x2 =

2,1648

12
2

2x2x2x2X2x2x2x2x2x2x2x2 =

4,1196

2x2x2x2x2x2x2x2x2x2x2x2x2 =

8,192

2x2x2x2x2x2x2x2x2x2x2x2x2x2 =

16,384

= 2x2x2x2x2x2x2x2x2x2x2x2x2x2x2 =

32,768

1
,~

2
2

2
3

2
4

2
5
2
6
2

2
8

2
9

2
1~

1.1

•

13
2
14
2

Powers of Two

Table 2-6

Positional Notation with Powers of Two

Powers of Two

1{6
2

32,768: 16,384 8,192 4,1696 2,JU8 1,1624

512

256

128

2
4

16
1

Place Values

J-".,".

...

fi:

!,lIf-

o,J

Since the binary system is a positional number system,

•

the rules are identical to those of the decimal and octal
systems.

The only difference, of course, is that the base

is ~ rather than ten or eight.
Determining the total value of any binary number is then
accomplished-in the same manner used for the decimal system:
applying the principles of place value, multiplication, and
addition.

The discrete value of the digit (¢ or 1) is multi-

plied by the value of the position in which it is placed, and
the resulting products are added.
This operation 1s presented in Table 2-7.

Table 2-7

15
l.

2
l.

12
2

8
2

11
2

Binary Number as the Sum of Powers

2
1

2
f4

2
1

f8
1

=
=
=
=
=
=
=
=
=

Ixl =

j6x2 =

lx4 =

lx8 =

.sx16 =

lx32 =

,QJx128 =

lx256 =

256

= lxS12 =
= j6x211 = .f6x1,1624 =
= lx2
= lx2,/l48 =
12
= J6x2 13 = /i1x4,/696 =
= {6x2
= .f6xS,192 =
14

512

2,/648

= lx2

16,384

lx2

1.
~2

= lx2 3
= 1x24
= j6x2 5
= lx2

= J1fx2 7
= f6x2 8
= lx2 9
= lx2

= lx2

9bt64

= lxl6,384 =

16
/I

= lx32,768 = +32,768
52,/613

The count and carry principle also applies to the
binary number system, and can be simply illustrated by
counting or addition.

In fact, one advantage of binary

notation is the simplicity of operation.

Since the system

consists of only the symbols zero and one, all the digits

•

are used merely counting to onel

Counting an additional

unit equals the base, ltand 1s represented as 11' (read as
"one zero, II not "ten").

You have counted until the base

was equaled, and then carried to the next colwnn.

As shown

in Table 2-8, this occurs quite often in the binary number
system I
Table 2-8

Counting in Binary with Decimal Equivalents
Binary

Decimal Equivalents
~

1
2
3
4
5
6
7
8

11'
11
12
13
14
15
16
17
18
19

2/4

Binary addition illustrates both the count and carry
principle and the operational simplicity of the system.

shown in Figure 2-2, there are only four possible individual
conditions.
Figure 2-2

The Four Possible Conditions for Binary Addition
Addend Addend

Sum

Carry

:tL

1[6

The procedure followed in the addition operation is
the same as that followed for the decimal systems

(1) Add

the digits in the column, (2) If the base 1s neither equaled
nor exceeded, record the sum; (3) If the base is equaled or
exceeded, divide by the base, record the remainder, and
carry the quotient to the next column.
Note the importance of recordinq the zero remainders
in the example below, "holding-I those places Within the sumo
carries.
1

111
,

fA 1

2.2.4

octal Number System
We may call the decimal number system the "numerical

language of the real world" because much computer input and
output data is in this form.

The binary number system is

considered the "numerical language of the computer" because
the majority of computers are designed to work With this
notation.

For the machine ~anquage and assembly language

user, the octal number system provides an easily handled
bridge between these two, and may be called the "numerical
language of the programmer."
As noted earlier, the binary system is mostcomrnonly

used with computers because its simplicity yields hardware
advantages; the components can be fast, yet relatively simple
and inexpensive to manufacture.

Computers, however, don't

have to IIlook li at the binary numbers they manipulate, and due
to speed of operation, work with them one at a time.

To the

programmer who must work With many cumbersome groupings, the
length of the numbers and the similarity of digits makes the
binary system far from ideal.

As we will discuss shortly,

there exists a quick and direct conversion between the binary
and octal systems, and for reasons given any numerical work
at the machine language or assembly language level is done
with the latter system.

The octal or base eight number system is comprised of
the digits zero through seven (~,1,2,3,4,5,6,7). Like the
decimal and binary systems, it too is a positional number
system.

Progressing from right to left within an octal num-

ber, the value associated with each position is an increasing
power or multiple of the base.

This place value principle

for the octal system is presented in Table 2-191.

Table 2-9
fir.

Powers of Eight

1 =

e =

axe =

8x8x8 =

512

8x8x8x8 =

4,~96

8x8x8x8x8 =

32,768

2
8
3

4
8
5

Table 2-1J~

Positional Notation with Powers of Eight
Powers of Eight

J
8

32,768 4,f196

512

Place Values

8
8

Since the octal system is a positional number system,
the rules are identical to those of the decimal and binary
systems.

The only difference, of course, is that the base

is eight rather than ten or two.
As presented in Table 2-11, determi~ing the total
value of any octal number is accomplished in the same manner
used for the decimal and binary systems.

applying the prin-

ciples of place value, multiplication, and addition.

The

discrete value of the d1qit is multiplied by the value of
the position in which it is placed, and the resulting products are added.
Table 2-1.1.

8
4

5
8

Octal Number as the Sum of Powers

8
5

a
6

6x8

Sx8

6xl =

5x8

4x64

256

3x512

1,536

= 2x4,~96 =

8,192

1
2

= 4x8
3

= 3x8
4

2x8
5

= lx8

= lx32,768 = +32,768
42,798

The count and carry principle also applies to the
octal number system, and can be simply illustrated by
counting or addition.

As shown in Table 2-1.2, you count

until the base is equaled, and then carry to the next column.

oj.

Table 2-1.2

Counting in octal with Decimal Equivalents
Octal
~

Decimal Equivalents
Ji1
1.

3
4
5
6
7
8
9

4
5
6
7
1~.

12
1.3

191

1.1.
12
13
14
15

1.6·

15
16

•
•

•

•
•

416
•

•

4¢
•

•
•
6[6

•

7¢

•

•
•

1~~

•

The procedure followed for addition is the same as
that followed for the decimal and binary systems.

(1) Add

the digits in the column, (2) If the base is neither equaled
nor exceeded, record the sum, ( 3 ) If the base is equaled or
exceeded, divide by the base, record the remainder, and
carry the quotient to the next column.
carriesJ

];

Again note the importance of the zero remainders
recorded in the sum, "holdinq" those places Within the
number.

2.3

CONVERSIONS

2.3.1 Introduction
It should by now be established 'that the binary number
system 1s good for computers, but little elsel

Therefore,

numerical data written in decimal or octal form must first
be converted to binary so that it can be processed by the
computer, and then the results converted back from binary to
decimal or octal so that they can be readily interpreted.
This process is represented by Figure 2-3.
Figure 2-3

The Conversion Process

Decimal Number

Octal Number

342341

J234567

Binary Number
I

9J~l¢1J6~111~9J1.~111~111

As noted earlier, these conversion processes are
usually performed by programs previously written and stored
in toe computer.

Specific conversion examples follow to

prOVide an understandinq of the processes.

2.3.2 Binary to Decimal Conversion

There are two commonly used methods for converting
binary numbers to decimal equivalents.

the Place Value

method and the Double Dabble method.
The Place Value method is simply the procedure used
,j

in representing a binary number as the sum of powers.

The

discrete value of each digit is multiplied by the value of
the position in which it is placed, and the resulting products are added.

An example of this method is presented

in Table 2-13.
Table 2-13

Place Value Binary to Decimal Conversion

543

222

1. (Binary Number)

l= lX/

= lxl =

= Ji1x2 . = pJx2 =

1
2

lX2

lx4 =

= lx8 =

= 91x16 =

=
3

= lx2
4

= 1Jx2
5

= lx2

= lx32 = ill...
45 (Decimal Equivalent)

To convert binary numbers to decimal equivalents

by means of the Double Dabble method, begin with the most
significant bit (left-most one bit) of the binary number.
~ouble that

bit, and if the next lower order bit is a one,

dabble (add one).
do not dabble.

If the next lower order bit 1s a zero,

Moving from left to right within the binary

number, repeat this process (doubling the

s~~

if the next

bit is zero, doubling the sum and dabbling if the next bit
is one) until there are no more digits.

An example of this

method is oresented in Table 2-14.
Table 2-14

Double Dabble Binary to Decimal Conversion

1~9J,--",,~~9J~1(Binary Number)
double

double

\dO
noJ\dabble/\dabble/'dOdabble
notAdabble
dabble
\/

•

\5/

\2/

\ 5 (Decimal EqUivalent)

2.3.3 Decimal to Binary Conversion
There are two commonly used methods for converting
decimal numbers to binary equivalents.

t he Subtraction of

Powers method and the Division method.
The procedure for the Subtraction of Powers method is
as follows.

(1)

Subtract the highest possible power of tw~

from the decimal number, and record a one in the apporpriate
place within the partially completed binary equivalent,
(2)

Repeat this subtraction process With the resulting dif-

ferences and descending powers of two (recording a one if
that power can be SUbtracted, recording a zero if it cannot
be subtracted) until the decimal number is reduced to zero.

An example of this method is presented in Table 2-1.5.
Table 2-15

Subtraction of Powers Decimal to Binary Conversion

(Decimal Number)

54321
2
222

Place

Value

(Binary Equivalent)

The procedure for the Division method of decimal to
binary conversion is as follows.

(1) Divide the decimal

number by two; the remainder is the LSD (Least Significant
Diait) of the binary equivalent, (2) Repeat this division

process with the resulting quotients (recording remainders
right to left within the binary eqUivalent) until the quotient becomes zero.

An example of this method is presented

in Table 2-16.
Table 2-16

Divisor Quotient Remainder
(Decimal
Number)
2

1.l

•

Division Method Decimal to Binary Conversion

16 1

1 (Binary

EqUivalent)

2.3.4 Octal to Decimal Conversion

There are two commonly used methods for converting
octal numbers to decimal equivalents.

the Place Value

method, and a method similar in principle and procedure to
the Double Dabble method for binary to decimal conversion.
The Place Value method is simply the procedure used
in representing an octal number as the sum of powers.

The

discrete value of each diqit is multiplied by the value of
the position in which it is placed, and the resulting products are added.

An example of this method is presented

in Table 2-17.

Table 2-17

Place Value Octal to Decimal Conversion

321 ~
888 8
6

(Octal Number)

L. 3X/
=

3xl ::

p1x8

= !6x8

=
~

= 3x8
Ae-

...

= 6x8

=
3

3x64 ::

192

= 6x512 = +3.'72
3.267

(Decimal
Equivalent)

The octal to decimal conversion method that 1s
similar to the Double Dabble method also begins with the
MSD .(Most

Significant Digit).

The procedure is as follows I

(1.) Record the MaD, (2) Multiply the MSD by eight, (3) Add

the next octal digit, (4) Repeat steps two and three until
the last digit of the octal number has been added.

An ex-

ample of this method is presented in Table 2-18.
Table 2-18

octal to Decimal Conversion
6

3 (Octal Number)

48"

+ 3.--------.1
5'l
x 8

4918

:j8~
x 8
3264

.1-------_.-#

+ 3 . .- - - - - - - - - - '
3267 (Decimal Equivalent)
&

•

2.3.5 Decimal to octal Conversion

There are two commonly used methods for converting
decimal numbers to octal eqUivalents I

the Subtraction of

Powers method and the Division method.
Using the Subtraction of Powers method for decimal to
binary conversion simply required sUbtracting powers of two
from the decimal number.

The additional digits of the octal

system create the need for more work when using this method
We may subtract not only

for decimal to octal conversion.

a power of eight, but up to seven times that power of eight
from the decimal number.

The procedure is then as follows I
n

(1) Subtract the highest possible value of the form

= ~-7) from the decimal number, and record the value

(where ~

of ~ in the appropriate place Within the partially completed
octal equivalent, (2) Repeat this subtraction process with
the resulting differences and descending powers of eight
(recording the value of ~) until the decimal number is zero.
An example of this method is presented in Table 2-19.
Table 2-19

S1.lbtraction of Powers Decimal to Octal Conversion
63~3

3267 (Decimal Number)
-3!72
195

3x8

~x8

R1x8

3xl

3x8

6xS12

3x64

(Octal EqUivalent)

192
3

1
~
3

3
~

The procedure for the Division method of decimal to
octal conversion is as follows.

(1) Divide the decimal

number by eight; the remainder is the LSD (Least Significant
Digit) of the octal equivalent, (2) Repeat this division
process with the resulting quotients (recording remainders
right to left within the octal equivalent) until the quotient becomes zero.

An example of this method is presented

in Table 2-216.
Table 2-2~

Division Method Decimal to Octal Conversion

Divisor Quotient Remainder
(Decimal
Number}
8

3267

4~8

~
6

3 (octal Equivalent)

~.
2.3.6 Binary to Octal to Binary Conversion
As the numerical language of the machine language and
assembly language programmer, the octal number system serves
as a convenient "shorthand ll notation for the binary number
system, numerical lanquage of the computer.

The unwieldy

strings of binary ones and zeros are converted to the more
workable octal notation by inspection, with 'no calculation
3

reqUired, because eight is an integral power of two (8=2 ).
As illustrated by Figure 2-4, three binary digits are the
direct equivalent of one octal digit; one ocbal digit is the
direct equivalent of three binary digits.

Three Binary Digits
9111

represent

One Octal Digit

Figure 2-4

Binary to Octal to Binary Conversion

Knowing the binary and octal equivalents (Table 2-21),
we can then represent any binary number as an octal number
by means of the following stepsi

(1) Beqinning with the LSD

of the binary number, group the bits by threes (filling in
leading zeros if necessary), (2) Convert these three bit
groupings to their octal equivalents.

An example of this

procedure is given belowl

Table 2-21

Binary and Octal Equivalents
Binary

Octal

~~~

f4111.

f41.16

1611

116/6

1161

1191

111

~I
As noted earlier, existing programs are available
for all conversion processes, but there may be several
occasions when you will need to make the binary to octal
and octal to binary conversion directly.
(1) Interpreting" reference texts and Instruction lists
Texts will often call upon the reader to make these
conversions when describing the computer, illustrating the
contents of various registers, and explaining instruction
formats.
(2) Hanually loading and verifying programs
While the machine language programmer must always do
this, it should be noted that even the most advanced computer systems usually have short preparatory programs that
must be so entered and/or checked.
(3) Avoiding binary notation
You may be involved in s1tuationswhere you must work
directly with numbers.

If any of the notation is binary,

convert to octal, operate, "and if necessary convert the
results back to binary.

2.4

ARITHM~~IC OPERATIONS

2.4.1 Introduction
No matter how complex the arithmetic problem, it is
eventually reduced to one of the four fundamental operations I
Addition, subtraction, multiplication, division (Figure 2-5).

ADDITION

•

SUBTRACTION

MULTIPLICATION

DIVISION

Figure 2-5

Four Fundamental Arithmetic Operations

As with the programs available to handle any conversion
process, there exist several arithmetic llpackages" that the
computer user may call upon to perform his calculations.

is imoortant to keep in mind, however, that any of these
~ckages

is a program comprised of instructions which are

essentially the four fundamental operations.

It is the pro-

gram which '-breaks down" the complex problem; the computer
receives only the simplest of instructions.

~""""

""'~

•

Many computers, including the PDP-l1, reduce the
four fundamental arithmetic operations to one, addition.
For reasons of hardware simplicity and efficiency, comple-

~ ... ~--

'\\.

mentary (negative) addition is performed rather than direct

~~-------._..

--- --- --'-_. -_.....--.-

subtraction.

Though we will later examine other methods

(rotating and shifting) when we later discuss the PDP-ll.
instruction set, multiplication can be accomplished by
means of repeated addition, division by means of repeated
complementary addition (subtraction).
--- --- '_.
_~_.-

- . -. ..
-

,~._--

We will therefore limit ,our discussion of arithmetic
operations to the following topicSI

Addition, complementary

addition, and (for comparision) direct subtraction.

Examples

in the decimal, binary, and octal number systems will be
given for each operation.

•

2.4.2 Addition
Our discussions on the count and carry principl~ of
positional number systems have also provided the steps required to perform decimal, binary, or octal addition.
review, the procedure is as follows.

•

(1) Add the digits

in the column, (2) If the base is neither equaled nor exceeded, record the sum, (3) If the base is equaled or exceeded, divide by the base, record the remainder, and carry
the quotient to the next column.
This procedure works with any positional number systern, and once the addition facts for the systems are learned
(see Tables 2-22, 2-23, 2-24), binary or octal addition becomes as automatic as decimal addition.
An addition problem is solved below in the decimal,
binary and octal systems.

Note that when working with more

than one number system. the base number 1s subscripted to
differentiate.
l.

carries I
1.
+5

f6
2

1.
8(1¢)
2 (116)

~(1.~)

1. 1 1 1.

1. 1. 1

1. 1. 1.

_ ¢ 1 1. 1. _ 1. 1. 1(2)

1. 6 7(A)

+1. 1 1 1 1 , 1. 1. 1(2)
1. __ 1. 1. _ 1. 1 1. -(2)

+7 6 7(8)
1. 1. 5 6(A)

Table 2-22

Binary Addition

114

Table 2-23

Octal Addition

1.11

1.{6

1.l

1J6 11

1.91

1.2

1{6

1.1.

1.3

1.4

1¢

11.

1.4

1.5

1.91

11.

1.4

1.5

Table 2-24

Decimal Addition

114

116 11

114

11.

111 1l

1.2

1.3

116 11

191

13 ·14

1.4

1.6

1.3

1.3. 14

2.4.3 Direct subtraction
/

The operation of direct subtraction is performed in
the same manner for all positional number systems, regardless of the base.

The procedure is as follows,

(1) For

each column, subtract the subtrahend from the minuend (if
the subtrahend is greater than the minuend, "borrow" a

power of the base from the next column and then subtract),
(2) Record the difference.
As with addition, binary or octal subtraction should
.'-

_ ........ - - _••,..•+...-- .• ----.-.,.

become as automatic for you as decimal subtraction.

The only

difference is the base, and you should keep this in mind; that
when you llborrow," you borrow a power of that base.
~

the example problems below.
borrows.

borrows I

V',.)2 (l~)
~Jt-~
- @ 1

- 2

4 (l~)

8 (lJl1)

f/J i !
V~~£~
1. m 1.
1.
1

borrows.
6

Reference

VltJ(8)

- 3

2 (8)

6 (8)

9.f {2}
@ (2) r--",

Jt1 (2)

.....

~~
!

"'-.,W~
f5(/V I:
,
v..,

2-6~
t--v JOo
~
~,r~
((

/r-i/ Y//

2.4.4 Complementary Addition
To understand complements, and thus the way in which
negative numbers are commonly handled in the computer, consider again the odometer of the automobile.

If the mileage

indicator is rotated backwards, it will eventually approach
and pass through zero, as shown below.
•

•
~ i' ~ /6 ~ 3

/6 /6 f4 ~ ~ 2
/I f6 f6 ~ f6 1
f4 f6

f6 f6 f6

9 9 9 9 9 9
9 9 9 9 9 8

9 9. 9 9 9 7
•
•
•

Considering zero to be a "boundary," we see that the
number 999998 corresponds to -2.

Applyinq this relational

concept to the operation of complementary addition, we add
the numbers 5 and 999998.

If we ignore the last generated carry, we have
effectively performed the operation of subtraction (5-2=3).

The number 999998 in the above example is referred to
as the ten's complement of 2.

For the complementary addition

operation, the term radix complement is defined as either of
two numbers which when added will result in a sum or zero
o

(last generated carry disregarded).

This concept is illus-

trated by the example below and by Table 2-25.
o

(It should

be noted that the term radix complement can by definition
I

apply to either a positive or negative number, but that it
is most commonly used to describe the neqative quantity.)

We can thus do away with direct subtraction; instead
of subtracting a positive number, we add the negative representation of that number.

In using a system of complements,

however g we omit the minus sign, and must therefore establish
what is and what is not a negative number.

For example, is

123456 a positive 123456 or a negative 8765441

With the

odometer as an arbitrary example, we have the ability to
represent one million numbers (/6 to 999999), and it would be
reasonable to use half for positive and half for negative.
ThUS, by convention, we would regard /6 to 499999 as positive
and

5~~~~¢ to

999999 as negative.

And this in fact is exact-

ly what is done With the computer; with a finite range of

binary numbers to represent, half are designated as positive
and half as negative.

Table 2-25

Radix Complements for the Decimal System

(-2)

(-1)

I f6f6J6fAfd3
I fI¢fAfAfA4 ¢¢J6J6J65.

••• 999995 999996 999997 999998 999999 ~flflJ6~J6 16[1[111161 f6~J6¢J62

(-5)

(-4)

(-3)

(+1) . (+2)

(+3)

(+4)

•

•••

(+5)

'IJ

We have establ1shen the followinq point!; conccrn1.n'1

the radix complements
(1.)

It is the negative representation of a
positive number.

(2) It is used because complementary notation

can be efficiently and simply handled by
the computer. All numbers can be treated
alike (added) in arithmetic operations;
complementary addition (add the negative)
rather than subtraction (subtract the positive) can be performed.

•

(3)

Signs are not required. The computer works
with a finite range of binary numbers, ann
a convention can be established such that
the number itself designates whether it is
poSitiv~~:·~~~~·eqattve. . ----............--.~-

We will examine the radix complement, the radix minus
one complement, and the complementary addition operation
first with the familiar decimal system, and then with the
languages of the computer (binary) and programmer (octal).

\_ /

The radix complement, which commonly takes the name
.of the base, is called the 1,'s complement in the decimal
system.

The procedure for radix (1."s) complement addition

is as follows.

(1) Subtract the subtrahend from the next

highest power of the base, the difference is the radix
(1~'s)

complement, (2) Add the radix (1~'8) complement to

the minuend, (3) Record the sum, (4) Disregard the last
generated carry (the next highest power of the base was
introduced in st.ep one and is IItossed out" here); this is
the final result.

231 (1')

Two examples are presented below.

direct subtraction
(as a check)

84 (1..0)

- 1.25 (1¢)

=2.2. (1~)

112 (191)

25 (1')

l¢~' (1.~) subtract the subtrahend from

next highest power of the base

- 1.25 (1.') the difference 1s the radix
875 (1.16)

(116's) complement

1.[6~ (191)

59 (1.~)

41. (116)

875 (1.¢) add the 1.[6·s complement to thE
41. (116)
minuend and record the SURl
+ 237 (191)
+ 84 (191)
disregard the· last generated
(1) 1.12 (191) carrYJ this 1s the result
:1.) 25 (1/1)

•

The radix minus one complement in the decimal system
is called the 9's complem~nt. 'l'he procedure for radix minus
one (9's) complement additiQn is as follows I

(1) Subtract

the subtrahend from the next highest power of the base minus
one; the difference 1s the radix minus one (9's) complement,

•

(2)

Add the radix minus one (9 I s) complement to the minuend,

(3)

Record the sum, (4) aring the last generated carry

around to the least significant digit position and add it to
the sum; this is the final result.

The same examples worked

with the radix (l_'s) complement are repeated below using
the radix minus one (9 I s) complement.

237(191)

direct subtraction
(as a check)

- 125(191)

- 59(1.0)

112(191)

25(116)

999(191) subtract the subtrahend from
next highest power of the base
minus
one
- 125(191)

::.....22(191)
4~(lpJ)

874 (1/6) add the 9's complement to the
minuend and record the sum

491(191)

bring the last generated carry
(1) 111.(1~) around to the LSD position and
add it to the sum)

>-1(91)
112(191)

99(191)

874 (191) the difference is the radix
minus one (9 I s) complement

+ 237(191)

84 (1.2')

this is the result

~(1p1)

(1) 24(116)

~1(1"

25(191)

The radix complement in the binary (base 2) number
system is callen the 2's complement.

But before we take up

the subject of 2'8 complement addition, let's relate ~ur previous discussion to the PDP-ll. 'and the binary number system.
The PDP-ll is a' variable word length machine, capable
of handling both 16 bltwords and 8 bit bytes.

For the pur-

pose of our discussion, let us consider it to be like many
other computers, a fixed word length machine.

This means

that all data processed by the computer will be in the form
of words (binary numbers) of the same length.

It should be

noted that from the programmer's standpoint words may be in
varied formats and lengths; we are here veiwing words as the
computer will ultimately receive them - in the form of fixed
length binary numbers.
Viewing the PDP-II as a 16 bit fixed word length
machine, it has a binary number range of
9J ¢¢¢ J6~J6 J6¢~ ¢J6J6 ¢f6{6(2) to 1 111 111 111 1.11 111(2)'

1.1

1.16

By convention, half of these numbers are designated as

positive (Y' J69JJ6 ~¢fI 161616 1616¢ 111116 - ¢ 111 111 111 1.11 111) and
half as negative (1 ¢¢¢ J6J6J6 J61616 ~J6J6 161616 - 1. 111 111 1.11 Ill. 111)
(Figure 2-6).

The bit 15 position then assumes the role of

sign bit; the number is considered positive if bit 15
negative if bit 15 = 1.

= 11 and

1 .~~¢ ~¢¢ ~¢~ f616~ ¢[616

SIGN BIT

[6 111 111 Ill" 113 I I I

SIGN BIT

= fA

(POSITIVE)

(NEGATIVE)

- l(lPJ)
1 111 111 111 111 111

Figure 2-6

[6 fA[6[6 ¢/Af6 f6f6¢ f6f6f4 161616

PDP-ll Positive and Negative Number Ranges (Binary)

Now that we know the application, let's look at the
operation.

The procedure for radix (2'8) complement addition

is as follows,

(l) Subtract the subtrahend from the next

hiqhest power of the base; the difference is the radix (2's)
complement, (2) Add the radix (2's) complement to the minuend,
(3) Record the sum, (4) Disregard the last generated carry

•

(tne next highest power of the base was introduced in step

one and 1s "tossed out" here); this is the final result.

example 1s given below.

~ 9J~{6 '11 1'[6 11' '161 (2)

- f6 W¢ tl.f! !,f6 1.W lpJl(2)
~ {A{A{6 {6{61

If6{6 ¢¢l 1.!6f6(2)

l¢ ~{6f6 ¢¢{6 ¢¢¢ ¢¢11 ¢~16(2)
¢

direct subtraction
(as a check)

Wi! p11p1 t!, Ip19J 19J1( 2)

1 III 1¢1 111 fAll {611(2)
1 III 1_1 111 .11 {611(2)
+ p1 @gf6 p!11 1M 1116 91911.(2)

(1) {6 ¢y1¢ {6¢1 1[6[6 [6{61 11616(2)

subtract the subtrahend from
next highest power of the base
the difference is the radix
(2'8) complement
add the 2'8 complement to the
minuend and record the sum
disregard the last generated
carry, this is the result
•

The radix minus one complement in the binary system
is called the lis complement.

The procedure for radix minus

one (lis) complement addition is as follows &

(l) Subtract

the subtrahend from the next h1qhest power of the base minus
one; the difference is the radix minus one (lis) complement,
(2)

A~d

the radix minus one (lis) complement to the minuend,

(3) Record the sum, (4) Dring the last generated carry

around to the least significant digit position and add it to
the su~; this is the final result.

The example worked with

the radix (2 1 8) complement is repeated below using the radix
minus one (lis) complement.

~ ¢¢¢ 161.1

If6Y1 l191 9J~l(2)

- ,!M t1,,'1 116' 1811.(2)

direct subtraction
(as a check)

¢ ~9fY1 t91l 1.f4~ 16161 If4J6 (2)
1 III III 111 111 111 (2)

- ! I'M '1' " , 1"

1p!1 (2)

1 111 IpJl 111 PIll 91111 (2)
1~ 111

1911 III 9111 911_ (2)

+ ¢ ¢lf4 [611 1"

11' "1 (2.)

(i2 ¢ f69116 J6f61 19116 16161 pJ11 (2)
+

~(2)

91 J6f6J6 91_1. 1f491 11161 11616 (2)

subtract the subtrahend from
next highest power of the base
minus one
'the difference is the radix
minus one (l's) complement
add the l's complement to the
minuend and record the sum
bring the last generated carry
around to the LSD position and
add it to the sum;
this is the result

Have you noticed something unsettling about our
complementary addition processes?

The reason given for the

use of complementary addition was that direct subtraction
could not be performed with the PDP-11, and yet direct
subtraction was used in all previous cases to obtain the
complements I

Let's see how the computer gets around this.

Note below that the bit patterns for any binary number
and its l's complement are exact opposites, and that the
2'8 complement is equal to the l's complement plus 1.
JfJ Ji1JfJ1 ¢1¢ ¢11 lf69J 11211. (binary number)

1 )]] ]]1 111 111 1]]
- ~ ¢p!] p!J¢ 9111

1@t1 1911.

]1¢ 1911 1¢~ ¢l1 ¢1¢

1. IIp! 1¢1

lp!~

911.1 ~19! (lis complement)

1¢ 91¢¢ ¢¢¢ ¢¢¢ ¢¢¢ ¢91¢
- ¢ FfflI pl1, ¢]l In 1911
1 IIp! Ipf1 l¢¢ ¢11 9111

1 11¢ 1911 11111 9111 9J11 (2 ' s complement)

The PDP-11 performs 2's complement addition, and therefore all negative numbers must be represented 1n 2'8 complement
form.

The PDP-Il 2'8 complements any binary number Without

direct SUbtraction; it obtains the lis complement by simply
chanq1nq all bits to their opposites and then adds 1.

In the octal (base 8) number system, the radix co~plement is called the 8 1 s complement and the radix minus one
complement is called the 7 t s complement.

Here too, the octal

system serves the programmer as shorthand notation for the
binary system; the l's complement is to the 7's complement as
o

the 2' s complement 1s to the 8' s complement.

Aqa in

j.t

should

be stressed that the programmer rarely works in the binary
number system; that any numerical work he must perform is done
in the octal system and only if necessary converted to binary.
If the 2 1 s complement is required, for example, the,programmer
obtains the 8 1 s complement (or the 7'8 complement plus 1) and
then converts to binary.
Using the direct conversion that exists between the
binary and octal number systems, the 16 bit f'DP-l1 word may
be represented by 6 octal digits.
n nnn nnn nnn nnn nnn (2)
N

(8)

The octal representation of the PDP-II fixed length number
range is then ~ ¢~~~~ (8) to 177777 (8);

the leading octal

digit will specify whether the bit 15 (sign bit) position
contains a zero or a one.
~ ¢~¢~? (e)

By convention, the numbers from

to ¢ 77777 (8) are designated as positive, and

the numbers from 1 ~¢¢¢~ (e) to 1. 77777 (8) are designated

as negative (Figure 2-7).

The procedure for radix (8 I s) complement addition is
as follows.

(1) Subtract the subtrahend from the next high-

est power of the base; the difference is the radix (8 I s)
complement, (2) Add the radix (8's) complement to the minuend,
(3) Record the sum, (4) Disregard the last generated carry
(the next highest power of the base was introduced in step
one and is "tossed out" here), this is the final result.
The example worked with the 2 1 s complement is repeated below
using the 8's complement.

pJ pJ~461(8)

direct subtraction
(as a check)

- R1 "2'45 (8)

l' 911414 ( 8)
l~ pJ¢¢pJ¢(8)
- ,

9!2f!45 (8)

7 75733(8)
7 75733(8}

+ t 913461 (8)
( 1) pI 911414 ( 8 )

subtract the subtrahend from
next highest power ~f the base
the difference is the radix
(8's) complement
add the S's complement to the
minuend and record the sum
disregard the last generated
carry; this is the result

•

, The procedure for radix minus one (7 1 s) complempl"'t
addition 1s as follows.

(l) Subtract the subtrahend from

the next highest power of the base minus one; the ~ifference
is the radix minus one (7 1 8) complement, (2) Add the radix
minus one (7 I s) complement to.the minuend, (3) Record the
sum, (4) Bring the last generated carry around to the least
significant digit position and add it to the swn; this is
the final result.
~ent

The example worked with the lis comple-

is repeated below using the 7's complement.

91 91 3461 (8)

direct subtraction
(as a check)

- ¢ ·pJ2¢45(A)
~ 91 1414 (8)

7 77777(8).
-

(1)

'28145 (8)

7 75732(6)

the difference is the radix
minus one (7'8) complement

7 75732(8)

add the 7's complement to the
minuend and record the sum

m813461(8)
~

pl1413(8)

~1(8)
~ pJ1414(S)

·r

subtract the subtrahend from
next highest power of the base
minus one

bring the last generated carry
around to the LSD position and
add it to the sum;
this is the result

SIGN BIT = ~

(Nl::GATIVE)

(POSITIVE)

SIGN BIT

- 1

(191)

1 77777 9J f6f6919J9J

Figure 2-7

PDP-ll Positive and Negative Number Ranges (octal)

2.5

r..r)GIC Of-EAATICNS

2.5.] Introduction
Computers use logic operations as well as arithmetic
operations in the e~ecut1on of ,programs.
tions we will discuss have a direct

The logic opera-

r~lationship to

algebraic system used to re~resent logic statements
as

3001e~n'algebra (named

an
~nown

in honor of George,Boole, EngYlsh

.nathematician and logician).

will be concerned with the

aoplication of two Boolean axioms to computer circuitry.
'I'hes~

are the two basic connectives'· used to

r~1;)tion6hip between

e:xpre~;s

the

two statements, the AND ann the CR.

We will specifically examine the AND,
and EXCLUSIVE OR operations.

I~CLUSIVE

OR,

Simple circuit diagrams,

truth tables, and applicational exan9les will be given to
help illustrate each of the operations.

1'f.

2.5.2 The AND Operation
The diaqram below (Pigure 2-8) 18 that of a simple
circuit with two sWitches.

Current 1s allowed to flow

throuah the switch .if it 1s closed, and is not allowed
to flow through the sWitch if it is open.

Therefore, in
o

order for the Function to occur, current must be allowed
to flow through the ent~re length of the circuit; both
sWitch A ~ switch a must be closed.

O~-~O

sWitch

function

Figure 2-8

Al~

Circuit Diagram

In computer logic, the closed switch (or true condition)
is represented as a 1, and the open switch (or false condition)

is represented as a~.

Expressing the AND axiom in terms of

our variables, we can say that A & B
sy~bolic

= F (when using PDF-11

language, the ampersand sp~.~f1e~._1:he At~D qp.era.t.ion).

If A is 1 (true), and B 1s 1 (true), then F will be 1. (true);
any o~her combination of the variables will result in a ~
(false) condition.

The relationship between the variables and

the resulting value of F is summarized in Table 2-26 below.
Table 2-26

Truth Table for the AND Operation

,
,
m

1
1

14
1.

When the AND operation is applied to binary numbers, a
hinary 1 will appear in the result wherever a binary 1 appeared
in the corresponding positions of the two numbers.

A binary ¢

will appear in the result wherever a binary ~ appeared 1n either
(or both) of the corresponding positions o~ the two numbers.
The A~D ooerationis commonly used to extract (or ~ ) a portion
of a 16 bit number.

In the example below, it is used to extract

the two least significant octal digits

the ten most sig-

nificant bits) of the number.

¢ 9f¢1 [61¢ .011 If4.0 lp11

(16 bit number)

& 91i!~p! !!@ f6'~ III 111 (mask number)
¢ ~,~ ,~~ f6~' l~~ l~l (result)

2.5.3 The INCLUSIVE OR Operation
The diagram below (Figure 2-9) is that of a parallel
circuit with two:switches.

Recalling that current is allowed

to flow through a switch if it is closed and not allowed to
flow through a switch if it is open, we see that current will
flow through the entire length of this circuit if sWitch A 2!:.
switch B 2£ both are closed.

The Function will be able to

occur as long as both switches are not open.

switch
A

function
F

SWitch
B

Fiqure 2-9 INCLUSIVE OR Circuit Diaqram

Recall that in computer logic the closed sWitch (or
true condition) is represented as a 1, and the open sWitch
(or false condition) is represented as a~.
lOR

Expressing the

axiom in terms of our variables, we can say that Al B = F'

(when using FDP-Il symbolic language. the exclamation point
_ . . -_ _

... ~

•

__ r

__ •

.•

,~.

f::Decifies the lOR operation).
............. _.-.--

_. __ .__.·w .'

.. ,,,•.

_~_._~.,,,,,,.,,.,,.,,,-,

'. _ • ..---.----- •• ----. --

If A is 1. (true), or !J is )

(true), or both are 1 (true), then F will be 1 (true); only
when both are ~ (false) will the result be ¢ (false).

The

relationship between the variables and the resulting value
of F is summarized in 'rable 2-27 below.
Table 2-27

Truth Table for the lOR Operation
A

When the lOR operation is applied to binary numbers,
a binary 1 will appear in the result wherever a binary 1

appeared in the corresponding position of either (or both)
of the two numbers.

A binary ~ will appear in the result

wherever a binary ¢ appeared in both corresponding positions
of the two numbers.

The lOR operation is commonly used to

set bits within a 16 bit number, where the present bit pattern
cannot be known.

In the example below, it is used to set bits

in positions ¢, 7, and 15.

1 111 ¢fl11 1191 ~1¢ 1911 (16 bit number)

1 1

P2?

1 111

n? 91? 999 991 (lOR value)
~9Jl

ll,eJ 9J1~ 1.911. (result)

2.5.4 The EKCLUSIVE OR Operation
The diagram below (Figure 2-1.~) 1s that of a parallel
circuit with two sWitches.

The dotted line between the

switches indicates they are mechanically connected such that
they cannot be simultaneously closed (1.e., close one, open

the other).

Thus one set of conditions (both closed) 1s

excluded in this OR operation, and the Function will be able
to occur only if switch A or switch B 1s closed.

10---.....
sWitch I
A

I
I
I

function
F

Figure 2-1~

EXCLUSIVE OR Circuit Diagram

•

Again recall that in computer logic the closed switch
(or true condition) is represented as a 1., and the open
sWitch (or false condition) is represented as a ¢.

Express-

1ng theXOR operation in terms of our variables, we can say
that A (J) 13
o

= F (when using PDP-ll symbolic language, the en-

circled --_.
exclama_!-Jon
point S2~~ifies.theXOR. <?e.~~~~-~~~L~_
-- . -_.-

--A is ) (true), or B is 1 (true), but not both are 1 (true),
_.~

then F will be 1 (true).

The relationship between the vari-

abIes and.the resulting value of F is summarized in Table
2-28 below.

Table 2-28

Truth Table for the XOR Operation

1
1

i f/I

,/
/Y-

When the XOR operation is applied to binary numbers,
a binary 1 will appear 1n the result wherever a binary 1 an~
a binary ¢ appeared in the corresponding positions of the
two numbers.

A binary ¢ will appear in the result wherever

binary f6's or binary 1. ' s appeared in both the corresponding

9Qsitions of the two numbers.

The XOR operation is commonly

used to set and/or clear bits within a 16 bit number, where
the

~resent

bit pattern is known.

In the example below, it

is usee to set bits iT! positions 7 and l5 and clear bits in

positions ~ and 6.

~ ~fll

;6111 fI~l fllfl flfIl (binary number)

<D 1 fiJI! !flJI fill fI¢J' f6~1 (XOR value)
1

991. 919 91.9 919 999 (result)

2.6

BXBRCISBS

The followinq examples based upon the content ot this chapter
are presented as an optional exercise for the reader.

The answers

can be found in Appendix A.

2.6.1 Decimal to Binary Conversion

Convert the followinq deci~l numbers to their binary equivalents
1.

11111

235

2.6.2 Binary to Decimal Conversion
Convert the followinq binary numbers to their decimal equivalents
1.

111111 111111 1111_ 11111 11111 11_

111111 111116 111111 1111/6 1116 1116

2.6.3 Decimal to Octal Conversion

Convert the followinq decimal numbers to their octal equivalents
1.

58$6

1111116

2.6.4 octal to Decimal Conversion

Convert the followinq octal numbers to their decimal equivalents
1.

1111/6742

2•

/6111/61611

2.6.5 Binary to OCtal Conversion

Convert the followinq binary numbers to their octal equivalents
1.

1111/1 111116 /1111 Ill' .'11 11111

/11111 1111/1 /116/1 1/11 111 11/1

2.6.6 Octal to Binary Conversion
Convert the followinq octal numbers to their binary equlvqlents
1•

$f161673 6

16115224

•

2.6.7 Binary Addition
Perform the indicated binary addition
2.

t'"

~16~ 16~f6 9111

1111

191¢ 1ftf]

'II 11.@ 111. III 11.91.

2.6.8 Binary Subtraction
Subtract using both the direct and the complementary methods

2.6.9 octal Addition
Perform the indicated octal addition
1.

9154362
9173441
+@6775!

16113321
9J~44167

+M5622

2.6.116 Octal Subtraction
Subtract usinq both the direct and the complementary methods
1.

1113421
-'12'54

1611234
-pI1!567

2.6.11 Logical AND
Perform the indicated AND operation

2.6.12 Inclusive OR
Perform the indicated OR operation
1.

16111. 91]91 9111 191' 1911. II'
1!1@ pI1.] 1M 1'1 11@ III

2.6.1.3 Exclusive OR

Perform the indicated OR operation
1.

111' ~11 19116 1111 1191 111
(1)11 111 [111 1@1 11.@ 111 !P![6

· .'

'I'be IOP-l1

3 .1

ampl OJtCWlJZATIpR

3.1.1 Introduetlon
We !av. cll.c.Bed ~be C)eDUal orqanlsat.lon of t.1le
cc.p*er In t.. . . . of t.he major Wllt.e (1apui:, IIGIIOry, cont.rol,

arlt.lllet.lc, out.pa1:), aDd Wit.h refereDCG t:o a baalc block dla-

gram.

We Vill DOW cSi.cWl. t.~e _jor UD1t.. ift IIOre detail,

aDd relat.e .peclflcally t.o t.he element. of t.he 81mp!lfled
PDP-11 Syat.. . orgaalAt.ioa diagram (Ple,ure. 3-1).

3.1.2 The VlIBUS
'I'be URIBUI 1• • • ingle, ee.DOD pat.h t.bat. CODftee1:. t.he

proc...or, IIAIIDOry, aDd all peripheral (iDpK aDd o=put.)
cI."ie. . , it. carrie. all lDforat,1on.
umBUS 1. a•• igD" all .&Sr...,

way.

Baeh devlee on t.he

aDd cOIIIDunlcat... 1ft the ••lDe

.,bi_..au tbat. perlphftal " . .le. . _y

MDlpu1ated •• IIAIIDOry.

be..

flexlbly

ftoa tbe Pl'0CJZ'8-.r" • •taDdpolnt.,

t.h1s 1e t.he 110ft lIIponant feat....e of the UltIBU8.

Mo8t.

cc.p*.- require a .eparate 11M (aDd ~hua • epeeial 1n.~loll .ub8et)

for lnput.-o1&JM& 4.neu.

Wit.h t.he PDP-l1

aid ita UIIlBUS, all of t.he poIIVfal 1~10D8 t.hat. caD be
app11e4 to dab 1n .-ory can be appl:1.e4 t.o dau in peripheral
clevic...

~
Q

I
I
C)

II
Hit
~H

....0 ,

..I
a

<II

«0
fI'4
r4

....

1
CQ

Jill

'N,

i.
•
...,
•
.!
CII

I
It

•
D

I
!

lire

3.1.3 JIEorY
"he 1Il8lllOry unl~ 1- uaed ~o s~or. lftforma~ioft Wl~11 1~
1. n• .sec.t.

Jat. •• you remember f.c~. concerning pa.~ an4

pr".D~ event.,

the 1Il8llOry of t.he computer et.or.. infonB-

tion for f1Kure referenco.

We My concep:uallze the COlIlp1&er .emory •• a ... i ••
of locat.lona, 1n a pigeon-bole or slot-l1ke arraftg__M,
wMr. _ch locat.lon la8 a binary &&Sr... aDd eont.alna binary
iftformat.ion (Pi4JUre 3-2).

ADDUS8

ft81ft'

Figure 3-2

Conceptual eoap1&er Memory SectiOD

iD8t.ruatloD, a~her (-forwarding-) _~dr"., or ~au.
~.....-.~ft.

,-.-,--..,~-- ...--r.---.-~_o-_ •._ . . .,----_·_--~,,~

%t.

v111 "epeDe! apon "heD (1ft wblch major 8t.a~.) and how (W11:h

Which addr. . .lng aIOd.) 1t. 1. acc••• ees by ~be COIlptKer.

'I'M PDP-l1 1. a vulable .,Id l.-.,th _chi• • world.ftCJ
V1~b eli:hK 16-1tl~ m. . . . calle&!

eall.. . . . .

sdI or 8-b1~ a".ber.

Any 16-bi~ wo~(bi1: pOII11:1ou '-15) Will ~heft

co_1ft of two 8-1»11: lIyt... 1:he.ll!..JZlS! (bit p08it1o. . "'7)
aid the ~ (bit poa1t10. 8-15) •

.-------JIIII~~-----~-t

·1

15 .

.19'11"- 3-3

. ,

PDP-ll.ems ane Byte ae1at1oMhlp

'I'he ba.lc IOP-11 ..-01'1' ualt co.i.~. of
'. I

4.'"

(111,• • oc1:al) _Ed locatio• • aat 1:hRefore 8,192
octal) ~. locatio• •

(211.,__

a. "'loDe4, _he _chiu i . capabl.

of baldllllCJ eli:bu I.-bit woda OZ' a-bit lIyt• • aad tbe . . . .

ory 1. thuefore byte ~_.e4 .0 thai: ~h foru CAD be
accOW*)CJate4.

'lIMa addr._ I'~"_ for tile -.'__ oc1:al byte

locat1oM 1. 1f-17777.
~---' --- .....-- ...;..••.--------. -~- ~.-.-.... -

A. 111_"at84 ],y rigw.. 3-4 aD4

--"--1

3-5. t'he PDP-l! .-ozy _Y _

aODe.~_li." ail el1:bu . . .

Jq\left1:1al word loaai:iou 01' . . . . . .t1.1 byte- 1oca1:1ona.

Mote

that word. aad low lIyt_ .e foaD4 . t " ' 8

1I19h

byt._ at o4d adIk• • • •

addr......

•

ill

16-b1t. word

••,.,
_"11"2

_11"1113 high b~. low b~.
111111"5 h1CJh b~. low byt.e 11__11/14

8-b1t. word

word{

law

b~e

11/1'11_•

high

b~.

'"_~

low byt:. 11/111.2
word{

word {

high

byte .~

low

byte

high

low

b~. 1117772

h1gb

byte 1117773

low byt:e Sf17774

1117773 h1CJb byt:e tow bY'e '17772

word {

high

1617777 high byt:_ low byte fI17776
Wo~ organ1zat.1on

b~. jIl7775

low byte 1117776

h19h byte low byte 1117774

P1gure 3-4

b~. • •5

word {

•I

I
I
t

~7775

,-,

_11__161 h1CJh byte low~.

•

word {

h1qh

P1gure 3-5

b~e 1f17777

Byte orgaDi"~1oD

3.1.4 Central PrOCM_or
The cem:ra1 proc...or (P1CJure 3-6) 1. eOJllpr1eed of

thr.. f~lonal block•• · The Coptre1 UUl and Ar1t.Mlet:&s,

l!D.&! (as 81.0 glven 1n our ba.lc computer block 41a9l""')'
and the General Purpo.e .!lleter..

A figure ei9ht i8 formed

by the data pat_ coDllectift9 thue unite, and d ••cribe. the

flow of dau t.hrough t.he proc•••or.

~M toUl

funcxion of

the proc••801' 1. to proc... data. to execute t.he proqram,

cODtrol111l9 operat.iODs froa beqlulng to eDd.

tJIIUS.
15

UNIBUS
COm'ROL
OlttT

876

543 2 1 _

G. . . .L
PORP08B
RBQI8'1'BRS

·0

J'lgure 3-6

PDP-Ii Central !Toc....or

'l'he UllIBU8 CoftUtol UDit. c!lrect.s t.he proc...lng by lIleaD8

of t.he following .equence.

(1) l'et.ch an (tobe 1l8Xt.) lnat.ruc-

t.ion froa ~he program st.ored 1ft t.he ~ry Unit., (2) Decode
t.hat. ift8t.ruetion, (3) If 4au is r8fl\l1red, obtain ~h&~ data

from t.he Memory Unit. or a peripheral 4. .100 and bring it t.o
t.he A(lt.1aftlc um, (4) Specify t.o t.he Arit.1Te!tle OM$- wbat.
operat.ion is to be performed apon the da~a, aftd (5) If required, st.ore t.he r ••\I1t of t.ho oporat.lon.
The PDP-II proce••or he. major stoat... of operat.ion,

and four are li.t.ed below t.o help 91ve ~he reader a basic
description of t.be ~oc•••or·. operat.ional flow.
Plft'CB - Obtain aid dec:ode an lD8t.rQC1:ion. .Whea ffteb 1~'!
It.
1. po88ibl. t:o go fZOCD fetch t.o any ot.her stat.., including back t.o fetch, a.peDCIiftCJ upon t.he t.ype of inst.ruct-lon
decoded.
.

ccmPiated, t.he procuaor eDt.ers &llOt:her _jor atat...

80tJRCB - Decode t.he 80arce addr... field of a double operand ift8t.ruct.ion (det8Z'll1ne t.he adar.tilIs of the dat.a), and
t.raufer t.hat. da" to tM arithmetic unit. The .oure. major .t.a~e i . eMered only if ~he lnmt.ruct.ioD i8 t.he double
opera" type.
DBftlra'1'IOB - Decode t.he dut.lnat.loft ad4re.8 field (determine t.be addr... of ~he data). aDc! ~raD8fer ~ha~ daQ 1:0
~he arit:~lc Wl1t..
'1'M d. .tlDa~loD major .ute 18 on-

1:er8l! for ~h 8lngl. aid double operand lnsUUC1:10D8.

BJalCt7IB - Perfoftl ~he lD81:ructlon. If dab 1s ~o be operat.ed upon, 1:he arltbaftlc an1~ 1s d1rected to . .ft1pu1a~e
~... da" aecordlD91YJ if the r8sul1: 1s ~o be ftored, 11: 1s
~raDllferred froa the arithmetic unit. to the appropriate
loca~:1on.

Although t.he major 81:a~. . given follow thtt .equence of

fetch, 8OaZ'ce, cleRina~1oD., aDd execute, DOt all are needed

for every iD8truc1:1oDJ the proc. . .or enter. only ~he .tat.e.

nec...ary to porfora ~he curr.n~ lnatruet.ion.

PrOC• •OF .Q~" wRd (1'19Ve 3-1) 1. a ••1f-4.1IC1'1~lv.
~ii:leJ

it. i . an addr. . . .b1. word loea~lon t.ba~ eonuln8 infor-

ma~ioft on ~be .aa~U8 of ~be proc...ing.

byte w1ll indicate ~he follovlftCJ.

Specifically, t.he low'

Current. priority 1.".1 of

t.he proc. . .o~ (bl~ po.i~1oft8 5-7), iDIJtructioft ~J:ap (bit. po.1t.ion 4), an&! r . .u1t of tlw pr. .10ua operat,ion (bit po.it,lou

'-3).
'1'he priorit.y 1. . .1 of t.he proc...or, Which can be _Dip-

ulat.e4 by t.he prograa at. any t,1ale, i . an ln~egr.l part of t.he
Automat.ic: Priorit.y Int.errupt 81'8t.- of t.he PDP-l1.

We Vill

look at. all of ~bl. in BIOI'. detail .hen .e lat.er discuss lnpui:out~

progr&mmtng.

be poftponecS.

Dl.CU88ioft of t.he ~ap iadlcat.or w1ll a180

It. role Will be .a1D1Mt1 When

w. pr. . . tAp
nt.

lD8i:ruct.lou during d1.e• •loft of t.be I'DP-l1 1natruct10D e81:.

w. w1l1 talk about. the four l . .t .19ft1t1can~ b1t.. of t.he
Proc:...or Sbt.U8 Word, c:alle4 the coDdUloD code bl~..

Upon

t.he CClllplft10ft of ~he _ec1&e _jor 8tat.. of aD 1aat.ruc:i:ion,
t.hese bit.. are condit.ionally modified ~ reflect t.he r . .u1t of

t.hat. lnet:r~ioft (~. t.he direct. 11ft. froa t.he Arit1aKic Unit.

t.o t.he Proc. . .or Stat.u Word).

!'he prograa may t.hen WI. ~h18

informat.ion t.o det.rmine 8~equeD1: a~ioD.

••

The.. bit.. are ••t.

follonl

C bit. (.) - if 1:bere va. a SU'~y froa ~he mo.t.
.1gD1ficaat bi~ ~l~loD

V bl~ (1) - if ~her. . . . arlt.~lc o!.rflow

Z bit. (2) - if ~be r . .u1~ . . . Zero
• bit. (3) - if t.he r . .u1t. va. !egat.l98

The central proc•••or a1eo contain. a set of eiCJ~
General Pur

po..

Jt!CIi.t.er8 (Figure 3-7).

(cOJIEOnly referre!l too .8 R_,

.1.

'l'he.. reqi.ters

R2 •••R7) are aeSdre••able

word locat,ion. w1~h epecia1 f . .~ur.e t.hat. great,ly enhance
the power and flexibility of t.be PDP-Ii.

RH
R:
R: ,
R.~

R·
R:
It
R

P1CJUre 3-7

18P)
(PC)

General Purpo•• Regi.~.r.

The regi.t,ers are called general purpo.. because each
may be used .a an.
ACCtJMtJLATOll

Where a

8U11l

18 accumulat.ed in t.he General Purpose R8CJiet.er
POII1'1'BR

Where t.he General Purpo.e R8CJ1.t.er ~ ~o the operand
(contaln. ~he addre•• of ~be operandl . .
AtJ'l'OIIlCRBMBlfl' RBGISTBR

'1-"

Where the General Purpoee R.,lst.er polnt:. ~o ~be operaDd
(cont.aiM t.he addre.. of tbo operand) J t.be addr... 1. uaec!
and t.hen a1Komat.lcaUy 1IlCrPlm:e4
AO'l'CI>BCUMlft IUlQISTBR

Where the General Pul'p088 &891.t... polnt.s t.o t.he operand
(conta1D8 t.he addr. . . of the operand). t.he addre•• 1. first.
aut.oauat.lcallY d8CX". .at.e4 aDd t.hen U8ecS
II1DBX ltBGIftBR

Where t.he General Purpoe8 R891.t.. . cont.aiM an lndex value
t.hat. 18 added 1:0 a ba•• addre•• t.o provide ~be addre.8 of
t.he ctperand
"

All 144re•• ina 'i\b tbl rpP-ll 1. ,csomp!1ehg4 ~b£oP9h
tohe General furpo.. '.1ftlE.. .nd t.hey t.herefor. play •

vital 1'01. in .ff1c1.M progr.-1DCJ of t.he _chin..

We have

only 11.t.ed t.he .ddre.81ftg feat.ur•• of t.he reg18t..r. bere,
an&! Will ex••lne them 1ft IIOr8 detail When

w. lat.er

dl.CU88

addre•• lng mod. . .

it·

It. .bould be not.eeI here t.hat. t.wo of the e1ght. reviet.ere
have unique capeb~11t.1•• J .7 'el'Ve8 a8 t.he Prosram COuM8r,
aDd R6 serve. •• the Suck Poit*er.
CWl8ec!

Both wl11 lat.er be 4i8-

in detail, but. • bl'ief •••crlpt.1on of each follove.
Proqram Count.er USl - Th1. r891.t..r IIl1ght be better

named the PrOCJram Point.er. i1; yil1

11_" SQPt;ain ,be 144r•••

of the next loc:a~loft to 1M rerer...ed.

It. 1• •utomat.1ca11y

up4at.ec! by t.he proe. . .or •• it. st.ape t.hrough t.he prOClram
(after aft lnstruc:t.ion 1. fet.chfK! from a location, t.he Pr~am
Count.er 1. .teppec! t.o contain t.he addre.. of t.he next. sequentlal locat1on).

St.ack Point..r l.W. - Dur1DCJ the rUftD1DCJ of • prOCJram,

there are • .,er.l circUlUt.anc.. that. caD ea.e a CMIl9. from
one .equence of In.t.ruat.1on. t.o aD01:bu' ( int..rru~. , t.rapll ,

error conditlons, ftc.).

'fhe proc•••or will automat.1cally

-rem.aber M where 1t. ... 1n ~he fir.t. 8equenc. of In.~ract.loft8
by "V1M a

ThUl R6,

return addr••• (COD~.1lt. 0' She PC) Oil t.he Stack.

a8 ~he S~ack Poillt.er, Will cont:aln t.he addr_s of

t.ba1: loea1:1on 'which hold. ~he ret.urn addr....

•

3.1.5 InpS-out.put; Devices

The Input; Device. a ••oclated w1 ~h a comp&er 8ytatem

enable aau and con~rol inforMtion t.o be entered into the
computer.

Some devlc.. require tbat t.he input: informat.ion

be in a apeeiel foZ'll (a card r ..der, for example, accept:8

only punche4 card.) J other device. do not require any previous preparat.ion of information (t.he Teletype allows 1nfor-

_t.ioft t.o be .1mply t.yped in).

In all case., t.he•• devic. .

t.ranslat.e t.he various forms of input informat.ion into a
form which can be handled by t.he computer.
The Ou1:put. Price. a ••oc1at.ed With a comput.er .ytlt.em
enable information (int.ermediat.e and final resulte) to be
received from the computer.

'I'M. out.put information may be

in any of ••veral forma, depending upon t.he device and the
cont.rolling program.
'l'he list of Input-outp,* devic.. for t.he PDP-l1 system
i8 a lODq one.

As example., .everal of t.he lIlore common de-

vices are described generally below.

The operat.or·. cODDole (1'19U8 3-8) proy14_ fUftCt.10n
1IV1t.che. to control t.he

.y.t:_

aD! in4ic:at.or8 1:0 1IOft11:or the

.tatU8 of the .~t_.

=..

."0'

'f":
.'~.

?~ J~

DAta

: . ..:.)
.

,. =': ":

_·...::ItUIt~,....:.~IUS~

..... '.

.JOU!!C! . eatlUmON

'.~

_.' ~ ~ Of.

P'!a me:

I ' ... '

IDCIItOS

Ii.

.',
,

......
1_ _.......

......_iliiiioo.-

--'

Figure 3-8 PDP-II Conaole
Althouqh 1t ca~ be 4_cribed •• all iD~-om:pUt. device,
t.he conaole 18 cSi.cue.tId here becaU8. 1t. do.. provide tobe opera-

tor wit.h a dir-* . . . . of input. aDd oapt&.
'1'0 input. informat.ion (DBPOSr1'), t.he prOcedure i • • • follOVll'

(1) Specify t.he II-bit. addr. . . wit.h bl~" po.lt.ioM /1-15 of t.he
SWITCH RBGIftBR (. .it.ch Uhl. ni~cll DOWR.,,)
(2) Depr. . . t.he ~ ADDaDI key (t".'.1'8 cont.ent. of t.he
SWITCH RBGISlfBR t.o t.he ADDItB88 RBGI8'1'BR)

(3) Specify t.he IS-bit. coKent.. wit.h bit. .p08it.ioft8 11-15 of t.he
SWITCH RBaIS'!'BR
(4) Rai•• t.he DBPQ8rl' key (t.ralUlf. . . coDt.ent. of t.he SWrl'CB RBGIftBR t.o t.he a4ck.. .peclfled 1n·t.lie AmaaS RBGlftBR.
Con1:ent.. 81.0 41.playe4 in laB DX8R.&Y RBGlftBR.

The eon.ole aerY.. a. a means of ou~put. in two way••
The funct.ion keylJ may be ueed to JlKAMINB locat.iona on t.he

UNIBUS, and t.he content. of General Purpose Register _ 1.

automatically d18played in the DATA DISPLAY RBGISTBR upon
the completion of any proqralll.

The BXAMID procedure 1. a. tollo...

(1) Specify the 16-bi1: addre88 wit.h bit. po8it.ions '-15 of the
SWITCH RBGIftBR

(2) Depree. 1:he LOAD ADDItB88 key (t.ransfers content of the
SWITCH RBGIS'.L'BR t.o t.he ADDRESS RBGIftBR)

(3) Depress the BXAIIID key (t.rall8fer8 content. of t.he address
8pecified in t.he ADDRB88 RBGIftBR t.o t.he DATA DISPlAY

RBGIftBR)
I~

.hould be noted that. t.he operat.or _Wit LoaD ADDRBSS

only init.ially if DBP08D'lng or BXAMINlnq sequent.ial locations.
The content of the ADDRB8. RBGIftBR 18 automat.ically updated
with each DBPOSIT or BXAMINB tUllC'tion.
The procedure for running a pr09rUl which has been input
i8 a8 follows,
(1) Specify the 8t.artlnq addr... in the SWITCH RBGIS'J.'BR

(2) Depress the LOAD ADDRBS8 key
I"

(3) Set the JDaBLB/HALT key to the BtaBLB po81tion (transfer.
cont.rol t.o t.he proce8.or)
(4) Depres. 1:he STAR'!' key (be91- proc•••or operation)
When the proqram 18 complet.ed, 1:he addr... of 1:he HALT
ins1:ruc~1on
.

Will. be in t.he ADDltB88 RBGISTBR, and t.he content.

of General Pur~. Req181:er _ (vtlieh can be t.he resu11:) will
be displayed in t.he taD DISPlAY RBGIftBR.

The !"Cd_I 33 Au1:C81tlc Se!!d-Recelve TeletYp! Unit

(Fiqure 3-9) 18 an input-out.put: device prov1de4 8S standard

equipment wit.h JD08t PDP-l1 syst.ems.
i

':"~J~r::!
~\

paper tape
punch

---~-..
paper tape

keyboard

reeder ~

'~-

Figure 3-9

ASR-33 '.l.type

Information i8 in~ in either of two ways.

Typed in

by mean8 of the keyboard (116 character. per second), or read

1n by means of the low speed paper tape reader (111 characters
per .8COn&!).

Informat.ion 18 a180 output in either of two way••

•

printed out. by means of the teleprint.er (1' charact.er. per
second), or punched out by ID8aIUl of the low 8peed paPer tape
punch (116 charae1:er8 per second).

The Higb Speed ,aper Tape 8..d~!, and Punch

(Figure 3-1_> i8 an 1nput-o~put device available for ~ho8e
user8 who require fas~.r paper tape reading and punching
.peed8 than those of ~he 8undard ASR-33 Teletype.
TAPE HOLD KNOB
READER INPt11' HOPPER

PUNCH FEED SWITCH

READER

fE~D

SWITCH

SPROCKET FEED WHEEL

Piqure 3-111

BiCJh Speed Reader and Punch .

Information 18 input: by meaDS of ~he h1CJb speed photo-

electric paper ~ape reaCler at the rate of· 3_11 characters per
second.

Information 18 out.put by means of t.he h1qh speed paper

tape punch at the rate of 5_ charac~er8 per second.

The High Sp!!d L1n, fE1pi;.,r (F1C)U1'e 3-11) 18' an out.put:
I

device available in several models for the user who requires
a fast:er pr i nt:1ftCJ _peed ~ban ~ha1: ot tohe stAndard &8R-33.

Figure 3-11
Using the
1nforma~1on 1s

H1qh Speed Line Prlnt:er

e_ column, 64 cbaract:er model as an example,

printed 0'* a~ the :followinq rates I

356 lines per m1n~e, colmDft8 1-8'

4616 11ne. per JD1nut:e, col UIIlIl8 1-6'
6511 11nes per IIl1nut:e, column. 1-411

11111 linea per minut.e, coltam8 1-2'

The DEetepe Unit (P1qure 3-12) is one of two maqnetic
tape options available for PDP-ll systems.
b1d1ree~lonal maqne~1c tape

It is a dual-unit

transport system for auxiliary

1ntorma~ion 8toraqe~

Pigure 3-12

DBCtape Unit

Information may be inpu~ (read) from or output. (writt.en)

on this device &t the rate of 5/1__ 16-bit words per seconC!.
The system stores information at: tiXed positions on the maqne~1c tape,

allowinq blocks of the information to be read,

written, or replaced without disturbing other previously re-

eorded information.

The RC-ll Disk Unit. (Piqure 3-13) 18 one of many •••

• t.or.g. devices available for PDP-1l 8YSt.ems.

Expandable di8k

ma.. st.orage syst.ems may be used 1n a number of comb1nat.lons,

and ranqe from ~he RC-11/RS-64 wit.h a haaie st.oraCJe of 65 t.hou.and word. (expandable t.o 262 t.'hoWland) t.o the RP-ll/RP-JlI2,
whlcb .tor. . up to 8/1 million words in an expanded c:onfiqurrat.ion.

Fiqure.3-13

RC-l1 Disk Unit.

Informat.ion may be inpu1: (read) from or out:put. (writ.t.en)
on t.he RC-l1 Disk at t.he rate of 62,5/111 16-bit words per second.
Information 18 st.ored at. fiXed po8it,loft8 on t.he d18k .ur~ae.,
al1OW1nq blOC::,kll of the intormation t.o be read, writt.en, or replaced without. dlst.urbinq ot.her previously record" informat.ion.

3.2 ADDRBSSIblG MODBS
3 • 2.1 I ntroduc:i:lon
A proqra. i . a ••ri.. of compU1:er words .equenced to
accompli8h a particular teak.

TheB. computer vords which

compri8e the prOCJraa may be divided lni:o t.wo major cateqorle••

Data Words or operands (the values to be operat.ed upon by t.he

lnstructlon words), and Instruction Word. (those which acee••
and manipulate t.he data word.).
The Data Word (P1qure 3-14) is quite 8traiqhtforwardJ it

ia lat..-pre_ •• a numerical value to be operat.ed. upon.

I------------I
lIUMBllICAL

VALOB

Figure 3-14

Data Word

The Inet..J;uc:t.ion Word (PiCJure 3-15), t.hough a180 numerical, must. be int.rpr~ed differently.

In order to manipulate

the data word, t.he 1ft8truction word must. ace••• it., aDd you

will recall t.hat. words in memory are always referenced by
addre...

The lft11tructioD word 18 therefore of t.wo part.s J by

convent.ion, certa1n bit.s specify t.he oeera~1on code (how the

data word 18 to be manipulat.ed), and t.he other bits specify
the addr••• of the dab word.

IOPBRA'l'IOIil CO!)B

PiCJUre 3-15

Dl'l'A ADDRESS

INitruct,ion Word

The PDP-l1 1n.~r~lon .et ha. ttfO ~ype. of lnatruct.ion
WOrd8

t.bat manipulate data, t.he !1ngl. Cps.1M! In.tEystioft

and t.he Double operand In8~ruct.ion.

The Single operand Inst.ruction (Wigure 3-16) implies
one operand, and follow8 the general format presenteeS earl1er.
Bit positiofta 6-15 8peclfy the operation code that

defines t.he instructlon ~o be executed.
Bli: poa1t.loD8 11I-5 8petfy the de*t.lna~1on.addr. . .· r.teW..·
(the addr.8. of the operand). 'I'M• • ix-bit aeetlnat,1on address
flel~

cODs1.t.e of tvo t.hree-bit eubfleld81

reg1ster .uhrlN! (b6t poeltloJl8 /1-2)

8p8ClfJ.•• ~irlileb of t.he eight
G.ft~r.l ,~~. Reglsters
18 t.o be . . .

mode aubfleld (bit poalt1one 3-5)
8peclfi.. b2! that General Purpose
Reg18t.er 1. t.o be uaed

15 14 D 12 11 116 9

e ·7 6 5 4

3 2 1

I : : : : :: : :: I ~ { f.i I
.t ._ _... ,.

...... ~_--.....~

1..

OPBRA'1'IOR

Flqure 3~16

COD.

DBITINlTlOB

ADmtBSS

PIBLD

Sinqle operand IDSt.ruct:1oD Format

•

The Double Operand Ift8~ruction (Fiqure 3-17) implies
~wo operand., calle(! t.he .oure. operand and i:he destination

operand.

The same qeneral format. 18 again followed, but

here there are t.wo addre.. field. (one for each operand),
arXI t.hus a shorter operation ccXie.

Bit po8ition. 12-15 specify the opera~ion cede that
defines the instruction to be executed.
Blt positions _-5 specify the destination address field
(the addre88 of t.he aeetinat10n operand).

Thi8 8iX-bit de8-

tination address rield COns18ts of two three-bit sUbfield••

the register 8ubflelCl (bit. po81tloft8 16-2), and the E.!
subfield (bit positloM 3-5).

Bit. poe1t.lona 6-11 8pecify t.he source addre8s field
(the ad4r••• of the source operand).
a~dre8. field

'l'hi. aix-bit 80urce

eon8ist.s of two t.hree-bit. eubfielda. the

resi8ter subtiela (bit. posit.lons 6-8), and ~he mode subfleld
(bii: poS1t.1ofts 9-11).
15 14 13 12 11 111 9 8 7 6 5 4 3 2 1

I :::I~+I:U+I~BI:R~I

...,-.....--. ,.__I__~ ,.._.......,__~ ,..__...J
v
V
V
OPBRATION
CODB

Figure 3-17

SOURCE
ADDRBSS

DB8'1'~.'1'IO!1

PIELD ADDRB88

PIBLD

Double Operand In8~ru~ioD Porma~

The meaning of our earlier .~~...Dt - all addres8ing

with the rPP-11 18 accompli 8 bed throusb ~he General PurFJD!.

R.,lat.... - 8bould now be clear.

In 8peclfyinq the aMr•••

of ~he data (ad4r88. f1eld), one of tho eight. r.,1ster. 1•

••1ect:e4 (rec;ri.ter .ubflel4) aloDq with one of .everal
addre8sing lIlOdea (1DOd. subfle1d).
The.. addr. . . inq mod.. enable the easy accese and

maft1pulatlon of dat.a.

They are especially efficient and

flexible in halXl11ng of etruct:ured data (tables, llst:s,

ebaraeter string_, etc.), since a great deal of the data

proc.ssed by the comput.r 18 organized in this manner.

w. w1l1 examine each of the addr... lnq mode. in 4eta11,
and use the following lD8tru~loft8 for l11uatrat1cnl
MNBMONIC COD"

OCTAL CODB

.SL! D8'1'

II!fDD

DBSCRIP1'IOB

CLeaR (replace wit.h zer08)
the contents of t.he

De8rlnat1on location

-INC D8T
g

SRC,DST

ADD SRC,DST

flnDD

IBCrement (add 1 to) t.he
conte~. of the
DesTination location

1188I)D

HoVe t.be SouRCe operand to
the D8Ifiaation location
(.oure. operand unaffected.
destination operaDd replaced
by the source operand)

f!88DD

the SouRCe operand to
theJ)eft':l.nation operand
(.oures operand unaffected,
De8'1':l.nat:l.oD operand replaced
by t:be .um)

Am)

*.ymbol1c code devi8ed for .... of recog~:l.t.1on and ret:ent.ion
which aust be COD9ert.e4 1:0 machine (bift8ry) cod. by .Ollle
device or rout.ln. before it. caD be execut:ed by t.he cOliputer

3.2.2 General Register Addr•••lng MOde.
There are eiqht General Reqi8ter Addre88inq McXIes, and

any mode may be us'" wit.h any of t.he General Purpose Reqisters

to acceS8 the operand.

ThoUCJh each of these eiqht modes 1s

unique, and we will discUS8 the specific applicat.ion ot eacb,
we may cateqorize them as tollows according to l!!! they use

the' General Purpose Reqistera
( 1) DIRBC'l' ADDRESSING

where the req1ster cont.ains t.he operand.
GPR

I OPBRAlm I

( 2) INDIRBCT ADDRESSING

where the register contains t.he address of the
operand (the effective addre8s).

( 3) DEFBRRED ADDRESSING

where the reqi8ter contains the addre88 of the
effect.ive addre8••
GPR

AllDRBSS

1-1

EPFBCTIVB
"ADDRESS

tJt

OPERANij

3.2.2.1 Direct Addre8.1ng
There 18 one mode where the regi8ter 1s us~ to

contain the operand.

RBGI8TBR MODB

RBGIftBR MODE

OCtal Code

Assembler 8yn~ax*

Rn
Req18ter Mode specifies ~hat the register con~a1n8 the
operand.

The register 1s thu. used to hold da~a While it is

manipulated.
Example. . The content of R3 18 increment.ed
Loca~ion 511/1

fer

contains ~he in8~ru~lon code

INC R3

/1111151616 1111165211131
Before Execution.

Example a

Af~er

Execution.

1----141 .

The con~.nt of a2 i8 added ~o ~h. con~.n~ of RS
Locat.ion 211/1/6 cont.aina the 1net.ruct1on code

tor ADD R2,R5

/11121111" [ 116/62111Sl
Before Bxecut1on.

Aft.er Execution.

""61

11111111'-61

R2 1111111

IIIIIJ231 1

RS J1111112371

*'l'he percent 8ign (,,) indicates a General Purpose Reqi.~er to
t.he PDP-l1 A8sembler, and may be used. Typically, however,
~be reql.~.rs are defifted a. follow8 r

a, .

"III

R1 • "1

.2
= "2
R3. g
R4 s

...

RS • ~5

sP. ~

PC • "7
Theee deflni~ioft. will be used throughout t.M. book.

3.2.2.2 Indirect Addressing
There ~hree mode. where the register 18 used to
contain the address of t.he operand (the effecti.e address).

. REGISTBR DBPBRRBD MODB - where the content: of the
reqister 18 used as a ·pointer- t.o the operand and 18 not
modified.
At11'OINCRBMD1T MOJ)B - where the cont.ent of t.he
req18t.er 18 used aa a "pointer- to the operand aDd then 1s
-automatically 8t:l*' aheat 80 that it polnte to the next.
sequentlai opera
in a table or 11.t.
At7l'ODBCRBMBNl' MODB - Where ~be content of the

reql.t.er 1. first automaticallY .~ep~ and ~hen U8ed

as a apolnter

to an oper.na~D • ~.~i.t.

Altbouqh INDBXBD MODB doe. not . . .t. our general

stat.ement. exact.ly (r8CJ18t.er contains the effect.ive addr••• ),
it. i8 included 1n t.Me catevory beeaus. t.he regist.er will
contain .e!!!! of the effect.ive address.
CWl8,

A8 we will 800n d18-

t.he register containa an index word whlch i. added t.o

a base address to fora t.he addre.. of the operand.

.BGIST. . DBnUBD MODE
A•• embler Syntax

octal Code

(Rn)

R8CJi8ter Deferred Mode specifies that the reqi.tar

contains the a~~ress of the operand (the effective addr.8s1.
The content of t.he select.ed req1at.er i8 not. affected·) it i8

used a8 • • pointer" t.o the operand.
Example.

The cont.ent of R4 i8 the addree8 of the operand.
replace the operand with EarOD

Location 316311 cantaina the 1ft8truct1on c0c5e

for

CLR (R4t

1111316311 1111651114 t
Before Execution.
R4

After Bxecution.

'lIJ17,,-t

,_711_11 1123456J
Example.

116_71116-1

/1117/11111 11111111111111
The content of R2 18 the address of the operanCS,
move the operand to R5

Location 5" contains the 1n8t.ru~1on code
for HOV (.2) ,.5

1111115•• 1111121151
Before Execution I
~

After Executions

)-111'11-1

1_1111191_1

RS 11116661

555

11111111111 JlJlJlSssl

Ifllll1"

1"91

1111111555

AOTOD1C1UU1B1'l' MODB

A8.embler 8yn~ax

octal Code

(Rn)+

Au~olncremeft~

~he addre••

MOde specifie. ~ha~ ~be reql.~.r contains

of ~he operand (the effective aMr.... >"

Reqi.~.r Deferred

Mode.

Jut a8 vl~h

The difference 1. ~bat. af~er t.he coM.n~

of t.he req18ter 1. used a• • pointer ~o the operand, 1t 1. auto-

matically 8tepped .0 tht t 'b! r'li.tt, lh'n ROinSI to \hi n"t
.!elUentlel operand. .

Autolncrement Mode thus provide. for t.he aut.omat.ic st.epping of a point.er t.broUCJh a table or 118t. of operand8.

Al~houqh

especially useful for ~h1. type of proc:•••i.n9, ~h1. mode i8 complet.ely qen.rel and . .y be used for a
Example.

vari~y' of

purpose••

Rl contaill8 ~he adc!r. . . of the operaNSJ
lncr...nt.tbe 'operand aRcS then step the content
of .1 by t.WO .0 tbat it Will point ~o the next.
r.equ.n~la1Vor4 operand (in a u])le of operallCS.)
LoCation 1111111 c:on~aiDII t.he iMtruc:t.lon code
for IRe (Rla

11161111111 1_1152211
Before Execution.
Rl

.11251616

'.11125' '1

I"..JlJllll

Aft.er Bxecutiona
Rl

111111251121

111112516/6 JlJlJfll121

In t.ypical operat.ion, t.he proqram would loop back and
repeat. t.hi8 . . . . inst.ruction a sufficient number of 1:1..- to

increment. each of t.he entrie. 1n ~M uble.

Assembler .yn~ax

Octal Code

-(ltn)

Autodecremen~

Mode specifies that the register contains

the address of the operand (the effect.ive address>"

and a8 with

Aut:olnerement HoeSe, this address will be modified.

'l'he differ-

enee is that With Autodecrement. Mode, the content of the reqlster i8 automat.ieally stepped back, and then used as a point.er

t.o the operand.

Autodecrement Mode thus provides for t.he pro-

ee88inq of structured data in an inverse direction.
We will

la~er di.cu..

how the.. post-increment (Autoin-

crement Mode) and pre-decrement (Autodecrement Mode) features

are used to manipulate dynamic tabl•• called Btacks.
Exampl..

Replace the operand. in Table A with the
operands in Table B (1n inver.e order)
R3 containll the, address of the first operand
in Table AJ R4 contalns the address of the
n~ sequential location after the 1.8~ operand in Table B
Location 5/1' contalns the instruction code
for NOV - (R4 ).....J R3 ).:t.

'1"5/1' 1!,14423l
The results of typical proqram operat.ion are shown below.
The proqram has looped back and repeat.ed the instructioD five

time. to accompli8h the task.

AlTBR BXBCU'1'ION.

UFORB EXECtn'ION I

"111_11161

l"3t'121

Table A

'$n'lI'
_"lpf_2 •.· .-;.:
"1~~46
_,1__

__1'1'

--I

"391__
"3~fl2

g. . . __3,1_
~II311114

1.]

1 1

"'1'"
_'_
,/11,_2 .
111111

"~,,,
11'3"'2 ~;i;1
44231

"'3"11
~1'13"2
"3121'4
,'3plpl6
_113'1'

11
"1"6
,_3"6
Table A

"3111'

/I"l'~

',_1111S4
/61'"

"1/6112

,/61/6_

Table

,,3_116

,/63_116

_~/61J6

IJl1,11J19

'l21pJ3fJII41

19ii3"2~

'table A

,¢,

~~iiI

'1'111'''2
11116'_ ~~~
~~!"'"I

__3,'2

/6_1111/6 ~~~

'_3$4111'

993

"1f1~4 ..-~"..
~~~

11¢3,pJ4 ~~riIII

"3"2 ~~l"!!t
"111'6
"3"6 ~'i,pjIj'"
'~U'l' ~ . .

',,3_'4
J3{4'11

,/6311_6

J11111111#1

J1I~~21

'1111 39191 21

IJl11S31J16~

Table

Table A
"lpl'~ 1oIiQ.~~
~'19Jgf2 ~~~

'113'1111

~J63~P2
'II31/f~4

"111114 ................w
~.;;:..;:.=. .... ,'f1311116 ,

.. "3111'

,_111164
16_1'_6

~~_

'111'1_ ,.~.

6 6

IJI

,,3_116

6 6 6

,_311,6

'14316164

"1'
11_ .-'_... ''/63'162
{4311"
16'1'/62

442.

'table

'fable A

I&iI

Table

'I'l'~ ~~"'i!iI
16/61._6
~~ ...

'¢3pJl~ ...-.~ ...

'$43_~

~~....
"1~92 ...-~ ....

16,3_/62
"311"

7 6

1"1111161

'111'"

"1"6
,sn,l,

1 I

Table

6 6 6

~"'1"61

Table A

R4 1,,3ffll

"3"6
"3'1'

t,,3,t2
6 'J3"'_
"1I3'1f4

2
11'3'~~
""3'114

Table

,/I1t_2
,nll"
"1/114

,22

Table

'''1111'

'113$1111 'C"'C-C-'

It,,1tt!1
R4·1"3,,61

,_3__ II
1
"1"6
11'"4
''1,lfJ'

"1"2

_~3'~

1_'1'1141
R4 1/lJ!3'1I6t

6
7,.. 6

""3"116

,,,lIDII I P~P~JO~'

1__312111211

Table

1"'3'1'1

__1"'6

6 6

16 1
6

,/l111tH

Table B

"1~1I9J 1
"1"2

111' '1_' 21

'labl. A

1' '111'21

Table A

"1'116
__1_1,

442 1
6

,_311_6

1t3

_'1'$64

Table

IBDBXBD MOJ)B

Assembler Syntax

Octal Code

X(Rn)

Indexed Mode 8pec1fle8 ~ha~ the r!ilster contains an
index word which 18 added to a baBe addres8 (contained 1n a
location following that of the instruction word) to form the
address of the operand..

Neither the index word nor the

base address word i8 affected.
Indexed Mode thus provides for the random access of
operand8 in data structures.' The index word (the content of
the selected reqi8ter) 1s modified by the program to access

the desired operands in the table or list.

Examples

Clear the third operand .in Table A
R5 contains the index word. location 1/62
contains the base address of Table A
instruction code

Before Execution.
RS

'/6/61611164 ,

, __7112

1_111111111111

[1[4167_2

Table

After Execution.

I--'IIJUI

1-_ 11l116t.

Table

. *This 18 the more common usage. Realize that at the proqralllmer 18 option, the selected register may hold the baae addre•••

3.2.2.3 Deferred Addressing
There are two modes where the register is used to

conta1n the addre•• of the effective address.

AUTOINCRBMBNl' DEFBRRBD MODS - where the content of
the req18ter i8 used
a pointer to the address of t.he opexrand
in a table of effective addr...... It 18 first used, and then
automat.icallY stepped ahead.
-

At11'ODECRBMBNT DEFBRaBD MODB - where the content of
the regi8ter 1s \18M! a. a pointer to the addr.ss of the operand
1n a table ot effective addr...... It is fir8t automatically
stepped back, and then \1884.
Although INDBXBD DBPBRRBD MODB 40es not Meet our qeneral
statement. exactly (regi8ter contains the addres8 of the effective addre8s), it i8 incluaed in this category because the req-

~'.J,
••

iater will contain ~ of the addres8 of the effeetive address.
The req18ter conta1ns an index word which 1s added to a base

address to form the address of the effeetive addre8s.

AtJTOINCRBMBNT DBPBRRBD MODB

Assembler Syntax

octal Code

.(Rn).

Aut.oinerement Deferred Mode specifies that the register
contains the address of the effective address.

The content

of the .elected reqi8ter is used a8 the address of the effective addre8s, and then i8 automatically stepped ahead to the
next sequential address.
Autoincrement Deferred Mode thus prOVide. for automatieally stepp11lCJ throuqh a table of addr•••8., commonly ealleCl
a dispatch table, a8 a meane of aeees.inq operands.

The

effect of the instruction 1s dispatched throuqh this table

to the operand.
Bxample,

Clear the first operand in Tables A, B, and C
R4 contains the address of the first effective

addreBs in the dispatch table

Location 5/111 contains the instruction code
for~

1116/151111 JJ1I51134 1
The result8 of typical program execution are shown below.
The proqram hae looped back and repeated the instruction three
time. to accomplish the task.

If!!"i2]

Dispatch Table
;11

;12

33
·1 iJ4 .

J )f5f'~ jJ

Table

Table C

Table A

Table

Table C

Table B

Table C

Table A

Table B

Table C

Table

22221
3333
4444

15555
166661

1__':1,61

Dispatch Table·

Table

Table C

Table A

Table

Table C

AUTODBCpt1BN'l' DBPgRBD MODE

Assembler Synt.ax

octal Coc.1e

e-(Rn)

Autodeerement. DeferreCI Mode specifies t.hat t.he register
contains ~he address of the effective address.

The content

of the selected reqist.er is fir.to automatically st.epped back,
arKl then used a8 the addres8 of the effective address.

Autodecrement. Deferred Mode t.hus provides for automati-

cally stepping through a table of addresses in an inverse
direction.

Example.

Increment the operand
The content of R·2 18 aut.omatically stepped
back, and then used as the ac5dres8 of the
effective address (the address of the addreS8
of t.he operand)

2___

Location
cont.ains the instruction code
for !!S...I::.{Yl

11112___ 1111152521
Before Execution.
R2

•

1-_l__2l

After Bxec:ut1on.
R2

IIIJII11Il'Jill

__1111111 ' __51111_1

_161111111 [lIIl1 SII"IIJ

11115111111 1.1111111

11115/6/6_ 1111112]

INDBXBD DBPBRRBD MODB

A••embler Syntax

octal Code

_(an)

Indexed Deferred Mode specifies t.hat the regi8t.er

contains an index word which 1. added to a base address
•

(contained in a location following that of the instruction)
3;0 form the addr... of the ef'-*ive atar.... Neither the
index word nor the ba• • •&Sr••• i8 affected.
InClexed Deferred Mode thu8 provide. for the random acce••

of operantS& in data structure. through a table of addresses.
Example.

Using- Dispatch Table A, add the first operand
in Table C t.o the content of RS

~ contains

the index word J location S/l2
contains the base address for Dispat.ch Table A

Location S__ contains t.he instruction code
for~

1111,5__ Iiii.i.nl
_11/65/12
Before Execution.
RS

Aft.er Execution.

}11l11_1_1

all JlJlIlI'J14 J

).1116"._1

Di8patch Table A

11111111

RSf 1'11'161141
1616_5162

Table

1611115112
Dispat.ch Table A

1-117'-1
Table

C
'1 1 f1

J: 2 2
toll

'. r!

3.2.3 program Counter Reg18t8£ Addre.81ng Mod••
You will recall ~hat Req18ter 7, althouqh a General
Purpose Req18~er, a180 functions as the Proqram Counter for

the PDP-ll.

In this role, 1t always contains the address

of the next location to be referenced, and i8 automatically
updated by the processor during program operation (after an

instruction -1. fetched from a loeation, the Program Counter
18 automatically stepped to contain the address of the next

sequential location).
Althouqh any of the e1qht General Reqi8ter Addressing

Modes we have just discussed may be used 1n conjunction with
~

any of the eight General Purpose Reqisters, there are four of

the8e modes with which the Program Counter can provide spec1al advantaqes for the hanClliDq of unstructureCI data.

These

are called the Proqram Counter Reqi8ter AddreSsing Modes.
It is important to remember that th••• -special effect-

modes, although cla••"

.eparately and CJlven unique names,

are 1n operation the same General Req181:er Addre.81nq Modes
we have discussed. the only difference is ~ha~ ~he register
selected 1s alwaY' Req18t:er 7, ~he Proqram Coun1:er.

IMMBDIATB MODE

A••ambler Syntax

octal Code

r21~
Au~oincrem.nt

MOde

Program Counter

Req18ter

IMMBDIATE MODB provides for fast access of an operana
in t.hat. the· opera·~. 1s 1n a location ZNMBDIAT31y followiftC1
that. of t.he lD8uue~lon word.

The operand 18 act.ually part.

(a second wor~) of the instruct.ion.
Thi8 mode uses t.o qood acSvantaqe t.he fact. t.hat t.he

processor automatically .t.~ the content of the Program
Counter (.0 t.hat it then point. to the operand) arter fetchinq the 1natruct,ion.

When the in.truction 18 executed (t.he

aeSdr••• field i8 Aut.o1ncrement Mode wit.h the PC), t.he operand
18 obtained and t.he content of the Program Count.er 1• •tapped

(because of the Mode) t.o the next. sequential location.
Example.

Move the value 1

to R4

Locat.ions 5411 and 542 contain the code
for~

./1/15414 IiWiil

_1111542 I!!!!!!J

Sefore Executlon.
R4 11234561
PC

I_II1lI54!1

/6111654_
/611/1542

lM
1

rpI

After Execution.

1_11111"111
PC ['11"5441

_161154'

111111542

•

Program Counter conta1na
address of 1n8truction---p~'
_ _54_

_127_4

Instruction 18 fetched J
content of the PC 1.
automatically 8tepped
by t.he processor---........._./lS42

16/611116' IMMEDIATB OPERAND

Instruction i8 execut.ed J
cont.ent. of t.he PC 1.
st.epped because of

Aut.o1ncrement Mode--.-..--Jl/l1I544

Piqure 3-18

16/65/6/62

Immediate Mode

INSTRUCTION

NEXT INSTRUCTION

ABSOLUTE MODE
Assembler Syntax

octal Code

~37~
Autoincrement
Deferred Mode

Program Counter
Register

ABSOLUTE MODE 18 Autoincrement Deferred Mode using the
ProqramCounter, where the location immediatelY following that
of the instruction contains the address of the operand.
The immediate data i8 called an ABSOLt1l'E address because
this address remains constant no matter where in memory the"
instruction 1s located and executed •. What is the implication?
With the PDP-Il, proqrams (and thus the instruct.ions which
comprise them) can be relocat.ed in memory for subsequent execution.
ABSOLtJrE MODE 18 used to specify the address of an operand when
it 1s desired .to have that address be ABSOLUTE regardless of. the
proqram (instruction) location at execution time.
As illustrated by Figure 3-19, the same instruction 1s
executed from different locations in memory, and because ABSOLUTE
addressing is used, the address of the operand remains constant.
I

•

I
I

/6/65161616
3
16/6516162 ~iIIiiiiioIiIIliiI1IiiiIIII

,111111
•

•

1111]1

Fiqure 3-19

Absolute Addressing

Example.

Move the content of location 7__ to R3 .
Locations 1
and 116_2 contain the code
for MOV 1!7@@,R3

__1/11111 Iiiiii3l

16161_162 ~
Before Execution.

After Execution,

11 23456'

R3 11111111

191_11691911

~1'917~~ 11111] 11

f8¢fl7f1~ 111.1l11 J

Example.

'!ffll@p!41

Move the content of location 71111 to R3

"
~.

Before Execution.

After Execution.

R3 11234561

R3 111111J

I!9J591¢fll

!l919J7~/I 11 1.1l111

J,@tJ5p!@4(

9J9JpJ7{1/1 11]11111

RELATIVE MODB
Assembler Syntax

oc:tal Code

~67~
Indexed
MOde

Proqram Counter
Reqister

RELATIVE MODE 1s used whenever direct reference i8 made to
a memory location, and 1. a ••embled a. Indexed Mode \Wing the
Proqram Counter.

Because the content of the Program Counter ls

ulled in the addre.. calculation, the address of the operand ls
~

absolute, it i. RBLATIVB to the address of the instruction.
Recall that Indexed Mode forms the effective address by

addinq the content of the specified reqister (index value) to
the content of a location following that of the instruction
word (baee address).

Relative MOde works in the same way, with

two distinquiehing points.
1.

The specified reqi.ter i8 the Proqram Counter, and
its content will be updated during the execution
of the instruction.

The addre•• in the location following that of the
instruction i8 here called an OPPSBT, because it
serves as an off8et to the content of the updated
Proqram Counter.
The following algorithms are used by the as.embler.
BPPBCTIVB ADDRBSS :s OPPSr:r + UP~TBD PC
OPPSft

Example.

= BPPBCTIVB AJ)DRBS8 - UPMTED PC

Increment the content of location TALLY
Locations 51616 and 5162 contain the code
for INC TALLY

16_1651616 Iii3Irn
1111115112 ~

Before Execution.
TALLY

~1234561

'UlS16"

After Execution.
TALLY 11234571
PC

1IIIIIISIMi

Proqram Counter contains
address of instruction-........ !'JlJlS!'JI 16165267

INSTRUCTION

Instruction is Fetched;
content of PC is
automatically stepped
by the processor - - -......... tlI1fAS1I2 fA~161674

OFFSET

Source state is entered;
processor gets the offset
and automatically steps
content of PC. It i8
this updated content of ...... 1611165114 XXXXXX
the PC which is then
added to the offset to
yield the effective
address. The instruction
1s then executed.

NEXT INSTRUCTION

BFPBCTIVB ADDRESS = OFFSBT + UPDATED PC

Fiqure 3-216

Relative MOde

Why have two modes (Absolute and Relative) which achieve the
same purpose?

Though each is used to specify the addre.. of the

operand, the method u8ed is ~ the eame, and that is the reason
for the existence of both.
If the program 1s always to be loaded and executed at the
same locations in memory, either mode may be usedJ there is no
particular advantage to using one in preference to the other.
If, however, the program is to be loaded and executed at various
locations in memory (relocated), then there i8 a preference,
Relative mode should be used.
The key to this usage preference i8 that when a program is
relocated, it should be as a complete entity.

This means that !!l

locations used by the program (instruetions ~ storage) should be
relocated, and that therefore no absolute references be made.
To help clarify this point, let's look at a simple (but not
terribly productivel) program wherein the same instructions are
used With both Absolute and Relative modee.
BEGINI CLR ",SAVE

BBGIN. CLR SAVE

HALT
SAVE.

SAVE,.

HALT
/8

As illustrated by Pigure 3-21, both proqrams were assembled
and loaded beqinninq at location 51111. and then relocated so that
they begin at location 311_11.
clear.

In relocating, the difference becomes

With Absolute mode. a location used by the proqram has been

left "danqlinq" far behind J with Relative mode, our program remains
a "compact" Whole.

~:: ,. ·Tcxtr'o·n
, i

,.... 350.

600

NET

V5,·

INCOME

.'.:, .',
.'

Index 100: 1963

SOOI----+---+----+--~-_1_-_+_-.J__4 -+-*-~

-4

200'

300

.- . -i

150

200~-+--4--:.l~-

CHART # 13

CHART # 12

$2.70
2.25
1.80
i

t-·

!
I

._~ .

.45

Ii
i

.__L-L_ i

_-L
71

CHART # 14

500·
Quarterly Clolings

400!
300;

200~

.20'

100,........,...........,.I
I

CHART 1115

I
70

CHART 1116

{~ .

I Assembled and loaded to beqin at location 51611 I
Absolute Reference
I

Relative Reference

I
J

I
f
I
I

I
I

J
I

I
I
I

•

------~I Relocated to beqin at location 39J9J9J I~-----
Absolute Reference

Relative Reference
t

•

5 67
2

Figure 3-22

Absolute Reference VB. Relative Reference

RBIATIVB DBPBRRBD MODB
Assembler Syntax

Octal Code

RELATIVB DEFERRED MODB 1s assembled a8 Indexed Deferred
Mode us1nq the Proqram Countero

It is similar to Relative Mode,

but with the additional level of deferral, the calculation
OFFSET + UPDATED PC yields the addres8 of the effective address.
Examples

Clear the location pointed to 'by the content of
location 1111111
Locations 5/111 and 5112 contain the code

for

CLR Ilfll

1616/15/1/6 IiimiI
/1/1/65162 l!!!!!!J
Before Executions
fill] IIlIfI

1'@2'.

16162111616 r1717171
PC

1_"16516"1

After Bxecutlons

/6/11/1_11

'"281

_/12/11616 liiuu~
PC

Iwsiij]

3.2.4

EXERCISES
The following examples based upon the discus8ion of General

Purpose Reqisters are presented a8 an optional exercise for the
reader.

The answers can be found in Appendix B.

3.2.4.1 General Register Addressing Modes
C~mplete the chart below.
Thi8 1s an instruction list, not a
program (consider the given values to be true for each instruction).
Given, (Rl):l1616/1, (R2)=2_J6J6, (2J6J6/6)=616J1f6~ (1776)=516JiJ14, (211611)=4161616

SYMBOLIC
CODE
HOV

OCTAL
CODE

SOURCE
DESTINATION
EFFECTIVB ADDRESS EFFBCTIVB ADDRESS

Rl,R2

HOV Rl, (R2)
MOV Rl, (R2)+

"~...Jj~~'
I.....

R:L

(R2)

2tlfb2,

MOV Rl,-(R2)
-~,

MOV Rl,lJ6I6(R2)
MOV Rl,el/6/6(R2)
MOV Rl,@-(R2)
MOV Rl,Gt(R2)+
3.2.4.2 Program Counter Register "Addressing Modes

Complete the chart below. This 18 an instruction list, not a
program (consider the given values to be true for each instruction).
Givens (R16)=7/616/6, (PC):5JiJ16, (123456)=316JiJII, (316_16)=31611
SYMBOLIC
CODE

OCTAL
CODE

MOV #123456,R16

...

MOV "'123456,R~
MOV 123456,RpI
MOV .123456,RI6

~--.r_

SOURCE
DESTINATION
BFPECTIVB ADDRESS BFFECTIVB ADDRESS

:1.%11$

•

(R,fj>

3¢rI4

3.3

INSTRUCTION SET

. 3.3.1 Introduction
Before you can write a program. you must have a working
knowledge of the instruction set which you are to use.

We will

then discuss the PDP-Il instruction set, but because this booK
is .1ntroductory in nature, we will limit our discussion to only
a part of the basic set.

(It is expected that you shall soon

become expert and seek the power of the complete instruction setl)
Those instructions we are to discuss will be first listed
by format. and then grouped according to function for. a more

detailed presentation.
Abbreviations. symbols. and other esoteric markings used
are as followsl

= general register (3 bits). _-7
SS or src = source address field
R or req

DD or dst = destination address field

1\ = AND

V = Inclusive OR
~=

Exclusive OR

lcc = arbitrary location

• = , for word; 1 for byte

xxx = offset (8 bits)
CONDITION CODE LEGEND

It' = Clear

= Set
* = Conditionally Set

- - Not Affected

3.3.2 Formats
3.3.2.1 Condition Code Ooerate Group

2 1

F; ; ; :~se: ~od~ : i·GEEEl
:

•

F1qure 3-23

Condition Code. Operate Format

MNEMONIC

INSTRUCTION

CLC

CLear C

CLV

CLear V

111616241
16__ 242

CLZ

CLear Z

111616244

- II

CLN

CLear N

J6f1_25J6

SEC

SEt C

fl1616261

- 1

SEY

SEt V

16/1/1262

SEZ

SEt z

1616[1264

- 1

SEN

SEt N

[/1161627_

CCC

Clear all

16 16 16 16

sec

Set all

OPERATION CODE

CONDITION CODES
N Z V C

- 16

- - fI -

- - 1

3.3.2.2 Single Operand

3.3.2.3 Double Operand

3.3.2.4 operate Group

Pigure 3-26

Operate Group Format

OPERATION CODE

CONDITION CODES
N Z V C

HALT

1611/6/6/6/6

- - --

RTI

ReTurn from
Interrupt

1616/616/62

* * * *

TRAP

1/644/6/6

* * * *

MNEMONIC

INSTRUCTION

HALT

to 1164777

3.3.2.6 Subroutine C!!!

Flqure 3-27
MNEMONIC
JSR

INSTRUCTION
Jwnp to

SubRoutine

Subroutine Call Format
OPERATION CODE
JlJ44RDD

~ONDITION

CODES

r·

3.3.2.7 Subroutine Return

Plqure 3-28
MNEMONIC

INSTRUCTION

Subroutine Return Format
OPERATION CODE

CONDITION CODES
N

R'l'S

ReTurn from
Subroutine

3.3.3 A Word About Bytes
You have already learned (and of course· ret.ainedl) some
import.ant. fact.s about PDP-ll memory organizat.ion and addressing.
1.

Memory is bot.h byte and word addressable;
each (16 bit.) word is comprised of t.wo (8 bit.) bytes.
(Refer t.o Figure 3-4)

Bit. 15 is the Most Significant Bit. and siqn bit
for both the word and the high (odd) byte;
bit 7 is t.he Most Significant Bit and sign bit
for t.he low (even) byte. (Refer to'Figure 3-3)

The instruction set. then, must have the capability of dealing
With both word and byte operands.
We will for the most part restrict ourselves in this book
t.o working with words, and t.herefore word instructions, but it is
important to remember that the PDP-ll instruction set includes a
full compliment of instructions which manipulate byte operands.
A good general performance guide is this.

Byte instructions op-

erate upon byte operands in the same way that word instructions
operate upon word operands.
In the lists of instructions on the previous pages, you
noted that those instructions used to handle both byte and word
operands were presented in the following manner.
MNEMONIC

OPERATION CODE

OPR(B)

mmNNN

The purpose was to indicate that both the mnemonic (symbolic)
code and the octal (or binary) operational code differ to specify
either the byte or the word operation.

As illustrated by the comparative examples below. the
coding procedure is as follows.
To specify a ~ operation.
do not append the B to the mnemonic coae;
use a zero (JI) as the MSB of the operation code
To specify a ~ operation,
do append the B to the mnemonic ·code J
use a one (1) as the MSB of the operation code

Example.

Clear Register _
Location 1"~ contains the instruction code
for CLR R@

__1~'J6 t'J65J6J6J61
Before Execution.

Example.

After Execution.

Clear the low byte of Register _
Location 1~~~ contains the instruction code
for CLRB RJ6

__1_1616 '1~5f6f6f61

"
t

Before Execution.

After Execution.

R1J 11111111

RJ' 11I1J6J6J6(

3.3.4 Condition Code Operate Group
You will recall that the four least significant bits of the
Processor Status Word are referred to as the condition code bits.

8 7 6 5 4 .3 2 1

unused in basic 11 ~r~or:it~

Figure 3-29

Processor Status Word

As was shown in the instruction lists presented earlier,
these bits may be implicitly modified by instruction execution

= 1 if the result was Negative
Z = 1 if the result was Zero
v = 1 if arithmetic oVerflow resulted
C = 1 if a Carry from the MSB position resulted
N

and will therefore in these cases reflect the result of the
previously executed instruction.

The information provided by

these bits can then be used by other instructions (i.e., Branches)
in the proqram.

These condition code bits may also be explicitly modified
by means of the Condition Code Operate instructions.

These 1n-

struct10ns are commonly used to make sure that certain bit(s) are
set (or cleared) before a given programming sequence is begun.

116

~
·0 : : I :~ : I : :~'~
'15

Clear
Se~

Piqure 3-316
Description.

Condition Code operate Instruction

Set or Clear condition code bit(s)
The bit 4 position specifies whether the
Condition Code Operate Instruction is a
Clear (bit 4'=/1) or a Set (bit 4
1)

Bit positions 16-3 of the Condition Code
Operate Instruction (correspondinq to the
positions of the condition code bits in
the Processor Status Word) specify whether
or not these bits are to receive the action
of the instruction (/6
no) (1
yes)

Be aware that many possible 'combinations
(other than CCC or SCC) may be selected for
use (1.e., ~J6/6243 Clears both C and V)
Condition Codes.
Examples.

All are explicitly Cleared or Set
MNEMONIC

INSTRUCTION

OPERATION CODE

CLC

CLear C

16/616241

SEC

SEt C

/1/616261

CLV

CLear V

161616242

SEV

SEt V

161111262

CLZ

CLear Z

111111244
,

SiZ

SEt z

Jl1I1I264

CLN

CLear N

/6111125{1

SEN

SEt N

/6161627/6

CCC

CLear all

16/6/6257

SEC

Sh all

16/6/6277

3.3.5 General/Arithmetic
3.3.5.1 Introduction

The qroup of sinqle and double operand instructions
which follows has been termed General/Ar1thme~ic because each
instruction could, dependinq upon a qiven usaqe, be listed in
either cateqory.

3.3.5.2 CLEAR

CLeaR DeSTination

116

~91:91:91 11 :91: 1

Piqure 3-31

Description.

Clear Instruction

Replaces the content of the destination
location With zeroes

Cond:Ltion Codes.

N
Z
V

Example,

Cleared
Set
Cleared
Cleared

Clear location l~__
Locat.ions 51616 and 5/62 contain the code
. for CLR @#l!@!

_~1651616 ~

16/1/15162 ~
Before Execution •

•

After Execution.

MOVe SouRCe,DeSTination

~~ :~ : 1 I

81SSDD
~

: : 8~C : : I : : dSf : :-I
8

P1qure 3-32
Descriptions

Condition Codes.

V
C

Move Instruction

Set if source operand less than zero;
Cleared otherwise
Set 1f source operand is equal to zero;
Cleared otherwise
Cleared
Not affected

Move the value 123456 to Reqister 3
Locations 5f6[6[6 and 5[6[62 contain the code
for MOV #123456,.R3
_165[6_[6 ~~ ....
_[65[6162

Before Execution.
R3

1~1~1$l11

__ 5J6~2 )1234561

Moves (a copy of) the source operand to the
destination location. The content of the
source location is not affected; the oriqinal
content of, the destination location is lost
(replaced by the copy of.the source operand)

Examples

After Execution.
R3 '11234561

__516_2 1123456~

3.3.5.4 TEST

.pJ57DD

TeST DeSTination

1KI~:~:~11:~: 1 1

116

1: : I : : d~t : : I

Piqure 3-33
Descriptiona

Test Instruction

Tests the content of the destination
(Specifically for Negative and Zero)
The content of the destination is not affected

Condition Codesa

N
Z
V

.Example a

Set if destination operand less than zero;
Cleared otherwise
Set if the destination operand i8 equal
to zero, Cleared otherwise
Cleared
Cleared

Test the content of Reqister 5
Location 7~pJ contains the code
for TST RS

1611/1791/6 I.@P!S791SI
Before Execution a

•

11777771

C
_.
1.
If 1

After Execution a

li777771

c
16

3.3. 5.5 COMPARE
0

82SSDD

CoMPare SouRCe.DeSTination

111

°9

~-:1:-I~:~:~3~c~:~:~I~~:~:_.d~F~:~:_1
Plqure 3-34
Description.

Compare Instruction

Compares the source and destination operands
by subtraction (source - destination)
Neither operand is affected

Condition Codes,

Example.

Set if the result is less than zero;
Cleared otherwise
Z
Set 1f the result is equal to zero;
Cleared otherwise
V Set if there was arithmetic overflow
(operands of opposite signs. s1gn of the
result same as sign of the destination);
Cleared otherWise
C Set if there was no carry from the MSB
position ofOthe result; Cleared otherwise
Compare the contents of Register 2 and Reqister 3
Location SI'I' contains the code
for CMP R2,R3

16~IIS_16 1'2~2~31
Before Execution

After Execution.
R2

r9J~9J9J9JS'

1177761"1

1_~9J9J1'51
R3 117776!

z
1

3.3.5.6 SWAP BYTES

SWAp Bytes DeSTination

Pigure 3-35

Swap Bytes Instruction

Description I

Exchanqes the hiqh order byte and low order byte
of the destination word

Condition Codes.

Set if MBB of low order byte (bit 7) of the
result is set; Cleared otherwise
Z
Set if low order byte of the result is equal
to zero. Cleared otherwise
V Cleared
C Cleared

Example.

Swap the hiqh order and low order bytes of R4
Location 2~~~ contains the code
for SWAB R4

Before Execution.
R4

After Execution.
R4

123456
87654321

•

~27247

3.3.5'.7 ROTATE RIGHT

Rotate Riqht DesTination

1.4 . 13

111

~ II /6 ~l

: 1 : 16

Fiqure 3-36
Description.

I 91: /6

: I : : aT : : f'
5

. 3

Rotate Riqht Instruction

Rotates all bits of the destination word one
place to the right. The content of 'the C bit
of the Processor Status Word is rotated into
the bit 15 position, and the content of bit ¢
is ··rotated into. the C bit position

D-+~-.......-........----....-...................-.~ .....

+-------~

A -17 bit connected serial shift- one position
to the riqht which facilitates sequential bit
testing and detailed bit manipulation.
Condition Codes a

z
v
C

Example.

Set if the high order bit of the result is
set (reSUlt less than zero); Cleared otherwise
Set if· all bits of the result are zeroes:
Cleared otherwise
Loaded with the Exclusive OR.pf the N bit and
C bit (as set by the completion of the
Rotate instruction)
Loaded with the low order bit of the destination

Rotate Right the content of Register /6
Location 1~~/6 contains the code
for Rca RPJ

lI/6ll1l1¢ ~{469J9J9J

Before Execution.

After Execution.

RII 11234561

RJ6 1~516271

C bit

3.3.5.8 ROTATE LEFT

Rotate Left DeSTination

..,

Pigure 3-37

Description.

Rotate Left Instruction

Rotates all bits of the destination word one
place to the left. The content of the C bit
of the Processor Status Word is rotated into
the bit 16 position, and the content of bit 15
is rotated into the C bit position

D....
C

~.....-.-..-

.............-....-.-. . . . .-..-....... . .-.

I....-.-------~
A -17 bit connected serial shift M one position
to the left which facilitates sequential bit
testing and detailed bit manipulation.

Condition Codes.

Z
V

•

C
Example.

Set if the high order-bit of the result is
set (result less than zero); Cleared otherwise
Set if all bits of the result are zeroes;
Cleared otherwise
Loaded With the Exclusive OR of the N bit and
C bit (as set by the completion of the
Rotate instruction)
Loaded with the high order bit of the destination

Rotate Left the content of Register 16
Location 1~16~ contains the code
for ROL Rf6

/lJ61~[6~ 1919161 i1911
Before Execution.

After Execution a

Rt6 11234561

RJ6 J ~47l34l

C bit

3.3~-S.9

ARITHMETIC SHIFT RIGHT

·Arithmetic Shift Riqht DeSTination

]

"
Fiqure 3-38
Description.

Arithmetic Shift R1qht Instruction
Shifts all bits of the destination location one
place to the right. The present content of the
C bit is lost as the content of the bit ~
position is shifted in and bit 15 is replicated
(to maintain the sign)

~-)(---------

The ASR instruction performs signed division by
two on the content of the destination location.

Condition Codes.

Z
V
C
Example.

Set if the high order bit of the result is set
(result less than zero); Cleared otherwise
Set if all bits of the destination are zeroes;
Cleared otherWise
LOaded with the Exclusive OR of the N bit and
.C bit (as set by the completion of the Shift
instruction)
~
Loaded with the low order bit of the destination

Arithmetic Shift Right the content of Register ~
Location 1~¢¢ contains the code
for ASR R@

16161~~~

¢{162¢{1

Before Execution.

After Execution,'

R/4 11234561

R/4 '1516271

C bit

C bit /4

3.3.!.1~ ARrTHMETIC SHIFT LEFT

Arithmetic Shift Left DeSTination

Figure 3-39
Description.

Arithmetic Shift Left Instruction

Shifts all b1ts of the destination location one
place to the left. The present content of the
C bit is lost as the content of the bit 15
position is shifted in and the bit ~ position
i8 (always) loaded with a zero

The ASL instruction performs s1qned multiplication
by two (with overflow indication) on the content
of the destination location.
Condition Codes,

Z
V

Set if the hiqh order bit of the result is set
(result less than zero); Cleared otherwise
Set if all bits of the result are zeroes;
Cleared otherwise
Loaded with the Exclusive OR of the N bit and
C bit (as set by the completion of the Shift
instruction)
Loaded with the high order bit of the destination

...

Example I
~

Arithmetic Shift Left the content of Register ~
Location l~~~ contains the code
for ASL R@

1616116[6[6 ¢¢63¢¢1
Before Execution.

C bit

After Execution.

C bit

ADD SoURCe,DeSTination

'6SSDD

:
SiC
:
:
:
m :-I
I : : d1 : : I
15

116

Figure 3-4_ Add Instruction
Description,

Adds the source operand to the destination operand
and stores the result in the destination location.
The source operand is unaffected, the destination
operand 1s lost (replaced by the result)

Condition Codes,

Z
V

Example.

Set if the result is less than zero,
Cleared otherwise
Set if the result is equal to zero;
Cleared otherwise
Set if there was arithmetic overflow
(operands of same sign; result of opposite
s1qn) , Cleared otherwise
Set if there was a carry from the MSB of the
reSUlt, Cleared otherwise

Add the content of Reqister 3 to' the content
of location S~~J6
Locations 7~~ and 7~2 contain the code
for ADD R3 ,@#5~{6~

JI!6!67~~ ~

__~ 7 _ 2 ~

Before E x e q u t i o n ' A f t e r Execution.
R3 1_~~1231

11115111111 ~fI~4561

R3 1~!6f61231

16115111111 flfl1I 6 f61 1

~
"J

3.3.5.12 SUBTRACT

SUBtract SouRCe.DeSTination

lltJ

:-1 .: :
Piqure 3-41
Descriptions

16SSDD

Subtract Instruction

Subtracts the source operand from the destination
operand (destination - source) and stores the
"result in the destination location
The source operand is unaffected; the destination
operand is lost (replaced by the result)

Condition Codes.

N
Z

V
C

Examples

Set if the result 1s less than zero;
Cleared otherwise
Set if the result is equa"l to zero;
Cleared otherwise
Set if there was arithmetic overflow
(operands of opposite signs; sign of result
same as sign of source); Cleared otherwise
Set 1f there was no carry from the MSB
position of the result; Cleared otherwise

Subtract the content of Register ·3 from the
content of location 5~~1tJ
Locations 7~~ and 7162 contain the code
for SUB R3,@#5~f6f6

•

_16167_~ miml
/616167[62 ~
Before Execution.
R3

IW1231

__5/61616 JW'!6¢1.1

After Executions
R3

IWP!123'

16/651616¢ l e¢¢456~

INCrement DeSTination

Piqure 3-42
Description.

Condition Codes.

Adds one to the content of the destination
location
N
Z
V
C

Example.

Increment Instruction

Set if the result is less than zero;
Cleared otherwise
Set if the result equal to zero;
Cleared otherwise
Set if the destination operand was _77777;
Cleared otherwise
Not affected

~
I

Increment the content of. Register 5
Location 3~~16 contains the code
for INC RS

._'3'16_ I[6p!s2p!sl
Before Execution.
RS

11234561

After Executions
RS 11234571

3.3.5.14 DECREMENT

DECrement DeSTination

: _ : _

116

~ _ : 1

1-:

Figure 3-43

Decrement Instruction

Descriptions

Condition Codesl

1.......:'-:-.....-..d1~t .-.-:.--:(

Subtracts one from the content of the destination
location
N

Z
V
C
Examples

Set if the result is less than zero;
Cleared 'otherwise
Set if the result is equal to zero;
Cleared otherwise
Set if the destination operand was l~~~~~J
Cleared otherwise
Not affected

Decrement the content of Reqister 5
Location 3161616 contains the code
for DEC RS

,,3'_, J 3 1

2[215 91 5

Before Execution.
•

J1234571

After Execution s

:3.-3.5.15- COMPLEMENT

COMplement DeSTination

_1651DD

Flqure 3-44

Description.

Condition Codes.

Replaces the content of the destination location
with its one's complement
N
Z

V
C

Bxample.

Complement Instruction

Set if the MSB of the result. is set;
Cleared otherwise
Set if the result is equal to zero;
Cleared otherwise
Cleared
Set

Complement the content of Register 5
Location 3~~~ contains the code
for COM RS

. 16_3161616 11616511651
Before Execution.
RS 11234561

After Execution.
RS

11654321' ·

3.3.5.16 NEGATE

NBGate DeST1na~1on

111

1
1
....
: ~......
: -..11 : ~ : ~ 1 .......
: .-...:_d;.....
~ ~: ~ : ~ 1.....
t

Piqure 3-45
Descriptions

Condition Codes.

_:_1

.....
:

Neqate Instruction

Replaces the content of the destination location
With its two's complement
N

Example.

Set if the result is less than zero;
Cleared otherwise
Set if the result 1s equal to zero;Cleared otherwise
Set if the res~lt is l~~~__ J
Cleared otherwise

'!'wo's cQmplement the content of Register 5
Location 3~¢¢ contains the code
for NEG RS

Before Execution.
•

R5 J1234S6~

After Execution.
RS

I ~543221

3.3.6 Logical
3.3.6.1 Introduction
The qroup of double operand instructions which follows
has been termed Loqical because the instructions are based on
the loqic operations discussed earlier (Section 2.5).
These instructions permit operations on data at the
bit level.

3.3.6.2 BIT TEST

BIt Test SouRCe,DeSTi~tion

P1qure 3-46
Description.

113SSDD

Bit Test Instruction

Performs a Loqical AND operation between the
source and destination operands
Neither operand is affected
This instruction is commonly used for status

checking; to determine whether or not certain
bit(s) are set (cleared) in a specified word
Condition Codes.

V
C
Example.

Set if the MSB of the result is set;
Cleared otherwise
Set if the result is equal to zero;
Cleared otherwise
Cleared
Not affected

Location 1775616 contains the status word for an
Input Device; bit 7 is set when a transfer of
information 1s complete. Check this Iidone bit.·
Locations 5~~¢, 5¢¢2, and S¢¢4 contain the code
for BIT #2@@r@!17756@

J616S¢¢[6
__SI6162 ~~~
16_516_4 ................
Before Execution.

After Execution,

16I1S16162 J ¢~¢2¢¢j

1775616 J¢Jl1¢f6¢~'

17756_1 ¢$!1¢9J0Jt11

Jl16516~2 ¢¢¢2~Jt'
V

11611

C
~

a4SSDD

BIt Clear SouRCe,DeSTination'

III

:-.-.Isj___..c:.....-..:~I

Fiqure 3-41
Description.

S·

Jt1

__
: .-. -:--.-dft......:......:__1

Bit Clear Instruction

Clears each bit in the destination operand which
corresponds to a set bit in the source operand
The source operand is unaffected. The original
destination operand is lost (receives the action
of the instruction); replaced by the result
This instruction is commonly used for a function
called Masking (getting rid of unwanted bits)
or Extracting (saving wanted bits)

Condition Codess'N
Z
V
C

Examples

Set if the MSB of the result is set.
Cleared otherwise
Set if the'result 1s equal to zeroJ
Cleared otherwise
Cleared
Not affected

Extract the two Least Significant Digits of the
content of location l~¢¢ for future action
Locations 6fi1fi1f6, 6J6fi12, and 611164 contain the code
for SIC #1777@@,@#1¢@@

1l1l6fi1Jt116
1116616162
11/1616164
Before Execution.

After Execution.

_/6616162 '1777@8d

JI{66J6J12 11777J?!P!1

/6/61/1fi1J1 '1234561

/lJl1/1J6[4 J¢11111156I

11616

_ 11 t6 16

3.3.6.4 BIT SET

BIt Set SouRCe.DeSTination

~
Fiqure 3-48
Description.

Bit Set Instruction

Performs an Inclusive OR operation between the
source and destination operands
The source operand is unaffected. The original
destination operand is lost (receives the action
of the instruction); replaced by the result
This instruction is commoniy used when it is
desired to set certain bites) within a given
word without affecting the other bits

Condition Codes.

Z
V
C
Example.

Set if the MSB of the result is set;
Cleared otherwise
Set if the result is equal to zero;
Cleared otherwise
Cleared
Not affected

Location l7756~ contains the status word for an
Input Device; setting bit 6 enables the Proqram
Interrupt Facility. Do this.
Locations 7~~~, 7~~2, and 7~~4 contain the code
for BIS #1~,@#17756~

J6J67J6~¢
16J67f6~2

1616716164
Before Execution.

After Execution.

~!67!6!62 If6919J1 ¢9J1

1616716162 1919Jf619J~1

1775616
V

I f691919J919Jl·

11616

1 775616
N

19J¢f619J!l

11 _ 16 /II

3.3.7 Program Con~rol
3.3.7.1 IntrOduction
The qroup of instructions which follows has been termed
Program Control because these instructions control the flow of
the program.

Typically, such instructions cause a chanqe from

one sequence of instructions to another.

JuMP DeSTination

Figure 3-49
Description.

Condition Codes.

Transfers proqram control to any location .~~.
memory (destination address calculated and
loaded into the Program Counter)
N
Z
V

Examples

Jump Instruction

Not affected
Not affected
Not affected
Not affected

Transfer program control to location LOOP
(arbitrarily defined as location l~~~)
Locations S~~ and Sli12 contain the code
for JMP LOOP

___5~1i1 ~
1f161i15~2 ~

Before Execution.

After Execution.

BRANCH INSTRUCTIONS

M A

O'P B R A.'l' I ON C O'D E

4
G

~
".-

0 F F S E'l'

OPBRATION
Branch
Transfer proqram con~rol to the location defined by the offset

Uncondi~ional

Conditional Branches
"
Check the appropriate Condition Code bit(s)
-If the condition(s) met, transfer program control to the
location defined by the offset
-If the condition(s) not met, pass control to the next
8equential location

OFFSET
The offset ~s a signed (two's,complement) displac&~ent to the PC
within the low order 8 bits of every branch instruction
It specifies the number of words from the updated PC to the
desired location

CALCUIATION
The PC expresses a byte address. Because the offset is expressed
in words, it is first converted to bytes and then added to the
PC to effect the transfer
The algorithms used arel

1&£= (OFFSE'l' x 2) + UPDATED PC

OFFSET=(LOC - UPDATED PC)/2

RANGE
The range of any branch instruction is limited b~ the offset
(forward 177 8 words; backward 2¢~8 words), but advantages are
that. (1) All branch instructions (except for BRANCH) are
conditional, and (2) The instruction itself will always require
only one word (versus the usual two for the Jump instruction)
RANGE
(WORDS)

-2"••

-1

•••
+177

OFFSET
(LOW 8 BITS)

2_'
•

••
376
377

11_'
"1

'Jl2
••

177

3.3.7.3 BRANCH
saanch (to) l~

~_:_:_I_:_:_~~:o~:_p~:p~:~s:~E~~T~:~1
1'1qure 3-516
Description a

Branch Instruction

Transfers program control unconditionally to
~he location defined by the offset
(See next paqe for qeneral operational information)

Condition Code..
Example,
" ... ".>

Not affected
Transfer proqram control to location LOOP
(arbitrarily defined as locati~n l~~~)
Location 51616 contains the code fo
for BR LOOP

Before Execution.

After Execut.ion.

* OS=(LOC-UPC)/2

OS=(1¢¢¢e- 5¢2s)/2
OS=276s/2
OS=137 8

3.3.7.4 BRANCH iP EQUAL ZERO

Jl1I14xxx

Branch 1f EQual zero (to) lac:

GJ,:,;,J-:';l
Piqure 3-51

Descriptions

Condition Codess
Example.

Branch if Equal InB~ruc~ion

.Transfers program con~rol to the location defined
by the offset!! the condition (Z bit set) is met.
If the condition is not met, control passes to the
n~ sequential location
Not affected
Compare the contents of Reqieter 1 and Reqister 2.
Branch to location SAME (arbitrarily 1~5~) if the
contents are equal
Locations 1161616 and 116112 contain the code
for CMP R1,R2
BEQ SAME

'1I1~J616 ~
11161_162 ~ * .
After Executions

Before Executions
Rl lp!p!¢59J~(

19J~~S9J9J1

IP!PJ[65f!!1

R2 1p!P!9J5p!9J~

Ii1pJlf!¢IlJ)

* OS=(LOC-UPC)/2

OS~pJ5¢8-19JJ64e)/2

OS=448/2
OS=22 8

I~~19J59J1

3.3.7.5 BRANCH IF NOT EQUAL ZERO
Branch if Not Equal zero (to) loc

Fiqure 3-52
Description.

Condi tion Codes.
Example.

Branch if Not Equal Instruction

Transfers proqram control to the location defined
by the offset ![ the condition (Z bit clear) is
met. If the condition is not met, control passes
to the next sequential location
Not affected

Compare the contents of Register 1 and Reqister 2.
Branch to location DIFF (arbitrarily 1~5~) if the
content~ are not equal
Locations If6f6[6 and 116[62 contain the code
for CMP Rl,R2
BNE DIFF

_16116[616 IOOiiI
_11116162 ~ *
Before Execution.

After Execution.

Imskm

Rl 1t1t1¢sf6!~

)wswl

JM¢ysi9J1

1.i19Jl¢.QJ~f

IJi1¢1YJ¢4 I

* OS=(LOC-UPC)/2

OS=(l~S~8-1f6~48)/2

OS=44e/2
OS=228

3.3.7.6 BRANCH IF PLUS
Branch 1f PLus (1:0) 1oe:

.13.

116

~_:_:_I_:_:_~~:~O:~F~:~F:~'S~:-E~:T~:~1
Plgure 3-53
Description.

Condition Codes.
Example,

Branch if Plus Instruction

Transfers proqram control to the location defined
by the offset II the condition (N bit clear) is
met. If the condition is nat met. control passes
to the next sequential location
Not affected
Test the content of Reqister 4; branch to location
PeS (arbitrarily 2~2_) if the content is positive ~
Locations 2~~~ and 2__4 contain the code
. for TST R4
.
BPL POS

_162'~'
~*
__216162 .~
Before Execution.

After Execution I

R4 11111111

R4 '1111111

PC .J~J62¢9J-"J

PC '161629J9J4(

" OS=(LOC-UPC)/2

OS=(2J62~8-2J6J648)/2

OS=14s/2
08=68

3.3.7.7 BRANCH IF MINUS
Branch 1f ~nu8 (to) loc

116

GJ _:_:_I _:- : - I 1--......:
1

'II

Pigure 3-54
Description.

Condition Codes.
Example.

0.....
: _P_ _
F :

s :_E.-.-:T__
:

Branch if Minus Instruction

Transfers program control to the location defined
by the offset!! the condition (N bit set) is met.
If the condition is not met, control passes to
the next sequential location
Not affected
Test the content of Reqister 4, branch to location
NEG (arbitrarily 2/62/6) if the content is negative
Locations 2~1616 and 216164 contain the code
for TST R4
BMI NEG

_162/61616 ~

_16216162 ~ It

Before Execution.

After Execution.

R4 t11l1111

R41111111)

-oar

I!9J2169J~1

PC J9JPJ291 2

* OS=(LOC-UPC)/2

oS=(2162f6e-29J~4e)/2

OS= 14 e/2
OS=6 e

3.3. 7 ~ 8 BRANCH IF CARRY CLFAR

Branch if Carry Clear (to) lac

8-:-:-1-:

116
1

Pigure 3-55
Description.

Condition Codes.

Example.

Branch if Carry Clear Instruction

Transfers program control to the location defined
by the offset!! the condition (C bit clear) 1s
met.. If the condition 1s not met. control passes
to the next sequential loc:at10n
Not affected
Add the contents of Reqister 3 and Reqister 4,

then check to see 1f a carry resulted. It there
was no carry, branch to CNO (arbitrarily S~')

Locations 5416 and 542 contain the code
for ADD R3,R4
BeC CNO
___5411 ~
__16542 ~*

Before Execution.

After Execution.

R3 .IJtl76543I

R4 1123456)

R4 '16222211

R3 [fJ76543

IM'54,Q!1 .

.. OS=(LOC-DPC)/2
OS=(59JJt's- 544 e)/2
08=-44 8/2

OS=-22 a

08:356 ,
8

t!I1P!544 ~

)

3.3.7.9 BRANCH IF CARRY SET
Branch if Carry Set (to) loc

lSll34xxx

•
Fiqure 3-56

Description.

Condition Codes.
Example.

Branch if Carry Set Instruction

Transfers proqram control to the location defined
by the offset!! the condition (C bit set) 1s met.
If the condition 1s not met, control passes to
the next sequential location
Not affected
Add the contents of Reqister 3 and Register 4,
then check to see if a carry resulted. If there
was a carry, branch to eYES (arbitrarily 5~_)

Locations 54~ and 542 contain the code
for ADD R3,R4
BCS eYES
__~54_ ~
J6Sl1~542 ~

Before Execution.
R3

After Execution a

Ip!76543 I

R3 191765431

R4 11234561

19122221'

PC (p19!9!54~f

I @f19!5~~1

* OS=(LOC-UPC)/2
06=(591918-5448)/2
OS=-448/2
OS=-22 a
OS: 356 e

. 3.3. 7 .111 BRANCH IF OVERFLOW CLEAR

Branch 1f overflow Clear (to) lcc:

116

GJ -:-:- 1- :

1 :-

Fiqure 3-57
Descriptions

Condition Codes.
Example.

9·

1- l~·.:. . :-p:......p.....:S.....:-B.....;T....:~l
o.....

Branch 1f Overflow Clear Instruction

Transfers program control to the location defined
by the offset !l the condition (V bit clear) is
met. If the condition is not met, control passes
to the next sequential location
Not affected
Rotate right the content of Reqister 3, and branch
to location OK (arbitrarily 5~3J6) if there was no
arithmetic overflow
Locations 5~~J6 and S_~2 contain the code
for ROR R3
BVC OK

_165161616 ~
11165__2 ~*
Before Execution.
·C bit

•

After Execution.

C bit

R3 19112345 ~

J9J9J5162~

PC ~J6J65Jt1.PJ~1

19J~59J9J4~

* OS=(LOC-UPC)/2

OS=(SJ63J6e-SJ6J64e)/2

08= 24 e/2
OS=12 8

3.3. 7 .11 BRANCH XF OVERFLOW SET
Branch if oVerflow Set (to) lcc

116

EJ -: -: _ J - :

1 : -

Figure 3-58
Description,

Condition Codes.
Example a

1 1--......:
1

~s:_E...-.:T......:

0.....
: _F:_ _
1' ......
:

Branch 1f Overflow Set Instruction

Transfers prog~am control to the location defined
by the offse~!I the condition (V bit set) is met.
If the condition is not met, control passes to
the next sequential location
Not affected
Rotate right the content of Register 3, and branch
location RESTOR (arbitrarily 5~3_) if there was
arithmetic overflow
Locations 5~~16 and 5~_2'contain the code
for ROR R3
BVS RESTOR

Before Exe~utiona

After Execution a

C bit

R3 J'12345~

Iplf!5162J

It1g15JtUt!~J

J,'fflst3kl

* OS=(LOC-UPC)/2

OS=(5¢3~8-5~~48)/2

OS=24a/2
OS:12a

'.01-,

APPENDIX

Answers to selected exercises
Chapter 2

1•

~[61

Ifl[6 1[616

1611 1161 [611

1.912

11'4

17591

1..

482

512

91/1/1576

2.6.5

2.6.7

911691 ¢[ll [611 1111 [41[6 111.

169JfI ¢¢l ¢11t1 Ilt11 1.f61t1 1611

1•

237773

9115552

[6/11345

1616[6445

2.6.1'
1.

2.6.11
1•

9Ji'f6 161616 16116 161616 lf6f6 11111

2.6.12
1. [Ill f611 111 1911 111 111
2.6.13
1..

9~1

III 11511 JIll 51'1 111

APPENDIX

Answers to selected exercises

3.2.4.1 General Register Addressing Modes
SYMBOLIC
CODB

OCTAL
SOURCE
DESTINATION
CODE . BPncrIVB. ADDRUS EPPBC'l'IVB ADDRESS

(R2)

R:J..

HOV R~,R2
HOV R1,(R2)
HOV a1,(R2).
MOV R1,-(R2)

MOV R1,1911I(R2)

R3.
121··

MOV. R1,.-(R2)

R:l..

R:L·

3.2.4.2 Program Counter Register Addressing Modes

SYMBOLIC
CODE

OCTAL
CODE

SOURCB
DBSTIMTION
BPl'BC'l'IVB:ADDRBSS BPFBCTIYB ADDRESS

(R_>

MOV #123456,R/I

1(,;

HOV "'123456,RJ

3f/4<1;

HOV 123456,Rt'

•

3f/4tIJ

MeV .123456,__

. .
...

mamaama

DIGITAL EQUIPMENT CORPORATION Maynard Massachuselts Telephone ,6171897·5111 • ARIZONA Phoer"llK • CALIFORNIA
5mnyva1e Santa Ana Los Angeles San Doege and San FranciSCO ,Mountam VIeW' • COlORAOO, Engelwood -CQNNECTIQ)T,
Meni1en • DISTRICT OF COLUMBIA, washington Rrverciale, Mel • FLORIDA Orlando. GEORGIA Atlanta. ILLINOIS Northbrook
• INDIANA lnchanapohs. LOUISIANA Metairie. MARYlAND, Riverdale. MASSACHUSETTS Cambndge and Waltham. MICHIGAN
A1V'I Arbor and DetrOIt Southfield • MINNESOTA MinneapolIS. MISSOURI Kansas Cty and Maryland Hetghts • NEW JERSEY
Fa.rloete' MetUChen and Pnncelon • NEW MEXICO AlbuQuerQue. NEW YORK Hunllnglon Slatoon Manhattan, New Yor1o; Syracuse
and !;'Xhe$lef. NORTH CAROliNA. Durham,Chapel HIli. OHIO Oeveland, Dayton and Euclid. OKlAHOMA. TUlsa-OREGON
Pc...-:
• PENNSYLVANIA Bluebell Paoh and Plnso...gh • TENNESSEE KooKv,lIe • TEXAS Dallas and Houston • UTAH Salt Lake
City • Y'oi&.SHIN(;TQN Betlevue • WISCONSIN MIlwaukee • ARGENTINA Buenos AIres. AUSTRAliA Adelaide BrIStlrane Crows
~> NoI'wood Perth and Symey •
AUSTRIA Vtenna • BELGIUM Brussels. BRAZIL RIO de Jane'fO sao Paulo
p.
A f'9fe
•
CANADA Alberla Vancouver British ColumbIa, Hamilton M,sslssauga and Onawa Qnlaoo. and
..etleC • o-tlLE $antl.tgO • DENMARK. Copenhagen and Hellerup • FINlAND HeolSUllu • FRANCE Grenoble and RoogtS •
r
Har'wlovef FrankfUrT MunICh and SIUt1gan • INOlA Bombay • ISRAEL Tel AvIV. ITALY. MIlano • JAP~
• MEXICO MeXICO C,ty • NETHERLANDS The Hague • NEW ZEALAND Auckland. NORMY Oslo.
•
PUERTO RICO. M,ramar and SMturce
• REPUBLIC OF CHINA. TaIwan • SCOTlAND Wesllottllan •
:u .nd Maond •
SWEDEN Solna and Stockholm
• SWITZERLAND Geneva and Zunch • UNITED
gham Sr stot EdInburgh, London, Manchesler ReadIng and WarwIckshIre
•
VENEZUELA. caracas