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DEC-15-YWZA-DN1

April 1970

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Document:	DDT Dynamic Debugging Technique
Order Number:	DEC-15-YWZA-DN1
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Pages:	70
Original Filename:	http://bitsavers.org/pdf/dec/pdp15/DEC-15-YWZA-DN1_DDT_Apr70.pdf

OCR Text

dynamic debugging
technique

digital equipment corporation

DEC-15-YWZ.~-DNI

ADDENDUM #1 TO PDP-l5
UTILITY PROGRAMS f1ANUAL

DDT
DYNAMIC

DEBUGGING

TECHNIQUE,

UTILITY

PROGRAM

NOTE
THIS MANUAL IS A COMPLETE REPLACEMENT FOR THE DDT
SECTION OF THE PDP-15 UTI LI TY PROGRAMS r1ANUAL
PRINTED OCTOBER 1969

FOR ADDITIONAL COPIES, ORDER No.

DEC-15-YWZA-D~1

FROM PROGRAM LIBRARY

DIGITAL EQUIPMENT CORPORATION, MAYNARD, MASSACHUSETTS 01754

PRINTED APRIL 1970

1970

BY DIGITAL EQUIPMENT CORPORATION

THE MATERIAL IN THIS MANUAL IS FOR INFORMATIONAL
PURPOSES AND IS SUBJECT TO CHANGE WITHOUT NOTICE.

THE FOLLOWING ARE TRADEMARKS OF DIGITAL EQUIPMENT CORPORATION
MAYNARD, MASSACHUSETTS:

DEC

PDP

FLIP CHIP

FOCAL

DIGITAL

COMPUTER LAB

PREFACE

Prerequisite
In the preparation of this manual, i t was assumed that the reader is
familiar with the Advanced Software System; e.g., its Monitor and
utility Programs, etc.
PDP-15/20/30/40 ADVANCED MONITOR SOFTWARE SYSTEM MANUAL, DEC-IS-MR2B-D
This manual provides descriptions of system programs including discussions of:

languages, utilities and application, operation , core

organization, and input/output operations within the Monitor environment.
PDP-IS UTILITY PROGRAMS MANUAL, DEC-IS-YWZB-D
The PDP-IS Utility Programs manual is comprised of a set of individual
manuals, each of which describes the operation and use of a PDP-IS
Utility Program.

The manuals which make up the Utility Program set

are listed in the following Application Guide.

In addition, the guide

also indicates the order number of each manual and the specific PDP-IS
Monitor Software Systems in which the program described may be used.
The Utility Manuals may be ordered either individually, by using the
title and order number given with each manual or as a set by referencing JlpDP-lS Utility Programs Manual, DEC-IS-YWZB-D JI •

iii

APPLICATION GUIDE
PDP-15 UTILITY PROGRAM MANUALS

PDP-15 Utility Program Manuals and the Application of Each

Applies to Monitor:

Manual
Title

Order Number
(DEC-15-YWZB-

DDT
Utility Program

DNl

CHAIN & EXECUTE
Utility Program

DN2

SGEN
ADVANCED Monitor

DN3

MTDUMP
Utility Program

DN4

PATCH
Utility Program

DN5

EDIT
utility Program

DN6

UPDATE
Utility Program

DN7

LINKING LOADER

DN8

PIP
ADVANCED Monitor

DN9

PUNCH
Utility Program

DN10

SRCCOM
Utility Program

DNll

SGEN

BASIC I/O

DOS

ADV

B/F

.;
.;

DN12

DOS Monitor
PIP
DOS Monitor

DN13

Disk SAVE/RESTORE
Programs

DN14

CONTENTS

SECTION 1

INTRODUCTION

1.1

General Information

1-1

1.2

Operation

1-1

1.3

Conventions and Special Symbols

1-2

SECTION 2

BASIC DDT

2.1

Loading DDT and User Programs

2.2

Examining Storage Words

2.2.1

Opening a Location

2-2

2.2.2

"Last-Opened-Register Pointer"

2-3

'2.2.3

Closing/Reopening Locations

2-3

2.3

2-1

Type-out Modes

2-3

2.3.1

Address Modes

2-4

2.3.2

Instruction Modes

2-4

2.4

Retype Commands

2-5

2.5

Modifying Storage Words

2-6

2.6

Input Modes

2-6

2.7

Sequencing

2-6

2.8

Breakpoints

2-7

2.8.1

Setting Breakpoints

2-7

2.8.2

Breakpoint Restrictions

2-8

2.8.3

Breakpoint Type-Out

2-8

2.8.4

Reassigning and Removing Breakpoints

2-8
2-8

Proceeding After a Break

2.8.5
2.9

Starting a Program

2-9

2.10

Stopping a Program

2-9

2.11

Errors

2-9

SECTION 3

DDT LANGUAGE AND SYNTAX

3.1

Command Structure

3-1

3.2

Arguments

3-1

3.2.1

Syllables

3-2

3.2.2

The Symbol Table

3-2

3.2.3

Expressions

3-4

3.2.4

Symbolic Instruction Mode
Octal Number Mode
Transfer Vector Mode

3-6

ASCII Text (Output Only)

3-8

3.2.5
3.2.6
3.2.7

SECTION 4

3-7
3-7

DEBUGGING WITH DDT

4.1
4.2

Loading a Program
Starting a Program

4-1
4-1

4.3
4.3.1
4.3.2
4.3.3
4.3.4
4.4

Register Examination and Modification
Type-Out Mode Commands
Special Symbols and Concepts
Register Examination Commands
Expression Retype Commands
Defining a Symbol

4-2
4-2

4.5
4.6

Search Operations
Breakpoints

4.6.1
4.6.2

Definition
Setting Breakpoints
Breakpoint Restrictions
Flow of Control at Breakpoints
What Happens on a Break
The Execute Command
Patch File Input and Output
Mass Storage Dump
Paper Tape Patch Files
Miscellaneous Features

4.6.3
4.6.4
4.6.5
4.6.6
4.7
4.7.1
4.7.2
4.8
4.8.1
4.8.2
4.8.3
4.8.4
4.8.5
4.8.6
4.8.7
4.9

Operate Link and AC
Make Subprogram Current (Header Command)
Initialize Memory
Loading DDT Without a Program

Restarting DDT
Typing Mistakes
Protect Mode Commands
Error Recovery

4-2
4-5
4-6
4-8
4-9
4-11
4-11
4-12
4-14
4-15
4-18
4-20
4"':22
4-22
4-22
4-23
4-23
4-23
4-24
4-24
4-24
4-24
4-24
4-25

C"'" .. ..._ _ _ _ .. .,.

_..s=

nnm

,, _____ ..:J_

vu..i.lULiaJ..:t

U.L

lJlJ.l.

~UHU.llCl.iJ.U~

Expression Operators

rl.L-.L

Al-l

Instruction Mode Commands

Al-l

Address Mode Commands

Al-l

Special Addresses in DDT

Al-l

Special Addresses in User's Program (Not Searched

on Output)

Al-2

Contents of Special User Locations

Al-2

Defining a Symbol

Al-2

Starting a Program

Al-2

Breakpoint Commands

Al-2

Al-3

Word Searches

Al-3

Patch File Commands

Other DDT Commands

Al-4

Error Messages

Al-4

Special DDT Symbols

Al-5

(I/O Monitor System Only)

Al-4

APPENDIX B

Mnemonic Instruction Table

A2-1

APPENDIX C

Patch File Format

A3-1

APPENDIX D

DDT Memory Load Maps

A4-1

Symbols Used in Text and Examples

1-2

TABLES
1-1

vii

SECTION 1
INTRODUCTION
1.1

GENERAL INFORMATION

DDT (Dynamic Debugging Technique) is a conversational system program
which is available in the PDP-9/PDP-15 ADVANCED Software Systems.

provides both MACRO and FORTRAN programmers with a convenient means
for debugging and closely monitoring the operation of their programs.
DDT commands entered via the Teletype{R) permit the user to:

1) start

a program, 2) suspend its execution at predetermined points,
ine the status of memory words, and 4) make additions and corrections
using either symbolic or octal code.

Under most circumstances the

user will be able to stop a "runawayli program.

DDT always resides in

core with the programs to be debugged, and may be considered as being
both a program supervisor and a binary editor.

The type of the input

information required by DDT or output (printed) from DDT
the user to be familiar with machine language programming.
mat of DDT Teletype

requires
The for-

input and output is similar to the format used

by the MACRO assembler.
1.2

OPERATION

When the appropriate request is typed to the Monitor, DDT is loaded
into memory (the top 1600

locations) along with the Linking Loader.
10
Upon command the Loader relocates and loads the user's main program
and subprograms (including symbol table if requested), all requested

user library subroutines, all requested I/O device handlers, and all
requested Fortran Object Time System routines.

After loading has

been accomplished, the Loader transfers control to DDT.
DDT uses the Monitor to communicate with I/O device handlers and to
trap errors during program execution.

While DDT is running, Program

Interrupt and (if available) Automatic Priority Interrupt are enabled.
The user converses with DDT via the Teletype

Teletype I/O is

done almost exclusively one character at a time in Image alphanumeric
mode.

This enables the DDT language to contain simple, concise

commands.
{R)Teletype is a Registered Trade Mark of Teletype Corporation,
Skokie, Illinois.
lIn the Background/Foreground System, DDT operates only in the
Background and communicates with the user via the Background
Control Teletype.
1-1

1.3

CONVENTIONS AND SPECIAL SYMBOLS

Table 1-1 lists special symbols which are used throughout this
manual to represent Teletype Keyboard Operators.
NOTE
In examples simulating TTY entry/response
operations, the character or text to be entered
by the user is underlined (e.g., xxx) to distinguish it from printed output.
--Table 1-1.

Symbols Used in Text and Examples

TEXT
SYMBOL

TELETYPE
ECHO

KEY(S) TO BE
ACTUATED

)

Non-Print

RETURN

Carriage-Return and Line Feed

Non-Print

SPACE Bar

Carriage is advanced one character space

Non-Print

LINE FEED

Platen is advanced to next line

Non-Print

SHIFT and
I/TAB

Moves carriage to next tab location (normally 8 spaces)

SHIFT and
N/

Control entry defined within programs

t plus
a character
(e. g. , t T)

Char

CTRL and
Character
Key
CTRL & T

Initiates a program or system
control operation defined within
the system

ALTMODE

Terminator whose use is defined
within the system program

SHIFT and
K/VT

Use defined within program

SHIFT and

Use defined within program

OPERATION
INITIATED

Character Rubout

SHIFT and
L/FORM

1-2

SECTION 2
BASIC DDT
This section introduces the main features of DDT to the uninitiated
user.

Many programs can be successfully debugged using only the basic

commands described below.

More detailed descriptions of the basic com-

mands and other less frequently used commands are given in later sections.
To simplify "the information presented in this section it is assumed
that only the main program of the user's system is to be tested, even
though subprograms and library programs may also have been loaded and
linked to the main program.

This assumption avoids having to describe,

at this point, how symbols in one program are differentiated from identical symbols in another program.
2.1

LOALING DDT AND USER PROGRAMS

In a basic Monitor (paper tape) environment, the Linking Loader is an
integral part of the DDT tape.
In the ADVANCED Monitor, or Background/Foreground Monitor, the Teletype

command

DDT

(or DDTNS) calls the Linking Loader as well as

DDT

(DDTNS prevents loading of the user symbol table in order to save

memory) .
The first response to the Teletype

, in the I/O and Keyboard Monitor

systems is:
LOADER
>
In the Background/Foreground system the Background Linking Loader
prints:
BGLOAD
>
The user program is then loaded by typing a command string to the
1
Loader.

lRefer to sections 2.1 and 2.2 in the section on the Linking Loader in
the utility Programs Manual (DEC-15-MR3A-D).
Refer to the Background/
Foreground System Manual (DEC-9A-MRZA-D) for a description of the
Background Linking Loader.
2-1

When loading is complete, DDT takes control and types:
DDT
>
to indicate its readiness to accept DDT commands.
NOTE
NS is printed before DDT if the symbol table could

no~ be loaded into available core.

In the ADVANCED Monitor system, .DAT slots -4 (user programs), -5
(user external library, if any), and -1 (system device for loading of
DDT and system library routines) must be assigned to appropriate devices for proper loading.

This applies also to the Background/Fore-

ground system with the exception that the system libraries are accessed from .DAT slot -7, which cannot be altered by the user.

In the

I/O Monitor system, DDT with patch file capabilities utilizes .DAT
slot -10 for patch file input and .DAT slot -6 for patch file output.
2.2

EXAMINING STORAGE WORDS

To examine the contents of a core word location, the user must
"open" the desired location, receive printout of its contents, and,
when finished, "close" the location.
2.2.1

Opening a Location

TO operi a storage word, 'the user must type its address terminated immediately by a slash (/).
EXAMPLE:

To open location ADR+l, type:
ADR+l/

DDT responds to an "open" entry by performing a tab operation, printing the contents of the addressed location and performing a second tab.
EXAMPLE:
Entry

Response

ADR+l
NOTE
The above examples assume that the program's symbol
table is available to the user; if not, the user
must enter the location address in an OCTAL form.
2-2

2.2.2

"Last..".Opened..,-Register Pointer"

Once a location is opened, DDT sets its address into a special pointer
register termed the HLast-Opened-Register-Pointer ii •

The pointer is

represented in DDT by a period (.)i thus when location ADR+l is opened
the pointer (.) is set to its address
2.2.3

(i.e., .=ADR+l).

Closing/Reopening Locations

Opened registers are closed by entering a carriage-return/line-feed
( ~).

EXAMPLE:

-f I.M:L,Tll1P-S -I )

ADR+ 1/

The last-closed register may be reopened at any time by using the
IILast-Opened-Register Pointer"

(.).

The pointer symbol is typed fol-

lowed immediately by a slash (/).
EXAMPLE:

DDT responds to a "reopen" entry by reopening the register whose address is stored in the pointer location and printing its contents.
EXAMPLE:

If ADR+l as given in the preceding examples
was the last opened register, the reopen
procedure would be:

Entry

Response

2.3

~LACL-I TEMP-S-J

TYPE-OUT MODES

The examples of 2.2 show DDT typing out the contents of a register in symbolic form (i.e., a symbolic instruction with an address
field relative to a symbol).
sumed by DDT.
The

symboL~c

This is the type-out mode initially as-

All numeric quantities are printed in octal.
mode is

~seful

if the user expects the opened register to

contain a machine instruction.

However, that register might be inter-"
l
preted instead, as octal data, as a transfer vector , or as symbolic
text.

Additional commands to DDT are available which specify the form

IThe term transfer vector means a word which contains a IS-bit address
pointing to some other register in core.
Transfer vectors are used
with indirect machine instructions because memory reference instructions cannot directly access 32K of memory.
2-3

in which data is to be interpreted and printed.

These mode commands

may be typed whenever control is in DDT and it is waiting for typed
input.
2.3.1

Address Modes

There are three address mode commands:
Typed Symbols

Meaning
Print addresses in symbolic form relative
to a symbol with the closest value, e.g.,
ADR+l.
Print addresses as absolute octal numbers
(IS-bit value) .
Print addresses as octal numbers relative
to the lowest register in the program.
F
signifies "floating" addresses, which correspond to the unrelocated values one sees
in an assembly listing.
These addresses
are printed as a pound sign (#), representing the program's load address, a plus sign
(+), and an octal constant.
For example,
ADR+ IMLA,C

2.3.2

#+ 3 3~

Instruction Modes

Four instruction mode commands specify the form in which the contents
of opened memory words are printed.
Meaning

Command

Print symbolic instructions; if the instruction is a
memory reference instruction, the address field will
be printed according to the current address mode.
Interpret words as transfer vectors, ignoring the
high-order three bits. Print the IS-bit value as
an address in the form dic~ated by the current address mode.

Print words as octal quantities.
mode has no effect.

The current address

Interpret data as packed 5/7 ASCII (lOPS ASCII) text.
When a register is opened for examination, the contents of both that register and the following register are printed as five ASCII characters.
EXAMPLE:

Assume that registers ADR and ADR+l contain, respectively, 206613
and 155102 octal.

2-4

>§Y

> ADR/ ~ LAC TEMP
> ADR/...j LAC #+ 4 2
> ADR/ -+I LAC 6613
> ~~~~·1 ~.;s r ~~MP

> nUL\./

£.UOO..L..)

> ADR/ ~ TEMP
> ADR/ ~ #+42
> ADR/ -+I !XYZ!

-+I

-tI

.-.\

...,

-"I

-.!

Since #+42 is equivalent to the IS-bit value 6613, the relocation factor (#) is 6613-42 = 6551.
2.4

RETYPE

CO~M~~DS

Often, while examininq reqisters in the prevailinq type-out
mode, the user finds data which should be interpreted in a different
mode.

DDT permits the user to request that the data be retyped in

another form, without changing the setting of the current mode.
There are four retype commands which, if no expression is typed
immediately preceding these commands, use as their argument the value
of the last expression typed out by DDT.
Retype
Command

Meaning
Retype the value as an octal number
Retype the value as a symbolic instruction
Retype the value as a transfer vector
Retype the value of the contents of locations
. and .+1 interpreted as a 5/7 ASCII text string.

EXAMPLE:
Assume that register TEMP is absolute location 6613 and XYZ is absolute location 26613.
Entry

Response

>$R)
>$0)
>ADR/

-t 226613

Entry

Response

-I ~LAC* TEMP

-I-=.. XYZ

-I

-'i ~ %XYZ%

or
>ADR/

~2266l3

or
>ADR/

4226613
or

>ADR/-.=OOOOOO

2-5

2.5

MODIFYING STORAGE WORDS

Once a word has been opened, its contents may be changed by typing the desired new contents immediately following the type-out produced by DDT.

A carriage return terminator commands DDT to make the

indicated modification and to "close" the word.
ABC-2/ 4JMP

BEG

~JMP

For example,

BEG+2)

After being closed, register ABC-2 contains JMP BEG+2.

If the regis-

ter had been closed without typing a modification expression, it would
retain its old contents.
NOTE
DDT does not permit the user to modify all core locations
in order to prevent him from inadvertently modifying the
Monitor or DDT itself. Should the user attempt to do so,
DDT will respond by typing a question mark.
When a user types an expression to DDT, as in the preceding example, it should follow the same format as is recognized by the MACRO
assembler.

However, instruction and address fields cannot be separa-

ted by a tab (-f) as allowed in MACRO.
2.6

INPUT MODES

There are no input mode commands.

Type-out modes are necessary ..

since DDT cannot guess which form the user considers most appropriate.
Where DDT expects an address quantity, user-typed expressions, such
as, ADR+3, are evaluated by DDT and truncated to IS-bit values.
wise, they are taken as 18-bit values.

Other-

There is no provision for typ-

ing in data to be interpreted as 5/7 ASCII text.
2.7

SEQUENCING

Once a register has been opened and (optionally) modified, it is
sometimes convenient to be able to open some other register without
having to close the current register and type the address of the new
register followed by a slash.
(a)

DDT contains sequencing commands which

modify the open register if an expression was typed,

that register, and (c) open a new register.

(b) close

A few of these sequencing

commands are listed below:
Command

Meaning
Opens the next sequential register

Opens the preceding register.
Opens the register in the same memory bank or page
as directly addressed by the address partl of the
last expression typed in or out.

IThe indirect and index bits are ignored.
2-6

EXAMPLE:
The following illustrates the usefulness of these commands.

ADR/ -;.J
ADR+l 4
ADR+2 -+j
-+I
TV+l 4
TV
-1

LAC L...J TEMP+ 1
DAC L...J rEMP 2
JMS*L-I TV
CAL * L...J ARG-IO
DACL-INTX
CAL * L..IARG-l 0

Opens registers ADR, ADR+l
and ADR+2

Opens register addressed
~by contents of ADR+2
$Y/
~

-=- SUBRl-l...±..7
t
---.y

Retype the value of the lastopened register as a Transfer
Vector and open the addressed
register
____~Openthe register preceding the
last opened register

2.8

BREAKPOINTS

One useful testing of DDT is its ability to start execution of
the user's program and, if one of several pre-set control points in
the program is reached, to suspend execution and "break" to DDT control.

Such control points are called "breakpoints".

symbolized by the command:
2.8.1

Breakpoints are

$B.

Setting Breakpoints

DDT allows up to four breakpoints which are entered as numbers 1, 2,

3, and 4.
EXAMPLE:

To set a breakpoint at location ADR, the user types
ADR$B (no number assigned)

2iADR$B

(#2 assigned)

When no breakpoint number is given, DDT chooses, if available, a
breakpoint not in use and assigns its number to the entry.
When the command is given to start execution of the user's program,
DDT replaces the contents of the assigned break registers with the
instruction JMS* 17, where register 17 is an auto index register which
contains a return pointer to DDT.

DDT can be made, if necessary, to

use some other autoindex register.

2-7

2.8.2

Breakpoin~_HR~st!ictions

Because breakpoint registers are modified so that control can return
to DDT, they should not be set on registers whose contents are programmodified

or which are used as literals.

The user should refrain

from placing breakpoints on CAL and XCT instructions until the implications discussed in Section 4 are understood.
2.8.3

Breakpoint Type-Out

When a breakpoint is reached during program execution, control is returned to DDT, which a) restores the original contents of all breakpoint registers, b) prints the number of the breakpoint causing the
break, and c) prints the value of the contents of the Accumulator at
the time of the break.
DDT automatically saves and restores the contents of the Link and the
AC when a break occurs and when resuming program execution.

The user

may examine and modify the saved contents of AC and Link by opening
registers A# and L#, respectively, e.g.,
>A#/

>L#/

-l 777777 -I
-t 000001 -I
NOTE

The instruction replaced by the breakpoint instruction
is not executed when the break occurs. It is executed
only when execution proceeds after a break (see 2.8.5).

2.8.4

Reassigning and Removing Breakpoints

Breakpoints may be reassigned or removed as follows:

2.8.5

To reassign a breakpoint, simply type in a new assignment statement using the new address; DDT
automatically deletes the previous assignment.

To remove all breakpoints, type:

To remove a specific breakpoint, for instance,
breakpoint 3, type:

Proceeding After a Break

After a break occurs, program execution may be resumed by typing:

The instruction which is located at the breakpoint is simulated by
DDT (except CAL and XCT) and execution proceeds from the register
following the breakpoint.
2-8

2.9

STARTING A PROGRAM

The command
ADR$G
will cause DDT to give control to the user's program and to GO to
location ADR.

If no argument is specified, the command

$G
will go to the starting address of the program.
2.10

STOPPING A PROGRAM

In spite of the fact that the user may have judiciously set breakpoints in a program, execution may never reach those points if,
for instance, the program enters an infinite loop.

The user may brea~~

out of such a loop and return control to DDT by typing the character CTRL T (iT).
2.11

ERRORS

If a typing mistake is made by the user and DDT has not taken
action, typing either RUBOUT or CTRL U will erase the entire input
and allow it to be retyped.
If control is returned to DDT, causing it to print DDT and version number, an illegal action was specified by the program either
by having a breakpoint in a routine operating with interrupt off or
by obtaining a Monitor error.

If an error of this type occurs, the

user program must be restarted from the beginning.
If a user enters an undefined symbol as part of a command string,
DDT will type the letter U and will ignore the entire string.

When-

ever the user attempts to perform an illegal command, DDT responds by
typing a question mark (?).

2.,..9

SECTION 3
DDT LANGUAGE AND SYNTAX

This section describes the rules governing the formation of DDT commands and of expressions, which are used as arguments for these commands.
3.1

CO!-!HAND STRUCTURE

DDT commands take none, one, or two arguments, depending on the
specific command.

When two arguments are required, they are

separated by a semicolon or by an open (left) parenthesis.
mand's arguments always precede the command characters.

A com-

The command

may be either a single control character (such as i, =, or I), or the
character ALTMODE (echoed as $) followed by another character, such
as, $A, $T, or ~.
The following examples illustrate the forms which DDT commands may
take:

Isingle argument commands

Ino argument
$B

Iboth args missing [two required for B]

arg$B

larg 1 missing

argi$B

larg 2 missing

argi arg$B

Iboth args

Note that the $B command is shown four times.
arguments.

It requires two

In the absence of either or both arguments, DDT supplies

a predefined value as a default argument for each missing argument.
The default arguments used depend on the specific command which they
accompany.
If DDT cannot recognize a command string, it will type a ? and ignore
the string.
Appendix A contains a detailed list of the commands which are
recognized by DDT.
3.2

ARGUMENTS

Arguments to DDT commands are, in general, symbolic expressions
which consist of syllables (symbols or numbers) separated by operators.

3-1

3.2.1

Syllables

A syllable may consist of one to six characters from the radix 50
octal character set:
a)
b)
c)
d)

A thru Z
0 thru 9
period (.)
% and #

The special characters single and double quote (' and ") may occur
anywhere within a symbol.
on.

Their significance is explained farther

Syllables are delimited by any non-radix-50 character.

A symbol must contain at least one non-octal character; otherwise,
it is taken as an octal number instead of a symbol.

Since DDT inter-

prets all numeric input, and outputs all numeric output, in octal
radix, the digits 8 and 9 are treated as an extension of the alphabet.

In a symbol of more than six characters, all characters beyond

the sixth are ignored.

In a number of more than six octal digits,

the last six digits are retained and those digits preceding the last
six are discarded.
The following illustrates symbols and numbers:
ITEM

3.2.2

TYPE

INTERPRETATION

A symbol

8 is not an octal digit.

%1.#

A symbol

• % and # are radix 50 characters.

A symbol

Symbols need not begin with a letter.

123456A

A symbol

Although the A is discarded because
it is the 7th-character, its presence
declares the character string to be
a symbol.

123456Z

A symbol

Same symbol as 123456A since only
the first six characters are retained.

A number

Same as 000007.

1234567

A number

Same as 234567 since the 1 is discarded.

The Symbol Table

In order to evaluate symbols within an expression, DDT must find the
symbol and its definition in a symbol table (see memory map in Appendix 4).

This symbol table has two parts:

one part contains

the definitions of standard machine instruction mnemonics (e.g.,
LAC=200000, ADD=300000), the definitions of special DDT symbols
3-2

Ce.g., A# [the saved accumulator] or L# [the saved Link]) I and the
definitions of symbols created by the user in commands to DDT.

This

part of the symbol table resides in the area of core occupied by DDT.
The second part of the symbol table resides in lower core and is
built there by the Linking Loader.

It consists of several groups of

internal symbol definitions, one group for each user subprogram loaded.
A "header" at the beginning of each group gives the subprogram's file
name and its load address (relocation factor).

(If the command $DDTNS

is given to the Monitor, the Loader will store the headers in the
symbol table, but not the internal symbols.

This is done to save

core space.)
Subprograms may have internal symbols which are also used as internal
symbols in other subprograms.

Thus, the same symbol may appear sev-

eral times in the DDT symbol table, each time with a different value.
The following explains how DDT, in the face of multiple-symbol definitions, decides which value to assign to a symbol when it appears in
a user-typed expression.
Normally, DDT searches the entire symbol table for the symbol and its
18-bit value and takes the first match it finds.
bol table in the following order:
(2) the special DDT symbols,
time,

(1)

DDT scans the sym-

the instruction mnemonics,

(3) the user symbols defined at run-

(4) the symbols for the "current subprogram", and (5) the sym-

bols in all other subprograms.
Initially, the current subprogram is defined as the main program (the
first to be loaded - the first to appear in the Loader's command
string).

The concept is that one normally debugs one subprogram at

a time; therefore, it is natural that when the user types a symbol
SYM, he is referring to SYM as defined in the subprogram on which he
is currently working and not as defined in other subprograms.

The

user can, by using the header command ($H), define any subprogram as
being "current" thus controlling the manner in which a symbol is defined by DDT.
The special character single quote (I), which may appear anywhere
within a symbol, declares the symbol to be a file name.

The value

of such a flagged symbol is the relocation factor for the subprogram
with that file name.

In such a case, DDT only searches for a header

with a matching symbol.
The special character double quote (H), which may appear anywhere
within a symbol, declares that symbol to be an address tag.

Speci-

fically, it declares the symbol not to be an instruction; thus, DDT
3-3

will bypass the instruction mnemonic table when searching for the value
of such a symbol. This provision is made because it is legal, in the
MACRO assembly language, to define address tags which have the same
mnemonics as instructions, e.g., JMP JMP, where the second JM~ is an
address. So, for example, if the user wishes to examine a register
labelled JMP, he types the following to DDT:

Both' and
are flags (not to be confused as bei~g symbol constituents) much as the character # in MACRO may be located anywhere within
II

a symbol to declare it to be a variable. Within an expression, any
symbol following a space is treated as if it contained the double
quote (") flag, that is, treated as an address tag. Thus, for example, in
JMP

3+ADR

the search for symbol ADR bypasses the instruction- mnemonic table.
The special characters ' and
octal numbers.

are ignored when they appear within

NOTE
Symbols defined in a MACRO program by the use of
(as in SYM=IOO) are not passed on to DDT in the
user's symbol table.
The user should be aware that FORTRAN IV passes local symbols to DDT
which are identical to global symbols. In each case these local addresses contain transfer vectors which are equal in value to the corresponding global symbol. These transfer vectors exist as long lists
toward the end of FORTRAN programs and are used in the same way that
MACRO uses external global transfer vectors.
The preceding text discussed evaluation of symbols typed by the user.
When DDT prints out expressions (for example, the contents of an
opened register)~it must perform the inverse process of taking a
numeric value and converting it into symbolic form, usually involving a symbol table search. The output form is determined by the prevailing type-out modes and is discussed below.
. 3.2.3

Expressions

Expressions consist of one or more syllables separated by operators
and terminated by a character which is neither a legal syllable constituent nor an operator. When calculating the value of a user-typed
3-4

expression, DDT evaluates the expression from left to right, combining values according to the intervening operators.

ASCII text, which

can be output by DDT, cannot be input by the user and is, therefore,
not considered as a form of input expression.

An expression may, of

course, consist of only one syllable.
DDT assumes, when evaluating an input expression, that the expression
is preceded by 0+.
two~argument

in a

This implies that the value of the first argument
command is zero when the argument separator is

typed with no preceding argument, e.g.,
;ADR$B is equivalent to OiADR$B

. If the argument separator is missing, DDT supplies a default argument.

The following operators define the ways syllable values may be combined.
OPERATOR

MEANING
Add the two values in two's complement arithmetic
(overflow is ignored)
Add the two values in two's complement; but, from
now until the end of the expression, do not change
the instruction part of the accumulated value (bits
o through 5) •

Add the two values in two's complement and "exclusive or" the result with 20000 (the indirect bit).
(Overflow is ignored).
Negate the following value if this is a unary minus.
Otherwise, subtract the following value from the
preceding value in two's complement.
(Overflow is
ignored. )

Form the "AND" of the two values bit by corresponding bit.
Form the "exclusive or" of the two values bit by
corresponding bit.
Form the '~nclusive or" of the two values bit by
corresponding bit.

The following subheadings describe input and output expression according to the various type-out modes.

Note, however, that the pre-

vailing type-out modes have no bearing on user-typed expressions.

3-5

3.2.4

SX~olic

Instruction Mode

To obtain the relocated value of any instruction in the current program as loaded by DDT, the user may type an expression identical in
most respects with the original MACRO instruction.

If all the symbols

are defined in the original program, new instructions may be formed.
Tab, however, may not be used to separate OP code from address field
(space is used exclusively).

The following are legal instructions,

provided that the addresses are defined:
1.

JMPL..,IJMP

(JMP is an address in the current program)

DAC....,l+AD

(Although AD evaluates to 15 bits, only 13
bits are used because of the space)

CLA!CLL

EAE+1002

(DDT does not contain EAE mnemonics)

IOT+314

(DDT does not contain lOT mnemonics)

IAWL..,I-l

(Same as LAW 17777)

XCT*L..,IAD

LAC L..,I C,X

CAL ..... 77S

10.

AASL..,I,...l

(PDP-IS only; indexed instruction)

(PDP-IS only; once AAS, AAC, AXS, or AXR is encountered, the remainder of the expression will not
alter the value of bits 0 through 8.)

On output if DDT is in symbolic mode, the following procedure is used
to output an expression representing an 18 bit computer word:

the

instructions are broken down into categories appropriate to their OP
codes.
1.

If the OP code (bits 0-3) is 00 (CAL), the instruction is
output as an octal number with leading 0 suppression (e.g.,
77S) •

If the OP code is 74 and the indirect bit is off, the instruction is output as an inclusive ORed microcoded operate
instruction (740000 types out as NOP, 7S4000 as CLA!CLL).

If the OP code is 74 and the indirect bit is on, the instruction is output as LAW L-rN, wher~ N is an octal' number representing ·the two's complement of the instruction (e.g., LAW~-l
for 777777).

If the OP code is 64, the instruction is output as EAE+N,
where N is an octal number representing bits 4-17 of the
instruction (e.g., EAE + 1002 for LACQ).
If the DDT symbol table contains an exact match, that match will be output.

SA.

On a PDP-IS, the OP code 70 must be broken down further
(an
exact match in the DDT symbol table will be output as in 4).
If the instruction is an AAS, AAC, AXS, or AXR instruction,
the corresponding mnemonic followed by space would be output,
followed by bits 9-17 as a signed octal number.
Otherwise,
the number would be output as IOT+N, where N is an octal
3-6

number representing bits 4~17 of the instruction,
IOT+314 for IORS and AAS~l for 720001) •

(e.g.,

SB.

If the computer is not a PDP-IS, the OP code 70 is output
(if no exact match is found) as IOT+N.

The remaining OP codes are memory reference instructions.
The corresponding instruction mnemonic is output.
If the
indirect bit is set, a * is output following the mnemonic.
Then a space is typed, followed by the 15 bit address referred
to directly by the instruction, typed in the current address mode. The bank bits for the address are the same as
the bank bits of IIpointll (the address of the last opened
register), unless an auto-index register is referred to indirectly (e.g., LAC*~lO). On a PDP-IS, the index bit is
checked.
If the index bit is set, the address is followed
by IX.
The following are exa~ples of memory reference instructions printed by DDT:
1.

JMP~JMP

(JMP is an address in the current program)

DA~D+l

(Relative address mode)

XCT*~AD

LAC~,X

(PDP-IS only)

DAC~2l2S3

(Last opened register is in bank 1; absolute address mode; same as 041253)

DAC L...J# + 2

(Floating address mode; address relative
to beginning of current program [#])

3.2.5

Octal Number Mode

The conventions for numerical input were explained in 3.2.1.

output, octal numbers without leading-zero suppression are used
to represent instructions in octal mode,
3.2.6

(e.g., 060010 for

DAC*~O).

Transfer Vector Mode

In this mode, bits 0-2 of the word to be output are ignored.

the address mode is· relative, the symbol table is searched according to the current header for the address which is nearest in value
to the value of bits 3-17 of the word to be output.

If its value

is within 100 octal of the word to be output, this address symbol
is typed followed by either + or - and the difference as an octal
number.

Since the same symbol in the symbol table may represent

more than one address, care must be taken on input of addresses.
Since transfer vectors in FORTRAN are defined locally and globally
with the same symbol, the following seemingly legal attempt to
restore the value of a location fails:

The contents of local symbol TAN now contains, not the global address of the beginning of the tangent routine, but the address of
the transfer vector TAN (its own address).

On input, local symbols

of the current subprogram always take precedence over globals.
3-7

In absolute address model the 15 bit address is output in octal.
In floating address mode, the address is output as relative to the
symbol #, which represents the first address of the subprogram
loaded into the area of memory containing "point"
register).

(the last opened

In symbolic address mode, the address may be printed

as in floating mode, if a symbol within 100 octal of the address is
not found in the symbol table.

Likewise, an address to be printed

in floating mode will be printed in absolute mode, if it is less
than the current value of #.
3.2.7

ASCII Text (Output Only)

The locations

and .+1 are interpreted as ASCII 5/7 packed data

if typed in this mode (e.g., ABCDE is output for 406050, 342212).

3-8

SECTION 4
DEBUGGING WITH DDT

All DDT commands and features are described in this section.

Refer-

ences to earlier sections are made to avoid redundancy.
4.1

LOADING A PROGRAM

See SECTION 2.1.
4.2

STARTING A PROGRAM

When the Linking Loader has loaded DDT and the user's programs and
gives control to DDT, DDT sets the value of the special symbol SA#
(start address) equal to the starting address of the main program.
To command DDT to start the

user~s

program at a given location,

type:

where K is an expression whose IS-bit value is the address to which
DDT will Go.

In the absence of !, the default argument SA# (start

address) is used.

To first change the value of SA# and then go

there, type:

If K is missing,

(i.e., i$G), SA# will not change.

enter his program any place between P#' and C#.
4.3.2E.)

The user may

(See Section

Any other address will be rejected and DDT will type a

question mark.
The following two points should be remembered when reading the subsequent section on the

~roceed

count" and "proceeding after break-

point":
1.
2.

The $G command always sets the proceed count to 1.
The $p (proceed) command will also start the program
when~here has been no breakpoint break from which to

proceed.
If a started program seems to be malfunctioning and the user wants
to go back to DDT, then typing tT (CTRL T) will cause the Monitor
to give control to DDT, which will give the go ahead signal (».

4-1

4.3

4.3.1

Type-Out Mode Commands

The type-out mode commands (see Section 2.3 and also Appendix 1, sections B and C) dictate the form in which DDT will print instructions
and addresses, unless specifically overridden by the command being
executed.

When first loaded, DDT assumes $S (symbolic instruction

mode) and $R (relative symbolic address mode).

The commands to set

the type-out modes do not take arguments; if arguments are supplied,
they will be ignored.

If a supplied argument contains an undefined

symbol, DDT will type:

-1 >

to indicate that the preceding symbol is undefined and that the command was ignored.

The undefined symbol is encountered, in this case,

before DDT has a chance to determine that the command requires no
argument.

This is because DDT does not buffer the command text but,

instead, interprets it as it comes in, character by character.

This

pertains to all command input to DDT.
When the type-out mode command has been accepted and executed, DDT
will respond by performing a carriage-return, line-feed and typing:
>
Whenever DDT is expecting command input, the type-out modes may be
changed without affecting other DDT operations, e.g., an open register remains open.
4.3.2

Special Symbols and Concepts,

The single character "point" (.) usually represents the
address of the last register to have been opened.
"Point"
is often used as a default arglli~ent to DDT commands.
Under
the following conditions, the value of "point" will be modified as indicated:
'
1.

Immediately after a breakpoint is reached and control
is returned to DDT, the value of "point" will be the
address of the register whose contents are typed out
after the break (a feature to be discussed later).

After tT (CTRL T) has been typed and control has returned
to DDT-,-the value of "point" is the address of the instruction that waS to have been executed., The user may
determine at what location program execution was suspended simply by typing:
or

4-2

which are two forms of retype commands.
The user may
restart the program at the point where it was interrupted by typing:

provided that the program was not interrupted while
control was in the Monitor.
Device handlers are not
loaded with the user program units or DDT.
3.

When DDT is started or restarted it prints:
DDT
>

At this time the value of "point" is set from the
value of the starting address, SA#, which is initially the starting address of the main program.
4.

After one of the search commands (discussed below)
has been executed, the value of "point" is set to
the address of the last register typed out during
the search.
If no register was typed out, the
value of "point" is not changed.

The "current location", which is represented by the symbol.
in MACRO programs, differs at times from the value of "point"
used by DDT. The current location does not have a special
symbol associated with it. When a register is opened for
examination by any command, except $Z, the current location
and the value of IIpoint" are the address just opened. The
values of "point" and the current location will differ when
the sequencing command ~ is used (see below).

The areas of core occupied by each user subprogram, including
the main program, are defined by the "header" file names
which appear in the symbol table built by the Loader and
which contain the relocation factors (load addresses) of each
subprogram. The free core space which exists immediately
above the Loader-built symbol table (see memory map in Appendix 4) is treated as a pseudo-program to which DDT has
assigned the file name P#' (recall that the' declares the
symbol to be a file name). This symbol represents the first
location (relocation factor) of the patch file area (free
core). This core area is used to insert additional code at
run-time.

The special symbol # represents the first location of the subprogram which occupies the area of memory containing the address "point". The one exception, which does not change the
value of #, is when "point" has been modified following the
execution of a search command. The sub-program containing
the address "point" will be referred to as the current subprogram. The implications of this are best explained by example, using some of the basic commands described in Section 2.
Each line in the example is numbered and the comments are keyed
to these line numbers:

4-3

Example
;>
1
2
3

4
5

6
7
8
9

>$F)
>PRGA ' =003000
~-J
>PRGB ' =004000
- ~ ~
>PRGB II -.tLAC
#+100-1 #=004000
> .=004000~
BEGIN
-I
>PRGA 1/ -I DAC
#+3 ~: #=003000
>. =003000-t
• : START
>BEGIN/ -I, LAC #+100
#=004000
>. =004000-t

T
7

In line 1 the command sets the address mode to floating (#+
number) .
In line 2, the value of the file name PRGA ' is 3000 (program PRGA starts at location 3000).
In line 3, the value of PRGB ' is 4000 (program PRGB
starts at location 4000)-.---On line 4, the register whose address is the same as the
value of PRGB ' (register 4000) is opened and its contents
are printed as a symbolic instruction with a floating address (#=100).
The value of # and. at this point is 4000,
which means that the current program is PRGB and that its
relocation factor (#) is 4000. Therefor~he LAC instruction, if executed, would load the accumulator with the
contents of location 4100.
On line 5, . is retyped as the symbol BEGIN. Therefore,
PRGB has a local symbol called BEGIN which refers to the
first location in that program. Assume for the moment that
PRGA and PRGB do not use any identical symbols.
On line 6, PRGA ' is opened. This changes the value of .
to the va 1 ue-or-PRGAI and makes PRGA ' the current program.
Since . is now located in PRGA and no longer in PRGB, the
value of # is changed to 3000, the relocation factor of
PRGA.
On line 7, • is retyped as the symbol START, a local symbol in PRGA:On line 8, the register BEGIN is opened. If BEGIN had
been defined locally in PRGA, the value of # would not
have changed (since . would still have been located in
PRGA). Since BEGIN Is located in PRGB, PRGB is made the
current program and! is changed accordingly.
When searching the symbol table for the value of a symbol,
DDT searches the local symbols of the current program
before those of other programs and uses the first match
it finds.

4-4

DDT will now allow core outside a specified range to
be modified by command l . The area that may be modified includes the auto-index registers and the core area
between the symbols Pi' and C#. Pi' is the address of
the lowest register in the patch area.
C# is an address
within DDT which contains the two's complement value of
the "proceed count ll •
Immediately below C# are other
special DDT addresses, such as, A# (the address of the
saved AC). All the special DDT addresses preceding
and including C# may be modified by command to DDT.
The core between the patch area and these special DDT
registers is where the user's programs are loaded. A
breakpoint may be placed anywhere in a user program or
in the patch area, but it may not be placed at an autoindex register nor wi thin DDT. When operating in t,he
Background/Foreground system,: the user must be aware that
the patch area may start below the hardware memory protect bound (whose value is stored in .SCOM+32). Attempting to start a program below the bound will result in
.ERR~32 (Memory Protect Violation).

If a register is opened which is below the patch area,
the current program will not change since no file name
exists for registers below Pi'. The user may, however,
define a file name with a value less than pi' for the
purpose of examining lower core.

The special symbol Q#, which is often used as a default
argument to DDT commands, assumes the value of the contents of the most recently opened register or the value
of the argument to the last intervening retype command.
Example:
>LOC/ ~

>.1

123456
123457

123456

Register LOC is opened and contains 123456. Note that
at this point the value of Q# is 123456. Then the contents of register LOC is increased by +1, utilizing Q#
to represent its current contents.
4.3.3 ,Register Examinatjon Commands
The basic register examination command is

Typing the command K/,

where! is an expression, will open register K and print its contents
in the current modes.

The default argument is IIpoint"

(i.e.,

L is

equivalent to ~.
If L is replaced by 1, printout is temporarily forced into octal
mode.

(If 1 is preceded by ALTMODE

(~),

then octal becomes the

permanent instruction mode.)
If

L is replaced by 1, printout is temporarily forced into symbolic

mode.

(If 1 is preceded by ALTMODE, then symbolic becomes the per-

manent mode.)
lSee section 4.8.7.
4 .... 5

L is replaced by ~; printing of register contents is omitted

until the next occurrence of a carriage return command.

(This mode,

called "type-in mode'! ,allows for rapid code insertion.)
If any register examination command is followed by?), an attempt was
made to open a non-existent register.
Examples:
JMP* SUBR
623115
JMP* SUBR
623115
623115
JMP* SUBR
..r

>ADR-l/
>. [

>./
>. $ [

>./
>• ]
>. <

ADR
ADR+l

4.3.4

(print in current modes)
(force octal)
(modes haven't changed)
(permanent mode change)
(mode now octal)
(force symbolic)
(contents are not printed out until
encountered)

Expression Retype Commands

Alternative representations of the contents of a register or of an
expression may be obtained by using retype commands (see Section 2.4).
Where K is an expression,
K=

means retype K in octal

K+

means retype K in symbolic

means retype K as a transfer vector

means re type the contents of "point" and the following register as 5/7 ASCII text (~ unused) .

In each case above, the value of Q# is set to the value of K.

The

default argument, if K is missing, is the current value of Q#.
Retype commands force a temporary mode change; they do not alter the
setting of the type-out modes.
4.3.5

The basic register closing and modifying command is carriage return
( ).

If K) is typed when a register is open, then the value of K

(expression) becomes the contents of the open register and the register is closed

(~becomes

the value of Q#).

ister is closed without modification.

If ~ is omitted, the reg-

If no register is open, none

is modified

and K becomes the value of Q#.

will type >.

The command ~also turns off type-in mode, the tempo-

4-6

Upon completion, DDT

rary mode which allows the user to insert code without having to wait
for DDT to print the contents of each register.
In addition to
)

)

there are "sequencing corrunands!!, which behave like

in that, if given an argument K, K replaces the old contents of

the open register (assuming one is open) and then that register is
closed.

Then a new register is opened.

For every corrunand listed be-

low, with the exception of ~, the newly opened register becomes the
current register. The significance of this will be explained in the
example.

K+

(Line Feed). Modify the open register, close it
and then open the next register in sequence.

Modify the open register, close it and then open
the register preceding the one just closed.
Modify the open register, close it and then open
the register whose address is contained in the address part of K. K is assumed to be a memory
reference instructTon. The address part (12 or
13 bits, PDP-1S or PDP-9, respectively) is taken,
and that address (in the same memory bank as the
now closed register) is opened. If K is missing
from the command, the address part of the contents
of the closed register is taken. In other words,
the argument to $Y is always Q#.
This command is identical to $Y with the exception
that the newly opened register-is not made current
(the current register and the value of . now differ) .
$Z is useful for examining literals without breaking
program sequence; that is, subsequent use of + or t
will refer to the current register, not the one
opened by ~.
This command is similar to $Y.. It modifies the open
register, closes it and then, treating-K as a transfer vector, opens that lS-bit address. -

If DDT types a question mark after any of these commands, either the
command was not properly received (unlikely) or an attempt was made
to alter a location outside the allowable range (autoindex registers
and P#' through C#)l.

In either case, the command is ignored.

Example
>LOC/
LOC+l
LOC+ 2

~ LAW - 1

-I LAC LIT12$Z
12
-I +
DAC TMP
~~
JMS * TV
-l $Y
DAC ARG-7~ -: SUBRl -+f $J

.:.t 0

-IT

SUBR1+1....( CLL!CLA -t T
SUBR1+~ SAD XYZ-2·-~ t

>S,(JBR1+l~ CLL1CLA

-I ~TL1CLA )

lsee Section 4.8.7.
4-7

Note that the addresses opened by the cowmands $Y, $Z and $J are not
printed by DDT.

Note that $Y after JMS* TV opened register TV and

printed its contents as if it were an instruction.

This did not look

right, so _ was typed to request reprint as a transfer vector.
The use of $Y or ~ following indexed and indirect instructions, such
as,
LAC* ..... TAB,X$Y
will not perform the indexing nor indirection to determine the address
of the next register to be examined.

In this example, register TAB

would be opened.
If the user wishes to effectively

i~sert

code in his program, he must,

in general, modify two registers in his program:

one to JMP* through

a transfer vector and the other to the transfer vector pointing to
patch space in free core (which might not be in the same core bank as
the user's program).

Patch space begins at the register P#' and goes

as high as the address stored in register 103 (.SCOM+3).

The user should

be aware that making patches in locations higher than the value in
103 will overlay the user's program.
Besides the method of opening and modifying registers there are
special instructions available for initializing memory between limits
to a constant and for updating the values of special DDT registers.
These commands will be discussed under later headings.
4.4

DEFINING A SYMBOL

In addition to the symbols passed on to DDT by the Linking Loader, the
user has the option of defining additional symbols at run time.

These

"DDT-time" symbols are added to the end of DDT's symbol table (see
Appendix 4).
The symbol S, which must be a unique symbol, is given
the value of the expression E.
(~) is equivalent to

o (S) •

The symbol S is given the value of "point".
Only these "DDT-time" symbols may be redefined.

Symbols may be

deleted as follows:
(no argument) will !ill (delete) all DDT-time symbols.
will Kill all the user's load-time symbols.
This would
only be done, presumably, to increase the available
patch space (P#' will be suitably redefined).

4-8

If the user attempts to define any symbol which is not a DDT-time
symbol, DDT will ignore the command and type an X.

If he attempts

to define a symbol when the DDT-time symbol table is full, DDT will
ignore the cOIT~and and type an 0

4.5

(Overflow).

SEARCH OPERATIONS

There are three search commands, each of which searches inclusively
between a lower core limit and an upper core limit for words or parts
of words which have or do not have a specified value.

The search com-

mands use as arguments the values in three special DDT registers:
M#

search mask

LO#

lower limit search address

HI#

upper limit search address

Each of these registers may
>M#/
LO#
R#
>

--t
~

be individually modified, e.g.,

777777
012252
025611

770000+
14000 +
14400 )

Notice that these registers appear sequentially in memory.

(The

symbol R# is equal to HI# and will be printed instead of HI#.)
The contents of LO# and HI# may be modified directly by command:

where A and B are expressions.
the value in HI# to B.

This sets the value in LO# to A and

The default arguments are P#'

(beginning of

free core patch area) and L# (the address of the saved value of the
Link within DDT).

A# (the saved AC) is the first register in DDT

and L# is the second.

LO# is initially set to P#' and HI# to L#.

The initial contents of the mask, M#, is 777777 '(all ones).
The first command is the Word Search:

searches through every core register between the limits set in registers LO# and HI#, inclusively, for words whose values match the
value of ~ (an expression) in those bit positions specified by lIs
in the corresponding bits in the mask argument M.

4-9

For

exa~ple,

to search for all words

locations

, ""

.J..VV

')""

&.VV,

equal in value to JMP BEG, type:
100i 2 00$L

777777iJMP BEG$W
Of course, if LO# already contains 100, HI# contains 200, and M# contains 777777, the user need only type:
JMP BEG$W
Specifying a mask in the $W command does not cause the contents of M#
to be modified.
To search for all words between locations 100 and 200 whose Op code
parts contain JMP, type:
100i 2 00$L
740000iJMP$W
Note that the mask is set so that only the high order 4 bits (the
Op-code bits) are tested for a match.
To print out the contents of registers 100 through 120, type:
100i120$L
Oi$W
Zero is used as a mask so that there is no possibility of a mismatch.
The second argument is immaterial and was, therefore, not "given.
The default arguments for MiK$W are the contents of M# (if M missing)
and 0 (if K missing).
The second command is the Not Word Search:

It is identical to $W except that the search is for words which
differ from rather than match K in the masked bit position

For

example, to print all registers between locations 100 and 200 whose
contents are non-O, type:
100i200$L

4-10

(Note that 0 is the default value of K.

It is assumed, in this ex-

ample, that M# contains 777777).
The third command is the Effective Address Search:

The default arguments for M and K are, as above, the contents of M#
and 0, respectively.

$E searches core between the limits specified

by LO# and HI# for all memory reference instructions which directly
or indirectly reference the address

(an expression), testing for a

match only in those bit positions specified by lis in the mask.

(Bits

0-2 are disregarded.)
The following example gives a good indication of the usefulness of
this command.

It is desired to know all the registers between loca-

tions 1000 and 2000 which reference auto-index registers:
1000i2000$L
777770ilO$E
To search for all references to ADR+l, provided that the mask and
limits contain the desired values, one need only type:
ADR+l$E
Since a PDP-15 has a hardware index register (whose value is saved in
and restored from XR#) , an effective address search which encounters
indexed instructions will use the value in XR# as the index.
During any of the three search operations whenever a condition is met
(match or no match), DDT prints the octal address of the memory word
which satisfies the search constraints, prints a tab, prints the contents of that memory location in the prevailing type-out modes and
types a carriage return.
limit is exceeded.

The search then continues until the upper

At the end of the search, the value of "point" is

set to the address of the last register printed in the search map
(the current register remains unchanged).

"Point" remains unchanged

if no printout occurred.
4.6
4.6.1

BREAKPOINTS
Definition

A "breakpoint" is a pre-selected point in a program where the flow of
the program is broken to allow the user to perform DDT functions.
4-11

Whenever it is about to give control to the user's program, DDT saves
the instruction at each breakpoint and replaces it with a JMS* 17 inl
struction.
DDT also stores a return pointer to itself in auto-index
register 17.

Thus, whenever a breakpoint is reached, control is trans-

ferred to DDT to allow the user. to examine and alter registers, search,
etc.

When the user signals DDT to continue execution of his program

($P), the instruction that was originally at the breakpoint location
is simulated and then DDT transfers control to the register following
the breakpoint.
Needless to say, the user's program must not modify the auto-index
register that DDT uses for breakpoint returns.
Up to four breakpoints may be set to facilitate debugging when there
is uncertainty as to which path a program will follow.
4.6.2

Setting Breakpoints

Initially, all four breakpoints are cleared (unassigned).

The

general form of the breakpoint command is:

where N is a breakpoint number (1 to 4) and ~ is an expression evaluated to a IS-bit address.
N

This causes DDT to assign (set) breakpoint

at location A (provided that the value of A is non-Oi see below) .

For example, to set breakpoint 2 at location ADR, type:
2 iADR$B

It is possible to reset a breakpoint to some other address without
first deassigning that breakpoint, e.g., if breakpoint 2 had been set
at location XYZ, the preceding command would supersede the earlier
assignment.
If the argument A is missing, the default argument is "point".

For

example:

This sets breakpoint ~ at location ADR because "point" has the value
of register ADR.

(3i$B is equivalent to 3i.$B~)

lA command exists to tell DDT to use some other auto-index register
for breakpoints (discussed later).
4-12

If the user does not care which breakpoint number is used when assigning a breakpoint, he can, simply by leaving out the first argument ~,
request DDT to assign an unused breakpoint number.
suming all four breakpoints

For instance, as-

to set a breakpoint at

location ~, type:
X$B

DDT, finding breakpoint 1 unused, then types a 1 to indicate which
number it selected, i.e.,

If all four breakpoints are already in use; DDT types:

and ignores the command.
If register X is outside the legal range of registers which may have
breakpoints (P#' to C#) ,1

DDT types:

X$Bl?

indicating breakpoint 1 is free but X is illegal, and then ignores
the command.
In addition to assigning and reassigning breakpoints, the user may
remove (clear) them.

This is signified by a value of 0 for the argu-

ment A.
The input

removes breakpoint N (1 to 4).

If N is absent, the input

removes all breakpoints.
When a breakpoint is set at a given location, the contents of that
location are not changed, e.g.,
>X/
>X/

LAC

TMP
TMP

The swapping of the contents of X with a JMS* 17 occurs only when
DDT gives control to the user program.

See Section 4.8.7.

4-13

4.6.3

Breakpoint Restrictions

It has already been mentioned that DDT will not allow breakpoints to
be set at locations outside the range P#' to C#.l
The user must not set a breakpoint at locations containing:
a)

instructions which are program modified

instructions which are used as constants
(operands of other instructions).

This is because the actual instruction in a breakpoint location' is
changed by DDT, prior to program execution, to a JMS* 17 (or some
other autoindex register).
If a breakpoint is set on a CAL or XCT instruction and a break at
such a location occurs, DDT remembers this fact.

If the command is

then given to proceed with program execution where it left off, the
breakpoint is removed and replaced by the original CAL or XCT.

This

is done because DDT cannot simulate the CAL (which has a variable
number of arguments following it) out of place.

The XCT cannot be

simulated out of place since XCT can execute a CAL instruction.

How-

ever, should control return to DDT via some other breakpoint, the
breakpoint on the CAL or XCT will then be reinstated.
Should the user wish to place a breakpoint on a CAL instruction, the
following practice will always ensure that the breakpoint remains set
on the CAL.
Example:
CAL 3
12
LAC BUFF

(set breakpoint 1)
(set breakpoint 2)

The CAL used above is the macro expansion of the'Monitor call:
.WAIT 3
Placing a second breakpoint on the register immediately following the
CAL and its arguments (the return point)
at the CAL will always be reinstated.

ensures that the breakpoint

An additional feature in the

breakpoint logic, which is explained later, allows the user to specify
breakpoint 2 (above) so that it never causes a break (breaks and then
immediately continues).

This makes breakpoint 2 "transparent" or

"invisible".
Isee Section 4.8.7.
4-14

4.6.4

Flow of Control at Breakpoints

When execution of the user's program reaches a breakpoint, control is
returned to DDT.

At this point, DDT executes what is called a "con-

ditional break instruction."

If this instruction does not cause a

skip (e.g., NaP), which is the usual case, then DDT decrements the
"proceed count".

If this causes the proceed count to be equal to

zero, a break occurs.

If not, DDT will simulate the instruction

which was replaced at the breakpoint (by a JMS* 17) and then return
control to the user's program at the address following the breakpoint
register.

In other words, the break does not take place.

There is only one proceed count for all four breakpoints.

As the

name suggests, the proceed count is a value specified by the user
indicating how many times breakpoints are to be reached before a
break is to occur.

It is particularly suited for specifying the

number of times a loop should be executede

The proceed count will

be discussed later on.
If the conditional break instruction causes one register to be skipped
(e.g., SKP or SPA with the AC positive) the program continues without
a break and without decrementing the proceed count.

This is the

"invisible" breakpoint discussed in section 4.6.3.
If the conditional break instruction causes two registers to be
skipped (explained below) DDT will always break, without altering the
proceed count.
Each breakpoint ~ has its own conditional break instruction, which
the user may examine and modify directly by opening register I#+N
(N = 1 to 4).

Initially, when a breakpoint is set with the $B com-

mand, the contents of the associated conditional break instruction
register, I#+N, is set to Nap.

Nap does not skip, so the break will

be determined by decrementing the proceed count.

After the $B com-

mand, one may modify I#+N to contain any instruction one likes
CLA!SNL or LAC* 10 or JMS* V#+N).

(e.g.,

The first of these will decrement

the proceed count only when the Link is zero and continue otherwise.
The second always decrements the proceed count but, in addition,
effectively inserts one instruction prior to the breakpoint.
The third allows the user to call one of his own subroutines when the
breakpoint is reached.

The subroutine, when it returns, can decide

whether to skip two, skip one, or not skip, simply by incrementing its
entry point

twice, once, or not at all.

V#+N, shown in the last of

the three examples, is a register for breakpoint N set aside by DDT
specifically to be used to store the transfer vector for a conditional
4-15

break instruction which needs to make an indrect memory reference.
So, for example, if the user wants to call subroutine TEST, when
breakpoint 2 is reached, he types:

1#+2<

....j JMS*

~ TESTl

V#+2$Z.J
-

(Recall that when opening a register using < as a command, DDT does
not type out the contents of that register.)

The user is warned not

to insert JMP .+3 in a conditional break register, expecting a double
skip.

"Point" will not have the correct value to be able to do this.

One may set the conditional break instruction directly, without having to open 1#+N, by using the command:

where N = 1, 2, 3 or 4 to indicate the breakpoint number.

This com-

mand also takes a second argument, but it is immaterial to this discussion.
(with argument! missing) sets 1#+N to NOP (no skip)
sets 1#+N to SKP (skip 1)
sets 1#+N to skip 2 locations
Only the last bit in argument X is used to determine what is placed
in 1#+N.
The effective flow chart for the break/no break decision is as
follows:

4-16

EXECUTE CONDITIONAL BREAK
INSTRUCTION:
XCT I#+N
1

SINGLE
SKIP

NOSKIP

DOUBLE
SKIP

DECREMENT

YES

~BlAKI
I

I
NO_ _ _ _ _ _ _ 1I

$p COMMAND (PROCEED)

CONTINUE
USER
PROGRAM

Flow Chart, Break/No Break Decision

4-17

What Happens on a Break

4.6.5

When a break occurs, DDT makes three checks:
a)

If the break comes from a location not known to be
a breakpoint (if the user's program accidentally
executes a JMS* 17), DDT reinitia~izes itself and
types:
DDT
>

It is not possible to proceed from such a breakpoint.
b)

If the break comes from a known breakpoint but PI
(program interrupt) is disabled, the same action as
above will take place.
Therefore, breakpoints should
not be placed in routines which operate with PI off.

Also, if API is active when the break occurs, EaT
will type the contents of the API status register:
AP I

-4 4XXXXX

One may not proceed from such a breakpoint. Any
other command is valid, including a command to
delete this breakpoint.
The tests for PIon and API inactive are not made in the Background/
Foreground environment, since the Background job (DDT and the user's
1
programs) cannot execute lOT's.
If a valid break occurs, DDT will type the breakpoint number N followed by a tab and the contents of a register (specified by the address in R#+N) in the prevailing typeout modes, e.g.,
1

-4 776403)

R#+N (N = 1 to 4) is a special register associated with breakpoint N
(as are I#+N and V#+N).

Initially, when a breakpoint is set with the

$B command, R#+N contains the address of Ai (the stored AC).

There-

fore, if'the user does not alter R#+N, the contents of the AC will be
printed when a break occurs.
The user may change the address in R#+N in either of two ways: one is
to open and modify the register,e.g.,

l~~,~"- S
ect '~on 4 . 8 . 7 .
4-18

the second is by using the command:

The first argument ~, changes the contents of I#+N (N = 1 to 4) as
discussed in section 4.6.4.

Argument ~ specifies the address whose

contents are to be printed when a break occurs at breakpoint N.

This

address is stored in R#+N.
For example,
0;4004$1
will set the conditional break instruction in 1#+1 to skip a location
when executed.

Since this means a break will never occur, the second

argument is unimportant.
1;4004$1
will set 1#+1 so that a break will always occur and print:
1

~ contents of 4004

The default argument for ~ is A#.

If 0 is stored in R#+N, when the

break takes place only the breakpoint number will be typed.
When breaks occur, DDT saves the contents of the AC, Link, MQ, Index
Register, and Limit Register in the special DDT locations addressed by
the symbols A#, L#, MQ#, Xi, and LR#, respectively.
The MQ, of course, is not saved on a machine which does not have EAE.
The Index and Limit registers are saved only on PDP-IS.
Once DDT has stopped typing after a break, the user is free to type
commands.

Typically he would examine certain program registers,

perform searches, set and reset breakpoints and make program corrections.

Then he may continue program execution at the point where

the break occurred by typing:

This comnland places the value of the expression ~ in the proceed
count (the default value is 1) and proceeds by simulating the instruction at the breakpoint (the one saved. and replaced by JMS*17) and
gives control to the user program at the location immediately following the breakpoint.

If control returns to DDT because the user types
4-19

l! (CTRL T), $P may be used to continue progr~u execution from where
i t was interrupted.

Normally, when no breakpoint has been reached,

the $p command will not be accepted since there is no address from
which to proceed.

However, after DDT has initially been loaded or

restarted and has printed:
DDT
>
The command $P (as $G) starts the user program at the location
specified in SA#

(the start address).

Recall that the $G command

always sets the proceed count to 1.
For convenience, the

symbols Bl#, B2#, B3#, and B4# are defined as

the addresses of the breakpoints.
will be defined as 0).

(If breakpoint ~ is not set, BN#

DDT never uses these symbols on output, but

the user may use them on input to DDT.

If breakpoint 1 is set at

register 4000, which contains instruction CLA, then the following commands yield the indicated results:
Bl#/ -ICLA
Bl#=004000
As already mentioned, DDT initially assumes the use of auto-index
register 17 for breakpoint returns.

The command:

where A is an expression which must evaluate to 10 thru 17, commands
DDT to use auto index register A for its breakpoint return.

For

example:

will change the breakpoint instruction to JMS* 11.
of the argument A i~
4.6.6

(~is

The default value

equivalent to 17$0).

The Execute Command

Built into the DDT breakpoint processor is the facility to allow execution of single instructions.

By giving the command:

the user may execute the instruction I
CLA!CLL$X
LAC ....SUM$X
4-20

(an expression).

For example,

The first example will clear registers A# and L# (the saved AC and
Link in DDT).

DDT will type ? and ignore the command if an attempt

is made to execute a CAL instruction, an XCT instruction or an EAE
instruction which requires a memory reference to pick up an argument
(e.g., MUL).

The default instruction, in the absence of !, is the

instruction at location "point" executed as if it were located at
"pointH.

The reason for this is that if i t is a memory reference

instruction, it must be executed in the proper memory bank.
If the executed instruction causes a skip, DDT indicates this by typing an extra carriage return.
to return to after a break.

However, i t does not alter the address
For example; assume that breakpoint 1

was set as indicated in the following code and that a break at that
location has occurred:

A
B

LAC (1
DAC TMP (breakpoint 1 set here)
TAD ADR

The break occurs after LAC (1 has been executed and before DAC
is executed.

T~~

If the user now types:

SKP$X
$P
program execution returns to location B after the DAC TMP is simulated.

The SKP does not cause a skip.

One may execute a call to a subroutine:
JMS SUBR$X
provided that SUBR does not expect to pick up arguments following
the JMS nor skip more than one location on return.
In the Background of the Background/Foreground system, one cannot
execute IOT'S,l HLT, or an operate instruction microcoded with OAS.
These will cause a trap to the Monitor, which will treat these instructions as errors.

Return to DDT after the error message .ERR 032

must be done by typing tT (CTRL T).

lsee section 4.8.7.

4-21

4.7

PATCH FILE INPUT AND OUTPUT

4.7.1

Mass Storage Dump

If the DDT user has a system operated from bulk storage (Keyboard or
Background/Foreground Monitor Systems) he may save his work from session to session by means of a tQ (CTRL Q) dump, which dumps all of
core onto a specified area of the system device.

The reader should

refer to the Advanced Software Systems Monitors manual (DEC-9A-MADO-D)
for a complete explanation of the Monitor commands associated with
tQ dump.

In the Background/Foreground System it is not possible to

reload such a core dump.
In the Keyboard Monitor system, the Monitor

command~,

(where n is

some unit on the system device) will reload core from the control Q
area on that unit.

Typing tT (CTRL T) after the transfer is complete

will return control to DDT, which will respond by typing

In a Keyboard Monitor environment, after DDT and the user's programs
have been initially loaded, i t is recommended that the user dump core
using tQ.

Then, if his program goes awry and "clobbers" core, he may

return to the Monitor by typing tc and quickly recall the dumped
code.
4.7.2

Paper Tape Patch Files

In a paper-tape-only system (Basic Monitor) a program must be link
loaded e~ch session at the computer.

It is possible, however, to

save patches from session to session and all DDT-time symbols defined.

The format of the patch tape produced is given in appendix 3.

The following patch file commands exist:

outputs the contents of register K (the default argument is "point").

outputs the contents of every register inclusively between the
limits

and ~ (expressions).

The default value of H is zero.

The

default value of L is the value of ~, i.e., dump only one register.

will dump (save) all the DDT-time defined symbols.
4-22

After all patch file output has been accomplished,

the user must type:

to close the patch file.
At the beginning of a new session, after loading DDT and his programs,
the user may type

to ~nput (read) the patch file tape.

Because of the patch file's

format, it may be loaded only by DDT and only to the same version of
DDT in the same amount of core as was used to punch it.

If a read

error occurs during input, DDT types a question mark.

Data up to the

point of error is correctly stored in memory.

The user may reposi-

tion the tape by moving it back one block so that typing ~ at this
point causes DDT to reread that block and, if no error, to continue
reading the patch tape.
4.8

MISCELLANEOUS FEATURES

4.8.1

Operate Link and AC

The command:

will set the Link (L#)

from bit 17 of the value of the expression M

and the AC (A#) from the value of N.

The default values of both

arguments is 0, so that $U will clear the Link and AC (equivalent to
CLA!CLL$X) .
4.8.2

Make Subprogram Current (Header Command)

The header command is used primarily to make a

p~rticular

subprogram

current, thus giving preference to its local symbols when DDT performs a symbol table search.

The

form is:

The argument ~ is usually a filename (e.g., PRGl'), but it may be
any unique address symbol.

This command makes address A the current

location, sets the value of "point" equal to A and makes the program
containing that address the current program.

4-23

4.8.3

Initialize Memory

The command

where N is an expression, will change the contents of all memory
words between the limits in LO# and HI# to the value of N.
fault argument is
4.8.4

The de-

Loading DDT without a Program

DDT can be used to create programs on-line and it is not necessary to
load any user programs when DDT is called in.

This is done by typing

tT (CTRL T) when the Linking Loader has typed:
LOADER
>

BGLOAD
>

When DDT is started, core has been cleared between P#' and C#, and no
load-time nor DDT-time symbols exist.
4.8.S

Restarting DDT

The use

of tT (CTRL T) to interrupt a user program and return con-

trol to DDT has been previously explained.

tT may also be used to

abort a search operation which is in progress.
4.8.6

Typing Mistakes

If the user discovers that he has made a typing error while inputting
a command, he may type tu (CTRL U) or rubout, both of which are
echoed as @, to delete the entire command.
4.8.7

Protect Mode Commands

In order to avoid serious errors, a subroutine in DDT is used to validate addresses before certain commands are executed.

The following

commands are protected:
a.

Proceed ($P) following a tT

Go ($G)

All register modifying commands

Breakpoint ($B)

Except for references to the autoindex registers in register modification commands, addresses used by these protected commands must fall
the range P#' to C# (excluding the bootstrap in a 12K,20K,
or 28K PDP-IS).

This restriction prevents the user from altering

the Monitor, DDT's symbol table, or DDT itself.
4-24

The following two commands affect the protect mode:
~

Disables the protection feature, thereby allowing
the user to modify and transfer to any location in
core. Nonexistent memory references will still be
detected. To be effective in the Background/Foreground System, the Monitor command $MPOFF must also
have been typed.

Reenables the protection feature.

4.9

ERROR RECOVERY

Sections 2.11, 4.8.5 and 4.8.6 explain how to correct typing errors
and how to stop a runaway program.
The following DDT-generated error messages are not fatal:
U
X

o
?

API

(undefined symbol)
(illegal symbol definition)
(DDT-time symbol table overflow)
(general error message)
(breakpoint reached with API level active)

Commands which cause these errors are ignored.

The "API" error mes-

sage signifies that one may not proceed from such a break.
Errors which are caught by the Monitor (.IOPSXX or .ERR XXX) mayor
may not be fatal.

In any event, in the Background/Foreground system

control may be returned to DDT, after the error message is printed, by
typing tT;

in the Advanced and Basic Monitor Systems, control re-

turns to DDT automatically.
If an error is trapped by the Object Time System (OTS) , which prints
.OTS XX, it will exit to the Monitor.
that point.

4 .... 25

No recovery is possible at

APPENDIX A
SUMMARY OF DDT COMMANDS
A.

EXPRESSION OPERATORS
Logical inclusive or.
&

Logical and.

Logical exclusive or.

Two's complement sum.
Two's complement (unary minus) or subtract.
(Space)
Two's complement sum, but prohibit change
of top 6 bits in this and remaining operations in
the expression.

*
B.

Two's complement sum and exclusive or indirect bit
(20000) to value.

INSTRUCTION MODE

CO~wmNDS

(Symbolic)

Print symbolic instructions.

(Octal)

Print instruction as six unsigned octal
digits.

(Vector)

Print bits 3-17 as a transfer vector in
the current address mode.

(Text)

Consider registers "point" and "pointplus-one" as 5/7 packed ASCII and print
the corresponding five characters.

ADDRESS MODE COMMANDS
(Relevant to instruction modes $S and $V and also to tag
printouts) .

(Relative)

Print addresses relative to the nearest
tagged (labelled) location.

(Absolute)

Print addresses as absolute IS-bit octal
numbers.

(Floating)

Print addresses relative to #.
# is the
relocation constant for the current program.

SPECIAL ADDRESSES IN DDT
A#

Where the AC is stored on breaks.

Where the link is stored (bit 17) on breaks.

MQ#

Where the MQ is stored on breaks.

LR#

(PDP-IS) Where the limit register is stored on
breaks.

(PDP-IS)
breaks.

Where the index register is stored on

Where the default mask for searches is stored.

LO#

Where the lower limit address of search operations
is stored.

HI#

Where the upper limit address of search operations
is stored.
Al-l

R#+N

Where the register to be printed on breaking
at breakpoint N is stored.

I#+N

Contains the instruction to be executed on reaching breakpoint N.

V#+N

Reserved for the transfer vector possibly used
by the instruction in I#+N.

C#
E.

Contains the 2's complement of the proceed count.

SPECIAL ADDRESSES IN USER'S PROGRAM (NOT SEARCHED ON OUTPUT)
#

Address of the first word in the current program
(same as pi' when patch area is current).
Address of the last opened register.

Pi'

Address of the first word in the patch area
(Pseudo file name)

SA#

Address of the starting location of the first
program loaded (originally).

Bl#

Address of breakpoint 1 instruction.

B2#

Address of breakpoint 2 instruction.

B3#

Address of breakpoint 3 instruction.

B4#

Address of breakpoint 4 instruction.

CONTENTS OF SPECIAL USER LOCATIONS
Q#

Represents the contents of the most recently
opened storage word (never used on output) or
value of last intervening retype command argument.

DEFINING A SYMBOL
E(S)

Symbol S given value E (expression).
as~,

same as

~).

Symbol S defined

Kill DDT-time user defined symbols.

O$K

Kill load-time user symbols.

STARTING A PROGRAM
AD$G

Start at address AD (expression).

Start at address SA#.

;AD$G Set SA#=AD and start there.
I.

BREAKPOINT COMMANDS
K$B

Put lowest available breakpoint at address K (must
be non-zero; default: .) DDT types breakpoint
number after the B.

O$B

Remove all breakpoints.

N;K$B

Set breakpoint N (default: 4) at address K
(must be non-zero; default: .).

N;O$B

Remove ?reakpoint N (Default:

MiK$N

Type out register K (default: A#) when breaking at
breakpoint N (1 to 4) (N is part of command and may
not be omitted.)
If M; is missing, normal procedure
will be followed at breakpoint.
If M bit 17 is 0,
breakpoint will never break.
If M bit 17 is 1, breakpoint will always break.
Al-2

4).

E$O

Use auto-index register E (10-17; default: 17)
for breakpoint instructions.

M$P

Proceed from breakpoint and put M (default: 1)
in proceed count.

)

Close any open register, depositing its argument in that register.
If no argument is given,
the register is unchanged.

Close any open register and open another.
If
an argument is given, it is taken as the 15
bit address of the next register to be opened
and made current.
If not, the register "point"
is opened.

LINE
FEED

Behaves like ) except that it opens the register following the current register and makes
the new register current.

Behaves like ) except that it opens the register preceding the current register and makes
the new register current.

Behaves like ) except that Q# is taken as the
12 bit address of a location which is opened
and made current.

Behaves like ) except that Q# is taken as the
15 bit address of a location which is opened
and made current.

Behaves like $Y except that the new open register is not made current.
Behaves like I, but forces the printout to be
in numeric mode.
Behaves like I, but forces the printout to be
in symbolic mode.

Behaves like I except that the printing of
register contents is omitted until )
is used
to close a register (type-in mode) .
Print argument (default:

Q#) as an octal number.

Behaves like

but causes symbolic printouts.

Behaves like
fer vectors.

but retypes expressions as trans-

Prints out contents of . and .+1 interpreted as
5/7 packed ASCII text.

WORD SEARCHES
M;N$L

Set the contents of LO# to M (default: pi') and
HI# to N (default: L#).

M;D$W

Search from address in LO# through address in
HI# (? if lower·than address in LO#) for words
which are the same as D (default:
0) in all
_.c 1\wlL\
J.'J.1t J •
contents U.I..
bits that are 1 in M (default;

Al-3

Mi D$N

Search as in $W f

MiD$E

Search as in $W and $N for memory reference
instructions effect~vely addressing (directly
or indirectly) an address which is identical
to D (default; 0) in every bit which is I in
M (default: contents of M#).

PATCH FILE COMMANDS (I/O MONITOR SYSTEM ONLY)
LiH$Q

Output contents of locations between address
L (lower limit) and H (upper inclusive limit)
into the patch' file.
Default conditions for
these arguments are meaningless.

K$Q

Output contents of location K (default:
"point") into the patch file.

Dump DDT-time defined symbols.

Close patch file.

Input patch file (from paper tape reader).

OTHER DDT COMMANDS
I$X

Execute instruction I (default: contents of
"point" as if located at address "point").

Wipe out current command.

RUBOUT

but print unequal words.

Same as tu.

C$H

Make the address C (e.g., FILE') current,
but do not open it.
(Default: SAil.

MiN$U

Update contents of L# to M (default: 0)
and the contents of A# to N (default: 0).

N$M

Initialize memory from address in LO#
through address in HI# to N (default: 0).

Interrupt -- transfer control to DDT.
Used to stop a runaway program or to
abort a long search operation.

Enable Protect mode.

Disable protect mode. Allow modifications of
and transfer to any location in core.

ERROR MESSAGES

General error indication.

Symbol definition command attempted to redefine a non-DDT-time symbol. Command ignored.

Symbol just used is undefined.

API

A breakpoint has been reached with API active
(cannot proceed).

Symbol table capacity has been exceeded by
symbol definition command. Command ignored.

AI-4

Command ignored.

SPECIAL DDT SYMBOLS
First argument delimiter for commands with two
arguments.
Same as ; when used with symbol definition
commands.
$

Indicates that the following symbol is a
command (used primarily with letters and
numbers, which ordinarily comprise expression
syllables) .

ALTMODE Same as $ and echoes as $.

Indicates that the corr~and has been performed
by DDT.
Same as
Indicates that the preceding symbol is a file
name (must be used with file names for correct
operation) .

Indicates that the preceding symbol is an address (omit searching the instruction mnemonic
symbol table).

Sets the indirect bit of an argument (20000).

(PDP-IS) Sets the index bit of an argument
(10000) .
Same as ,X.

Al-S

APPENDIX B
MNEMONIC INSTRUCTION TABLE
OPERATE CLASS

MEMORY REFERENCE
CAL
DAC
JMS
DZM
LAC
XOR
ADD
TAD
XCT
ISZ
AND
SAD
JMP

NOP
OPR
CMA
CML
RAL
RAR
IAC**
SMA
SZA
SNL
SKP
SPA
SNA
SZL
RTL
RTR
SWHA**
CLL
STL
RCL
RCR
CLA
CLC
GLK
LAW

000000
040000
100000
140000
200000
240000
300000
340000
400000
440000
500000
540000
600000

EAE GROUP
EAE

640000

INPUT/OUTPUT
lOT

700000

740000*
740000
740001
740002
740010
740020
740030
740100
740200
740400
741000
741100
741200
741010
742010
742020
742030
744000
744002
744010
744020
750000
750001
750010
760000

PDP-15
AAS
PAX
PAL
AAC
PXA
AXS
PXL
PLA
PLX
CLX
CLLR
AXR

720000
721000
722000
723000
724000
725000
726000
730000
731000
735000
736000
737000

* DDT interprets 740000 as NOP.
** PDP-15

A2-1

APPENDIX C
PATCH FILE FORMAT
DDT outputs the patch file in four-word blocks (including the twoword block header used by the lOPS system) with blank tape showing
between the blocks.

Each block carries the address and the contents

of one memory word.

(See figure below).

The dump DDT-time symbols

command ($D) punches the additional symbol table area in the same
manner.

The $C command punches an lOPS end-of-·file block.

WD 0

WORD PAIR
COUNT AND MODE

WD 1

CHECKSUM

WD 2

)

lOPS BLOCK HEADER

ADDRESS OF
PATCH

WD 3

CONTENTS OF
PATCH

As many files as desired may be produced by using the following
sequence of commands:
LliHl$Q
(as many as desired)
LMiHM$Q
$D
$C

(optional)

A3-l

APPENDIX D
DDT MEMORY LOAD MAPS
'rOP
OF CORE
-

DDT

DDT
SYMBOL
TABLE

C#..:.::,;r

A#~

"T"''1\.T

T"'\

I
I

THIS AREA PLUS
THE AUTO-INDEX
RRGTC::'T'RRS -MAY
BE MODIFIED

• scni+ 3---./

PATCH
AREA
SYMBOLS
SUBPROGRAM N

SYMBOLS

·
·
SYMBOLS

i
..... ~

SUBPROGRru-1 N

f
LOAD-TIME

I/O MONITOR
ONLY

START HERE, UNLESS
INSTRUCTIONS BYPASSED

"'"

P#'~

CONTINUE HERE

SA# -Y~. __-=PSUBPROGRAM
.:;. ;:U\.~J.~l\l~.I:':. . .:R~V:. =\.:J. : ; RAM. ; . . . ;1;:. l-----l

USER
PROGRAMS

FREE
CORE

BOOTSTRAP
REMAINDER
OF DDT
DDT-TIME SYMBOLS
SPECIAL DDT
SYMBOLS
INSTRUCTION
MNEMONICS
SPECIAL DDT
REGISTERS

ORDER FOR SYMBOL
TABLE SEARCH

.............. .-..o...J_.

I
!

il\

FINAL SEARCH HERE

SUBPROGRAM 1
SYMBOLS
!~MAIN PROGRAM HAS
3 INITIAL PRECEDENCE
MAIN PROGRAM
(CURRENT PROGRAM)
I/O HANDLERS
PPA. AND PRA.
FOR DDT PATCH FILES
RESIDENT
MONITOR

BOTTOM OF CORE

*On a PDP-IS with 12K, 20K or 28K or core, the bootstrap is located
at the top of the next to highest 4K page.

A4-1

DDT IN BACKGROUND/FOREGROUND
TOP OF CORE
DDTI

USER
PROGRAMS

BACKGROUND
JOB'S
CORE

.SCOM+3 ____________~~~
FREE CORE

.SCOM+32-----------.. - - - - - - - .L
2 4
Hardware '
LOAD TIME
Prot ect
SYMBOLS
Boun d
...
. SCOM+31----------~~~
Software
USER'S I/O
Prote ct
HANDLERS
Boun d
.SCOM+25----------~~~
FOREGROUND
PROGRAMS
RESIDENT
MONITOR
BOTTOM OF CORE
NOTES:
1.

In Background/Foreground, DDT is loaded to the top of core
(overlaying the bootstrap).

Memory references below the hardware protect bound cause an
interrupt. The Monitor screens all such references to determine which are legal and which are not.

Free core may dip below the hardware protect bound. The area
below the bound may be used for data storage but not for
executable program code.

If the Monitor command MPOFF was issued in the Foreground, the
hardware protect bound is set at register zero.
If the command
$@ is given to DDT, the use~ may then issue 'IOT I S in the Background
and modify and transfer to any location in core.

A4-2

HOW TO OBTAIN SOFTWARE INFORMATION
Announcements for new and revised software, as well as programming notes,
software problems, and documentation corrections are published by Software
Information Service in the following newsletters.
Digital Software News for the PDP-8 & PDP-12
Digital Software News for the PDP-II
Digital Software News for the PDP-9/15 Family
l - r .. ,
T'-'ese
O'\ewslI'WII
.... ~~e I.. s -""
.... ~a:I I....I :I I...I Ic~
.. m ... ~:~ ... "'p_I:
__ L..I_ "0
-or.&...
.. ,... .. - - •. -:I-b IC
l l om
III
~VIII
VI I UIIVII U
tJll~UJJIC
I
;)
IIVV""IC UVUIIU
II

Digitalis Program Library, Articles in Digita I Software News update the
cumulative Software Performance Summary which is contained in each basic
kit of system software for new computers. To assure that the monthly Digital
Software News is sent to the appropriate software contact at your insta lIation,
please check with the Software Specialist or Sales Engineer at your nearest
D igita I office.
Questions or problems concerning Digitalis Software should be reported to
the Software Specialist. In cases where no Software Specialist is available,
please send a Software Performance Report form with details of the problem to:
Software Information Service
Digital Equipment Corporation
146 Main Street, Bldg. 3-5
Maynard, Massachusetts 01754
These forms which are provided in the software kit should be fully filled out
and accompanied by teletype output as well as listings or tapes of the user
program to facilitate a complete investigation. An answer will be sent to the
individual and appropriate topics of general interest will be printed in the
newsletter.
Orders for new and revised software and manuals, additional Software Performance Report forms, and software price lists should be directed to the
nearest Digital Field office or representative. U.S.A. customers may order
directly from the Pro~r(]m Library in Maynard. When ordering, include the
code number and a brief description of the software requested.
Digital Equipment Computer Users Society (DECUS) maintains a user library
and publishes a catalog of programs as well as the DECUSCOPE magazine
for its m-embers and non-members who request it. For further information
please write to:
DECUS
Digital Equipment Corporation
146 Main Street, Bldg. 3-5
Maynard! Massachusetts 01754

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