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PDP-8/I DDT
DYNAMIC DEBUGGING TECHNIQUE

PROGRAMMER’S REFERENCE MANUAL

For additional

copies specify Order No. DEC—08—CDDB—D to Program Library,

Digital Equipment Corporation, Maynard, Mass.

DIGITAL

EOLJIF’IVIENT

CORPORATION

MAYNAFIO,

Price:

$2.00

MASSACHUSETTS

Ist Printing July I967
2nd Printing October I967

3rd Printing January I969
4th Printing (Rev) August I969

Your attention is invited to the last two pages of this

manual. The Reader's Comments page, when filled in
and returned, is benefical to both you and DEC. All
comments received are considered when documenting

subsequent manuals, and when assistance is required, a
knowledgeable DEC representative will contact you.
The Software Information page offers you a means of

keeping up-to—date with DEC's software.

(available from Program Library, address on title page):
Paper Tape System User's Guide DEC-OB-NGCC—D

Documents Referenced

PAL III Assembler,

Programer's Reference Manual

MACRO-8 Assembler,Programer's Reference Manual

The following are registered trademarks of Digital

Equipment Corporation, Maynard, Massachusetts:
DEC

PDP

FLIP CHIP

FOCAL

DIGITAL

COMPUTER LAB

DEC—O8—ASAB—D
DEC-08-CMAA-D

CONTENTS

Chapter

P_ag__e_

INTRODUCTION

USE

OF DDT-8

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

........

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

Preparations for a Debugging Run

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

The Basic Functions of DDT:: A Sample Run

2-3

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

3-I

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

3-I

Symbol Table Tapes

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

3-2

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

3-2

Mode Control

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

Cross-Page Addresses

3-3

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

3-5

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

3—8

Program Examination and Modification

Using Combined Operate or IOT Class Instructions

Output

2-2

3-1

Storage Requirements
Loading Procedure

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

2-1

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

THE FUNCTIONS OF DDT-8

Definitions

1—1

.........

3—8

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

3—9

Special Registers

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

Program Execution and Control
Restrictions on the Use of

Word Searches

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

3-9

3—I0

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

3-13

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

3—13

Defining New Symbols

Breakpoints

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

Making a New Symbol Tape

Punching Binary Tapes

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

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

3-16
3-17

3-I8

Appendix
A

SUMMARY OF COMMANDS

INTERNAL SYMBOL TABLE

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

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

A-I
B—I

PREFACE

This manual describes the keyboard—oriented dynamic debugging program For use
on-line with the PDP-8 family computers.

Chapter 1 explains the purpose and efficiency of DDT (Dynamic Debugging Tech—

nique).

In Chapter 2, the reader is taken through an example debugging session

which acquaints him with the ease of debugging with DDT.

The use of each DDT

command and special character is explained in Chapter 3 and summarized in an

appendix.

CHAPTER1

INTRODUCWON

Users of most computers,

especially large-scale ones, are familiar with the procedure of sub-

mitting a new program for a computer run, waiting for it to be processed (which may take anywhere from a few hours to several days), and finally receiving the compilation and/or assembly

listings, a list or dump of the contents of each 'core memory cell at the time the run was terminated, and perhaps a storage map giving the addresses of the symbols used in the programs. The
may get a few remarks from the computer operator regarding the failure of the program to

user

run

properly.

If the user is present in the machine room when his program is processed, he may

get additional information from the console lights, motion of tapes, etc., but his correcting
must be done away from

considerable time

DDT

the computer.

Getting a program to work under these conditions takes

(Dynamic Debugging Technique) helps shorten this debugging time by allowing the user

to work

his program atthe computer,

to the program

its data. For example,

of coding is a laborious and frustrating

control its execution, and to make corrections

tracking down a subtle error in a complex section

job-by hand; but with the breakpoint facility of

DDT-8, the user can interrupt the operation of his program at any point and examine the
state of the

machine. In this way,

sources

of trouble can be located quickly.

Using DDT-8, the programmer can make corrections or insert patches in his program and try
them out immediately.

If his corrections work, the user can have the corrected sections and

patches punched out on the spot in the form of tapes which can be loaded along with the program the next time it is run, thus eliminating the

necessity of creating new symbolic tapes and

reassembling or recompiling each time an error is found.
When working with DDT-8, the user has with him a

listing of his program and of his symbols.

In making corrections, he may refer to variables and tags by their symbolic

names or

by their

octal values; he may add new symbols and delete old ones (for purposes of debugging,

l—l

symbol

changes do not carry beyond the immediate debugging run). If he writes in the permanent
corrections on his program

listing, he can keep a record of the debugging and eventually

make a new symbolic tape incorporating all his corrections and patches.

The first chapter of this manual explains a typical debugging run.
describe the functions of DDT-8 in detail.

1-2

The succeeding chapters

CHAPTE R 2

USE OF DDT-8

This chapter is designed to introduce the basic operations of DDT-8 to the person unfamiliar
with on-line debugging.

Although some elementary concepts are explained in detail, any

essential information which appears in this chapter about the DDT commands is also presented
in Chapter 3, where it is accessible for easy reference.

DebUgging a new program can be, and has been, done on varying levels of detail and sophistication.

On the crudest level, the programmer can simply load the program and let it run until

it stops unexpectedly.

Then, using the console switches and keys, he can try to find the error(s)

by interpreting the console lights.

There are two hazards to this approach.

the program stops, the error may have caused all pertinent information,

First, by the time

including itself, to be

altered or eliminated; the program may have stopped by the simple process of self—destruction.

Second, the program may not stop at all; it might continue to run in an infinite loop. Such

loops are not always easy to detect.
If the programmer plans his debugging attack beforehand, he can, using the computer console,

place strategic halts in his program before starting it.
ous

registers and alter their contents

After each halt, he can examine vari-

again using the console.

As long as he remembers to

replace each halt with the original instruction before proceeding, he might find sources of error
more

readily than if he iust let the program run on. However, errors seldom appear where ex-

pected, so a strategic halt may be of little real use.
Added to these problems (of console debugging) are the difficulties of interpreting binary con-

sole displays and translating them into symbolic expressions related to the user's program listing.

Further, adding corrections to a program in the form of patches requires seemingly endless man—
ipulation of switches and keys.

In all

this, the chance of programmer error at the console is

large and is likely to obscure any real gain made from the debugging session.

2—1

What is needed is a program which will assume the tasks which the programmer would have to

perform if he used the console. Such a program would allow the user to examine registers,
change their contents, and make corrections, without having to manipulate switches and keys.
It would allow the easy placement and removal of halts,

even to

execution of instructions which the halts had displaced.

Most important, it would allow the

programmer,

the automatic restoration and

using the keyboard and printer, to do all examination and to make all corrections

in the symbolic

language of the listing,- the debugging program would perform all the necessary

translation to and from the binary representation.
DDT-8 is such a debugging prOgram.

Descended from a line of programs that includes a version

for every computer produced by DEC, it performs all the tasks described above, and many more,

making the programmer's burden light enough to allow him to concentrate on the actual correction of his program.

PREPARATIONS FOR A DEBUGGING RUN

By the time the prOgrammer is ready to start debugging a new program at the computer, he should
have the following materials:
1

The binary obiect tape of the program.

The symbol definition tape which was part of the assembly output.

()1th

A complete symbolic

listing of the program.

A list of the symbols and their definitions.
A binary tape of DDT-8 (usually provided at the console).

To begin the debugging run, first ascertain that the BIN Loader is in memory.
it

If it is not, load

using the procedure described in the Paper Tape System User's Guide, for loading

Rl'M tapes.

When the BIN Loader is in the computer, load the programmer's binary program

tape(s) using the same BIN loading procedure.

After it has been read successfully, load the

DDT-8 binary tape.

In the machine, there now is:

the DDT program, which occupies upper
memory between registers 5240 and 7600, inclusive; 2) the User's programs

which must not overlap the area occupied by DDT-8 or its permanent symbol

table; 3) a table of symbol definitions, extending downward from location
2-2

5240 to 5000. This table includes the definitions for all of the PDP-8 memory reference instructions, operate class instructions, the ten basic lOT instructions, and the combined operations CIA and LAS.
Since DDT is to perform all translation between binary and symbolic representation, it must

have access in memory to the user's symbol definitions.

To load a symbol tape,

perform the

following steps. Note that the high-speed reader may not be used to load symbol tapes. Only
the reader on the ASR 33 console may be used.

Turn the reader off; insert the symbol definition tape.

Type the keys ALT MODE (or ESCape ) and R ([R)! in that order, and then

turn the reader on.

When the computer stops,

turn off the

reader; press CONTINUE.

DDT

will type out the address of the lowest register in memory which is occupied

by a symbol definition.
'The symbols for the user's program are stored in memory immediately below DDT's permanent

table.

These symbols, and any others which are entered from the console, comprise the exter-

nal symbol table; these definitions may be removed at any time (see Chapter 3) without harming the permanent table.
With DDT, the program, and the symbol definitions now in memory, the programmer is ready
to begin debugging.

Figure 2-1 is a listing of a program ready for debugging; the remainder

of this chapter will describe the process.

THE BASIC FUNCTIONS OF DDT:

A SAMPLE RUN

As soon as DDT has typed the address of the lowest extension of the symbol
for debugging work.
sum

table, it is ready

The prOgram to be checked out is a subroutine which accumulates the

of the first n integers.

For testing purposes,

short calling sequence has been included

which provides the integer limit of the sum as an item of data.

The first task is to place an in—

teger in the register which holds this datum; namely, the register labeled INT in the calling
*Some teletypes have a key labeled ESC

(escape) in place of the ALT MODE key.

DDT will accept either and perform the same function.

2-3

program.

By typing the address of the register (which in this case can be done by typing the

address tag), followed by a slash, the user indicates to DDT that he wishes to examine the contents of that register.

Thus he types:

INT/

/INTEGER SUMMATION SUBROUTINE
INTSUM,

o
CLA

/GET DATA

TAD l INTSUM
DCA N
DCA PSUM

LOOP,

/MA|N COMPUTATION

TAD PSUM
TAD N
DCA PSUM

/DECREMENT INDEX
/NOT FlNISHED
/F|NISHED. PUT RESULT IN AC.

ISZ N

JMP LOOP
TAD PSUM
ISZ INTSUM

IEXIT,

JMPI INTSUM

N,
PSUM,

/RETURN

*400

ITEST,

/lNTSUM TEST PROGRAM

CLA

JMS INTSUM

INT,
RTN,

/PUT ARGUMENT HERE

HLT

$
Figure 2-l

DDT Sample

Program

DDT responds to the typing of the slash by typing an expression which has the value of the con—
tents of the

(Note:

specified register.

In this case,

C(INT)

O, and the line now appears as follows:

In all of the examples below, information typed by DDT is underlined to make it dis-

tinguishable from that typed by the programmer.

In actual operation,

underlining is present.)

INT/0000
After typing the contents of the register, DDT types five spaces and waits.

open, which means that its contents are available for modification.
2-4

The register is now

The programmer decides

that the first test integer is l0.

arithmetic.

This must be an octal

integer since DDT performs no decimal

With the register open, he types the number 10.

types a Carriage Return

Then, to close the register, he

() ) immediately after the number.
[NT/0000

10)

Further access to this register is now denied until he opens it again.
If it works, it should

Having provided his data, the programmer is ready to start the program.

stop almost immediately with the sum of the first TO 8 integers, which is 44 8' displayed in the
AC

lights.

To start the program from

DDT, he types the following command.
ITEST[G

The left bracket (D is the character printed when the ALT MODE ( or ESCape) key is struck;
its function here is to

identify the succeeding character as a DDT command. The letter G

specifies the action to be performed, which in this case causes DDT to transfer control to
the test program at location ITEST.

The programmer has typed the command; his program starts to function.

Immediately he observes

that something is wrong, since it does not stop almost instantaneously, but runs for a very short,
but observable, time.
to

When it does halt, the contents of the AC

lights are definitely nglequal

448.

At this point, he knows something is wrong, but he is not sure where the error lies.

If he could

interrupt the program during its operation, he might get some idea of the nature of the difficulty.
For instance, if he could verify that the data was transferred to the subroutine correctly, he

could eliminate the calling sequence as a source of error.

The DDT facility which allows the programmer to interrupt the program at any time is called the

breakpoint.

As its name implies, it allows him to break into the program sequence at some point

and return control to DDT.

He can specify a breakpoint by typing the address of the instruction

where he wants to interrupt the sequence; and after this address he types the breakpoint command.
If he requests a breakpoint at location INTSUM+3,

the program will be interrupted when the

datum is in the AC, but before it is deposited in the working register N.

2—5

INTSUM+3[ B
When this command is given, the information is retained by DDT until the
start command is provided.

At that time, the instruction in the register

specified is removed and placed in a temporary storage location in DDT.
In place of this instruction, 0 JMP is substituted which returns control to
DDT.

To ascertain that the error did not destroy the item of data in the calling program, check it by

opening the register

INT/9919
Having ascertained that the datum is correct, again start the program.
ITESTlG
Almost immediately, the breakpoint is encountered.
occurs, DDT saves

the C(AC).

Control returns to DDT.

It then types the address of the breakpoint,

When the break

right parentheses,

and the contents of the AC which have been saved.

IN TSUM+OOO3)OOl O

The programmer sees that the transfer is correct.

In similar fashion, he moves the breakpoint to the end of the subroutine at

the location lEXlT/.

itself.

He discovers that at this point the error has manifested

He knows now that the trouble is in the initialization or the main

loop.

He can investigate the

cover

that the datum is placed in register N as desired.

loop by placing a breakpoint at LOOP, to disNow he moves the

breakpoint to the end of the loop.
LOOP+3[ B
ITE srr G
LOO P+0003)(_)01 o
At the end of the first pass through the
At this point,

loop, the C(AC) are equal to the starting value of N.

however, the C(N) itself have iust been changed.

properly, the C(N) should now be equal to 7 8

N/OOll

investigates:

If the subroutine is working

By this time the programmer realizes what has been wrong with the program.
save

space by using the datum as a counting

the contents of a register.

In attempting to

index, he forgot that the ISZ instruction increments

What he needs is a counter that starts with a negative value.

Real-

izing this, he ends the debugging run.
The sample program above was simple; the error was obvious.
and with

This is seldom the case; however,

long or complex programs, several debugging runs may be required. However, DDT,

with its facilities for handling symbolic expressions, allows the programmer to work entirely in

the language of the Assembler (either MACRO-8 or PAL III), thus shortening the time required
to arrive at a

correct, workable program.

A detailed explanation of every function of DDT is provided in the next chapter.

For those

interested, a correct subroutine for performing the integer summation will be found in the PAL III

Programmer's Reference Manual.

CHAPTER

THE FUNCTIONS OF DDT-8

STORAGE REQU lREME NTS
The operating portion of DDT-8 occupies storage in upper memory from location 5245. to location

7577, inclusive.
to

The permanent symbol table extends downward in memory from location 5237

location 5000, inclusive.

This table contains the definitions of the mnemonics for all the

basic memory reference instructions, the operate class instructions of both Group 1 and Group 2,
the combined instruction CIA and LAS and the symbol I for indirect addressing, and the basic
lOT instructions:

KCC, KRS, KRB, KSF; TSF, TCF, TPC, TLS; ION and lOF.

Appendix 2

lists all the symbols and definitions in the permanent table.

Space is reserved for the user's symbol table immediately below the permanent table.
mum

of 250 such external symbols is allowed; hence if the user's table is filled, the lower limit

of space occupied by DDT is 3030.
to

A maxi-

the user.

However, space not used for external symbols is available

Each new symbol defined on line uses four locations in the external table.

During operation, DDT uses location 4 on page 0 for the breakpoint link; thus this register is
not available to

the user.

LOADIN G PROCEDURE
To load DDT, the BIN Loader must be in memory.
set the

switches to 7777, then press LOAD ADDRESS and START in that order.

has been read, the status of the AC
all out,

Place the user's binary tape(s) in the reader,

lights will indicate any error in loading.

When the tape
If the

lights are

loading was successful; if any lights are on, there was a checksum error and the tape

must be reread .*

After the user's binary tape(s) is in memory, DDT may be loaded using the BIN Loader.

*For more information about the BIN Loader and binary tape format,

System User's Guide.
3-1

see

the

Paper Tape

SYMBOL TABLE TAPES
Part of the punched output of a MACRO-8 or PAL assembly is a tape containing the symbol definitions of the assembled program.

The definitions from a symbol tape are entered into the DDT

external table by the following procedure; only the ASR 33 tape reader may be used.

Turn the reader off; insert the symbol tape.

Type ALT MODE (or ESCape) then R ([R) on the keyboard and turn the reader on.

When the computer stops, turn off the reader, then press CONTINUE.

DDT will type out the address of the lowest register used by the external sym—

bol table

If more tapes are to be entered, bit 0 of the switch register (SR) must

be down; repeat steps l-3, for each tape.

Reading will continue until the end of the tape is reached or until a total of 250 symbol definitions have been read.

If this maximum limit is reached,

the table until some have been deleted.

further symbols may be added to

Even if the limit is reached in the middle of a tape,

however, the user may proceed with debugging by typing EOT, then turning the reader off and
pressing CONTINUE.

The remaining symbols left unread will not be in the table.

DEFINITIONS
A symbol is a string of up to six letters and numerals, the first of which must be a letter.

following are legal symbols: FIMAGE, K2, X464PQ, PMLA.

mum

value of

The following are not aceptable:

4WD

Daes not begin with a letter

F2.8

Contains an illegal character

AN PRC

A space cannot be imbedded in a symbol

GANDALF

More than six characters

A number is a string of up to four octal digits (integers).

Hence, a number may have a maxi—

77778. The digits 8 and 9, however, may be used only

3—2

The

characters in a symbol.

expression is a symbol, an integer, or a sequence of symbols and integers separated by any

of the following operators:
An operator designating addition (arithmetic plus).

An operator designating subraction (arithmetic minus).

An operator which indicates that the remainder of the

space

expression is to be treated as the address part of an instruction

(see MACRO—8 Programmer's Reference Manual.)

All other characters, except those used for DDT control commands,

If two or more spaces appear in succession, all but the first

TAD

are

illegal.

ignored.

Thus,

TEM and TAD TEM are identical expressions.

DDT will

respond to an extra CR with CR, LF; the extra CR‘s are otherwise ignored.

The following errors will cause DDT to type a question mark (?) and ignore all the information

typed between the point of the error and the previous tab or CR.
1.

AWN

Undefined symbol;

illegal symbol.

Illegal character.
Undefined control command.

Cross-page addressing.

MODE CONTROL

Any expression containing a symbol is symbolic; an expression containing only integers is octal

The user of DDT is free to use whichever mode is most convenient for the information he is typing
in.

On output, DDT will type exclusively in one mode or the other,

determined by one of

the commands described below.

NOTE: When DDT is first set into operation, the output mode is

symbolic

3—3

This command causes DDT to print any subsequent item
of information as an octal

symbolic or octal

integer.

Typed input may be

If LOC=2642z

Example:
1

LDC/1263
2642/1263

This command causes DDT to print any subsequent item
of information as a symbolic expression.
may be symbolic or octal

Typed input

If LOC=2642 and C(LOC)=

TAD DATA+4:

Example:
1.

LOC/TAD DATA+OOO4

2642/TAD DATA+OOO4

If the user wishes to find the octal value of a symbolic expression typed by himself or by DDT

without changing the output mode, he may use the following command.

Typed immediately after a symbolic expression, this will
cause

DDT to print the value of the expression as an

octal integer.

Example:
1.

LOC=2642

LOC/TAD DATA+OOO4

=1263

In the second example above, the prevailing output mode
is symbolic and remains

after the use of the equal sign.

PROGRAM EXAMINATION AND MODIFICATION
These commands and operations allow the user to examine and change the contents of any register in the PDP-8 core memory.

CAUTION
Be careful not to open and modify any register within the DDT symbol table or program itself. DDT does not protect itself against such

intrusions, which will inevitably cause errors in operation.

This is the register examination character.

Typed immediately

after an expression, it causes DDT to print the contents of

the register whose address is specified by that expression.

Example:

If the user types:

LOC/
DDT will type out the contents of LOC, followed

by 5 spaces, thus:
LOC/TAD DATA+OOO4
The user may now change the contents of the

LOC/TAD DATA+OOO4

pl (CR)

JMP LOC+IO

This causes DDT to close the opened register after making the

specified changes (if any) in its contents.

Example:
LOC/TAD'. DATA+OOO4

JMP LOC.+I 0

)

Typing additional CR's will have no effect on the operation
of DDT.

If, after examining and/or modifying the contents of a register, the user wishes to open the next register in sequence,
he types a line feed instead of a CR.

3-5

The open register is

closed, and DDT then opens the succeeding register, typing
the address, a back slash to indicate that the register was
not

opened by the user, the contents of the new register,

and another five spaces.

After examining and changing the contents of

Example:

LOC, the user wishes to examine the contents
of LOC+l

LOC/TAD DATA+OOO4

JMP LOC+10 (LF)

MDCA DATA
The register LOC+T is now open.

The line feed may be used at any time,

even

if the last reg-

ister examined has’ been closed or if other operations have

For example, if the following sequence of oper-

intervened.

ations occurs:

LOG/W

JMP

.+10)

.+5[B

(LF)
DDT will still open register LOC+l

The breakpoint address

has no effect on the counter within DDT which keeps track of
the last opened register.

lf instead of changing the contents of aregister, the user

wishes to examine the register addressed by those contents,
he types

follows:

LOC/TAD DATA+OOO4

DATA+4\ OPR+337
The register DATA+4 is now open.

3-6

Note that this operation is intended for use with unmodified

registers.

If the user types it after typing some modifying

information, the register addressed will be the one which is
For

changed.

example, if the following sequence occurs:

Loc/TAD DATA+OOO4

JMP

Loc+iol

the information will be placed in DATA+4,

that the next

line, printed by DDT, will look like this:

DATA+4\JMP LO‘C+OO]O
The register LOC will not be changed.

An indirectaddressmodifier will not be

operation.
TAD |

interpreted by the f

If, for example, the register LOC contained

DATA-t4, and the user typed f as in the previous

example, DDT would still open the register DATA—i4.
-

(period)

The period is used as a symbol whose value is the address
of the last previous register opened.

several ways

Example 1:

It can be used in

To check the results of a modification.

LOC/ TAD DATA+OOO4

JMP LOC+I o

)

./JMP LOC+OOIO
2.

To refer to the currently open register.

LOC/TAD DATA+OOO4
3.

JMP .+10

To execute any command starting at an ad-

dress relative to the last opened register.

LOC/ TAD DATA+OOO4
.

-5[ G

3-7

JMP.+l 0

)

<—

(back arrow)

An error may be deleted by typing a back arrow.

All infor—

mation between the 4—— and the previous tab or CR is ignored;

DDT responds by typing a tab.

LOC/TAD DATA+OOO4

For example:

JMP .+IO

JMP LC4—

CROSS—PAGE ADDRE SSE’S

When the user types an instruction to be placed in an open register, the address of that instruction must be in the same page as the address which contains the open

If such a cross-page address is attempted, DDT will signal an error by typing land ignoring
the information

Example:

If LOC

2642 and XPAG =2770, the following sequence would result in an error

indication:

LOC/ TAD DATA+OOO4

DCA

XPAG+20)

The expression XPAG+20

3OIO, which is outside the page containing LOC.

The register LOC will be closed without modification.

Conversely, an expression containing symbols defined outside the page is acceptable if its value
is in the current page.

Example:

If LOC

2642 and XPEG

30l0, the following sequence is acceptable, since

XPEG-20 has a value which brings it within the current page:

LOC/TAD DATA+OOO4

DCA

XPEG—ZO)

USING COMBINED OPERATE OR IOT CLASS INSTRUCTIONS

Except for CIA and LAS, combined Operate Class and IOT instructions are not defined in the
DDT-8 permanent symbol table.

To enter such instructions into an open register, the combin-

ation must contain no more than two mnemonics, the second of which must be CLA.

combination will be treated as an error, and the information will be ignored.

3—8

Any other

Example:

The following attempt is an error.

XPAG/ELA

CLA

CMA

)

This attempt is correct.

XPAG/CLA

CMA CLA

If the desired combination does not include CLA, the user may do one of two things.
define the combined operation as a new symbol

combined operation code.
say,

He can

(see SLmbol Definition) whose value is the

For example, the operation CLL RAR can be defined as a symbol,

CLAR, whose value is 7110.

Alternatively, the user may enter the combined operation as an expression containing the symbol OPR.
use

For example, the operation CLL RAR can be entered as CPR-+110.

He may similarly

the symbol lOT in entering new l/O combinations.
OUTPUT

When operating in symbolic mode, DDT—8 will always attempt to make a symbolic expression
out of the contents of an opened register,

be such or not.
ter will

regardless of whether the contents are intended to

For example, if register DATA contains the number 6i l5,

opening the regis-

result in the following line:

DATA/IOT+Ol T5
The user can use the equal sign to ascertain the octal value:

DATA/IOT+OI 15

=6] 1
-———-

SPECIAL REGISTERS

There are five registers contained within DDT which hold information of interest to the user.

These registers may be opened and their contents may be changed.

When a breakpoint is encountered, the

C(AE) at that

point are placed in this register.
[Y

When a breakpoint is encountered, the C(L) at that point
are

placed in this register.
3—9

This register contains the address of the lower limit of a
word search.

Initially, C([ L)

000i

This register contains the address of the upper limit of a
word search.

Initially, C([U)

5000.

This register contains the mask used in a word search

Initially, C(lM)

7777.

PROGRAM EXECUTION AND CONTROL

The commands described in this section allow the user to control the execution of his program.

This command causes DDT to begin the execution of the
user's program,

starting with the instruction inithe register

whose address is specified by the expression k.

If a break-

point (see below) has been requested, it is inserted just
before control is passed to the user's program.

Example:

If the user types

BGINlG
DDT will transfer control to location BGIN.

Likewise,
FILI-5l G
will cause the user's program to start in the fifth

Using [(3 without an argument is an error.

DDT will

ignore

the command, and type _?_ to indicate the mistake.

k[B

This causes DDT to insert a breakpoint at the location speci—
fied by the expression k.

mediately, however.

3-10

The breakpoint is not placed im—

When this command is typed, DDT

stores

the value of the address indicated by E.

Then, when

the user next types either a [G or a [C (see below) command,
the breakpoint is placed just before control passes to the
user's program.

At that time the sequence of operations per-

formed by DDT is as follows:

The contents of location _k_are saved in a special register.
In place of the instruction in location
l_<_, DDT substitutes

the instruction, JMP l 4.
a

3.
to

Location 4 contains the address of

special breakpoint handling subroutine within DDT.
After the breakpoint has been placed, DDT passes control

the user's program.

When, during execution, the user's program encounters the location containing the breakpoint,
control is immediately passed (via location 4) to the breakpoint subroutine in DDT.
and C(L) at the point of interruption are saved in the special registers

The C(AC)

[A and [Y, respectively.

DDT then types out the address of the register containing the breakpoint, followed by a right

parenthesis and the contents of A as an octal number.

Control has now returned to DDT, and

the user is free to examine and modify his program.

Only one breakpoint may be in effect at one time.

As soon as the user requests a new break-

point using the B command, any previous existing breakpoint is removed.

To eliminate the

breakpoint entirely, the command is typed without an argument, thus:
[B
When the breakpoint is removed, the original contents of the break location are restored.
After the breakpoint has occurred and the user has examined his program and made the changes
he wishes, he can cause his program to continue from thngoint of the break by means of the

following command:

This continue command causes DDT first to execute the instruction which was originally in the break location, and

then pass control to the next location in the user's program.

The breakpoint remains in effect.

Example:

This example illustrates the use of the three commands just described.

The comments

explain the events.
FlLl+7[ B

Breakpoint request.

BGIN[G

Program execution is initiated at BGlN. Program runs until

breakpoint location is encountered.
Fl LI+OQO7)7721

DDT types the address of the break location and the contents
of the AC at the time of the break.

Note that location FlLl+7

is not opened.

The user performs such examination and modification as he
desires.

The user‘s program continues, beginning with the execution

of the instruction originally in FlLl+7.

The breakpoint re-

mains in effect.

Oftentimes, the user would like to place a breakpoint at a location within a loop in his pro-

loops can run to thousands of repetitions, some means must be available to pre-

gram.

Since

vent a

break from occurring every time the location is encountered.

This is done using the [C

command; after the breakpoint is encountered the first time, the user can specify how many times
the loop must be executed before another break is to occur, as follows:

Example:

After the first breakpoint occurrence, the user wishes to wait for 250

repetitions

before the next break.

FlLl+7[B

The break is requested.

BGIN[G

The break is placed; the program begins.

F l Ll+0007)772]

250[C

The first break occurs.
The program continues.

The next break will not occur until

the location FlLl+7 has been encountered 250 times.

3-12

The next break occurs after 250 times through the

FlL|.+OQO7225§£)

loop.

Restrictions on the Use of Breakpoints

The user must not place a breakpoint at any of the following places in his program:

Within any section of the program which operates with the program

interrupt enabled.
2.

At any location that contains an instruction which is modified during

the course of the program.

For example, if the prOgram contains a sequence

which includes the following instructions:

152 B

TAD A

breakpoint may not be inserted at location B.

When the user's program comes to a halt, control may be returned to DDT

by setting the Switch Register to 5400 and pressing LOAD ADDRESS and
START,
3.

in that order.

In a register containing a subroutine

one or more

arguments for that subroutine

jump (JMS) which is followed by
.

A breakpoint may be inserted at the point of a subroutine call if the JMS instruction is not fol-

lowed by any subroutine arguments, but the breakpoint may not be removed until control has
returned from the subroutine to the calling program.

WORD SEARCHES

The searching operations are used to determine if a given quantity is present in any of the registers of a

particular section of memory.

The search is initiated by the following command:
3—13

k[W

DDT will perform a word search and print the address and contents of
every register in the desired section of memory whose contents are

equal to the value of the expressionlg.
a

If the expression lg is omitted,

search for the quantity 0000 masked by C([ M) is assumed

The conditions for any search are set by the following criteria:
l

The contents of every register searched are masked by the contents of the

special register M, using the Boolean AND operation.
product is then compared with the value ofl_<_.

The resulting logical

If the two quantities are

identical, the address and contents of the examined register are printed on
the teletype-writer

The search is conducted over that section of memory whose lower limit is

given by the C([ L), and whose upper limit is given by the C([U), except for
the special case described in the next paragraph.

If the C([M)

7777 and the expression l_<_ contains any symbol in its address

part (for instance, ISZ FlLl+5,- FlLl is the symbol), the search will be conducted

only on the page for which that symbol is defined, regardless of the search
limits specified by C([ L) and C([ U).

For any other case,

including that where

the address tag of l_<_ is defined for page 0, the search is conducted according
to

the limits set.

A search never alters the contents of any register examined.

Addresses and register contents are printed as symbolic expressions or octal
to

integers, according

the mode at the time of the search.

Example:

Search locations 2600 to 3000 for all occurrences of the expression TAD DATA,
where DATA
itial values.

2740.

The C([M)

7777.

The C([ L) and C([U) are at their in—

[L\OOOT

2600

[U\5000

3000

’)
)

TAD DATA [w

LociTAD DATA
LOC+0015\TAD DATA
FILI+0002\TAD DATA
NoTe ThaT in This example, The
sion

C([L) and C([U) could have been leTT alone, since The expres-

l<_ conTained The symbol DATA in The address parT.

Had The user requesTed a search for The

expression TAD 2740, he would have had To seT The limiTs as shown for The desired search

Example: Search locaTions 2000 To 4000 for all occurrences of an ISZ insTrucTion.

[L\(_)92_l_
[U\_5_009_
[M\Z_7ZZ

2000)
4000
)
7000
)

ISZEW
The addresses raTher Than Tags are Typed ouT when symbols are noT defined.

2002\ISZ 2l35

2053\ISZ 2l35
2] l l\ISZ 0017

(eTc .)
The search will conTinue unTil all regisTers conTaining an 152 are found.
of The mask limiTs The

invesTigaTion To The firsT Three biTs of each regisTer,

Tion codes are considered

NoTe ThaT The seTTing
so

ThaT only insTruc—

Example: ObTain a dump of any secTion of memory. The search is conducTed beTween The limiTs
seT, and The addresses and conTenTs of all regisTers in The searched secTion are prinTed

[ L\_000l_

2600

[U\§9_<_$_5_
[M\Z_7_7_Z

3000

[ W

)
)
)

2600\ 0000
2601\ CLA

2602\TAD 2610
(etc .)
The search will continue to the specified limit,
the following points:
same,

i.e., O.

printing the contents of every register. Note

The mask is set to 0 to insure that results of every comparison are the

The search is conducted for all registers containing 0,

each comparison are equal to the desired quantity, 0.

that the results of

Always remember that the contents of

the registers themselves are 99.“ altered.

DEFINING NEW SYMBOLS

Often, during the course of a debugging run, the user will want to add new symbols to the external table.

This is especially so when he adds a sequence of instructions to his program as a

patch elsewhere in memory.

The patch is usually identified by a symbol which is the address

tag of the first instruction in the patch.

In order to use the symbol in subsequent debugging

operations, he must add its definition to the external table as follows:
l.

Set bit 0 of Switch Register down.

Type [R (ALT MODE or ESCape, and then R).

Type carriage return, line feed, in that order.

Type the symbol, at least one space, and an octal integer whose value

is the definition of the symbol.

If more than one symbol is to be defined, repeat steps 3 and 4 for each

definition.

After the last definition, type carriage return, line feed,

Press CONTINUE.

nal table.

EOT,

in that order.

DDT will type out the new lower limit of the exter-

Example:

To define the symbols PATCHI and PATCH2, the
operations will appear as follows

(Assume that the current limit of the table is 4775):

,1 (If)
PATCHI

610) (If)
PATCH2
620) (If)

(EOT)
(Press CONTINUE)
(new limit of the table)

4665

If the user makes an error while typing a definition, he cannot
mation.

use4— to

eliminate the infor—

The erroneous definition must be entered.

A symbol already in the table may not be redefined.

Only new symbols can be added.

NOTE: Extra carriage return, line feed pairs may not be inserted
between definitions; they will cause errors in subsequent table lookup when DDT is operating.

To completely expunge the external
run

symbol table (for instance, when starting a new debugging

with DDT already in memory), the following command is used:

[X
On receipt of this command, DDT removes all definitions in the external table.

table is unaffected

The permanent

MAKING A NEW SYMBOL TAPE
DDT may be used to make a new symbol definition tape.

If a number of new symbols have been

defined in the course of a debugging run, the user can put these definitions on tape for future

debugging purposes.
I.

The procedure is as follows.

Place the ASR 33 console OFF

Only the ASR 33 console punch may be used.

LINE; turn on the punch.

Punch a

length of leader tape by the following method: strike and hold

down_i_n orderthe keys SHIFT, CTRL, REPT,@

When enough tape has

been punched with leader-trailer code, release the keys in reverse order.

Type RUBOUT.

Type CR,

Type the symbol, at least one space, and the definition (an octal number).

Repeat steps 4 and 5 for each definition required.

After the last definition, type CR,

LF.

LF, EOT. (Note that steps 4—7 are

identical with steps 3-6 described in the preceding section,

Defining New

Symbols.)
8.

Punch a length of trailer by repeating step 2.

Turn the punch off; place the console ON LINE.

from the punch

Remove the new tape

A tape punched in the above manner can be read into DDT's external table by the method de-

scribed under Symbol Table Tapes (page ll).

Punching Binary Tapes
After making the desired corrections and changes, the user may punch out a new binary tape
of his program.

This allows the debugged preram to be used immediately, without waiting

for the programmer to incorporate the corrections in a new symbolic tape and reassemble the
program.

The punching procedure given below may be used for either the Teletype console

punch or the optional high-speed punch. The device is indicated by the setting of“ bit 0 of
Switch Register.

Bit O of Switch

Device to be Used

Console punch (low speed)

down

High—speed punch

3—18

In the following description, instructions in parentheses apply to the use of the console punch.

If the high-speed punch is used, these instructions for turning the punch off and on may be

ignored.
[T

This command is used to obtain a segment of leader~trailer.

a;b[ P

This command causes DDT to punch a block of binary tape with
the information contained in the section of core memory designated by the expressions 3 (lower limit) and _b_ (upper limit),

inclusive.

gand bmay be any kind of acceptable terms.

This command is used at the end of punching operations and causes

DDT to punch a checksum block, fol lowed by a length of trailer tape

The punching procedure,
I

using these three commands,

is as follows:

(Turn the punch off), type [T.
and press CONTINUE

Turn punch

When punching is completed (turn punch off) type the lower limit,

semicolon, the upper limit, and [P.
4.

(Turn on punch), press CONTINUE.

To punch more blocks, repeat steps 3 and 4 for each block.

After the last block has been punched

(Turn on punch), press CONTINUE. The computer will punch a check-

sum

block and a length of trailer.

When the entire operation is finished

(turn off punch) type [E

(turn off punch) remove the tape,

depress CONTINUE. This binary tape may be loaded by the BIN Loader.

CAUTION
The user should not try to punch the section of memory between
5000 and 7600 which contains DDT.

If the user wishes to restart DDT before he has punched a complete tape

(i.e., between data

blocks) he must set the console switches to 540i to preserve the checksum. Subsequent restarts
must also

be to 540] until the checksum block has been punched.

3-20

APPENDIX A

SUMMARY OF COMMANDS

Character

Action

space

Separation character.

.1.

Arithmetic plus.
Arithmetic minus.

When it follows the address

printed.
carriage return

Make modifications, if any, and close register.

line-feed

Make modifications, if any, close register, and open next se-

quential register.
When it immediately follows a register printout, it causes the

Type last quantity as an octal integer.
.

(period)

Current location

‘.___

Delete the line currently being typed.

Sets DDT to type out in symbolic mode

Sets DDT to type out in octal mode.

N[W

_V_V_o_r£l searchforall occurrences of the expression N masked with C( [M)

k[B

Insert a breakpoint at the location specified by lg.
is specified,

remove

any breakpoint.

Continue fromabreakpoint n times automatically.
it is assumed to be i

If no address

If n is absent,

Qgto the location specified by k.

liegd symbol table into external symbol table or define symbols
on

line.

Punch leader-trailer code.

A-i

Oibl P

PM binary tape from memory bounded by the addresses a and b.

Punch End of tape:

The following

symbols

are

checksum and trailer.

the address tags of certain registers in DDT whose contents are avail-

able to the user.

Accumulator storage (at breakpoints).

Link storage (at breakpoints).

Mask used in search.

[.Qy_e_r limit of search.

[4999; limit of search.

A—2

APPENDIX B

INTERNAL SYMBOL TABLE

AN D

CMA

7040

TAD

1 000

CM L

7020

IS Z

2000

RAR

701 0

DC A

3000

RAL

7004

J MS

4000

RTR

701 2

J MP

5000

RTL

7006

IOT

6000

lAC

700]

0 PR

7000

SMA

7500

C LA

7200

S ZA

7440

KC C

6032

S PA

751 0

K RS

6034

SNA

7450

KR B

6036

SN L

7420

TSF

604]

SZL

7430

TC F

6042

S KP

74] 0

TPC

7404

T L5

IO N

IOF

6044

0 SR

6046
6001

H LT

7402

C IA

7041

6002

LAS

7604

KSF

6031

400

C LL

7] 00

B—1

INDEX

Accumulator
Address Tags

Addressing

A-2

3-17

3-9,A-2

A-2

Control

3-8
Cross- Page
3-7
Indirect

ALT MODE Key

Output

3-9, 3-18

Program

3-10

2-3, 2-5

Debugging
B

Back-Arrow Key (see Left-Arrow Key)

With DDT 2-2
Without DDT 2-l

Binary Loader (BIN)
Loading Programs

Breakpoint

2-5, 3-1 I

A-2

Command 2-6, A-I
Restrictions 3-13

Equal Sign

2-5, 3-5, A-l

Characters, Special

Closing Registers

A-I

3-4, A-I

ESCape Key (see ALT MODE)
3-3

External Symbol Table
Contents

2-3

Deleting

3-17

2-5, A-l
F

Commands
A
B

Functions, Basic

3-9, A-2
3-10, A-I
3-12, A-I

Input/Output

Examination 3-5
3-10
Execution
G
3-10, A-I
L

3-10, A-2

2-3

3-10
3-19, A-2

Control
E

3-3

Error Messages

Expression

Carriage Return

2-2

Combinations 3-9
Control 3-4, 3-18
Instructions

3-] , 3-]

Instructions

Mode Control

3-3, -4

Combined

Modification
3-4
O

3-5

3-1, 3-1
Operate 3-1, 3-8

3-19, A-2
R
3-16, A-l
S
3-4, A-l
Summary A-l
T
3-19, A-l
U
3-10, A-2
W
3-14, A-2

IOT

Integer
-2
,

3-8

3-2

Internal Symbol Table

2-3, 3-1, 3-8, 3-]
Locations 2-2, -3

Contents

INDEX

(Cont)

Leader/Trailer Tape
Left-Arrow Key

Left Bracket

3-18, -I9

3-8, A-I

2-5, A—I

LINE FEED Key

3—5, A-I

3-16

Key

3-8

Punching Binary Tapes

Read-In Mode Loader (RIM)
2—2

2-2, 3-]
Procedures
3—], —2
2-2
Programs
DDT

2-2

2-3, 3-2

Symbol Tapes
Locations

2-2

Loading BIN
Register

Closing 2-5, A-I
Opening 2-4, A-I
Special 3-9
Requirements

3-1

Breakpoint
DDT

Plus Sign

RIM

Table)

2-2

Loaders, Tape

BIN

Permanent Symbol Table (see Internal

2-2
2-2

SymbolTables

Software

2-2

Storage

2-2, 3-I

Restrictions, Breakpoint
M

(see Carriage Return)

RETURN Key
3-3

Messages, Error

3-13

3-18

RUBOUT Key

Mode Control

Octal

3-3

Symbolic

2-4

Sample Program
Search Command

Number

Slash Key

2-4, 3-5, A-I

3-2

Storage

OctalMode

Breakpoint

3-4

Symbol Table

DDT

2-4, A-I

3-3

Operator

Binary Tape

2-2

Summary, Command

A-I , -2

3-18

Input 3-2
Output 3-18, -I9

3-18

Symbol Table

Internal

3-17

Storage
Tapes 3-2

External

Period Key

2-3, 3-2

Switch Register Options

3-9

Device

A-2

Opening Registers

Output

3-13, A-2

2-3, 3-2, 3-16
2-2, 3-1 B-2
2-2, 3-2
,

Symbol

IN D EX

3-4

Symbolic Mode
Symbols

Defining New 3-16, -l7
Tape 2-3, 3-2
T

Tables , Symbol

External
Infernal

2-3, 3-2, 3-16
2-2, 3-I B-l
,

Tape
BIN

2—2

DDT

2-2, 3-1

Leader/Trailer

3-18

Ou’rpuf

3-18

Program

2-2, 3-2

RIM

2-2

symbol

2—3, 3-2, 3—18
U

3-6, A-l

Up-Arrow Key

User-Defined Symbols (see External Symbols)

W
Word Search

3-13

(Cont)

DDT
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