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DEC-XV-UDDTA-A-D

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Document:	DDT XVM Utility Manual
Order Number:	DEC-XV-UDDTA-A-D
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Pages:	72
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OCR Text

DDT XVM UTILITY
MANUAL
DEC-XV-UDDTA-A-D

digital equipment corporation • maynard. massachusetts

First Printing, December 1975
The information in this document is subject to change without notice
and should not be construed as a commitment by Digital Equipment
Corporation. Digital Equipment Corporation assumes no responsibility
for any errors that may appear in this document.
The software described in this document is furnished under a license
and may be used or copied only in accordance with the terms of such
license.
Digital Equipment Corporation assumes no responsibility for the use or
reliability of its software on equipment that is not supplied by
DIGITAL.

(S) 1975 by Digital Equipment Corporation

The postage prepaid READER'S COMMENTS form on the last page of this
document requests the user's critical evaluation to assist us in preparing future documentation.

The following are trademarks of Digital Equipment Corporation:

DIGITAL
DEC
PDP
DECUS
UNIBUS
COMPUTER LABS
COMTEX
DDT
DECCOMM

DECsystem-lO
DECtape
DIBOL
EDUSYSTEM
FLIP CHIP
FOCAL
INDAC
LAB-8

MASSBUS
OMNIBUS
OS/8
PHA
RSTS
RSX
TYPESET-8
TYPESET-10
TYPESET-ll

1/76-15

- -"

CONTENTS

Page
PREFACE
CHAPTER

CHAPTER

vii
1

INTRODUCTION

1.1
1.2
1.3

GENERAL INFORMATION
OPERA.TION
CONVENTIONS AND SPECIAL SYMBOLS

BASIC DDT

2.1
2.2
2.2.1
2.2.2
2.2.3
2.3
2.3.1
2.3.2
2.4
2.5
2.6
2.7
2.8
2.8.1
2.8.2
2.8.3
2.8.4
2.8.5
2.9
2.10
2.11

LOADING DDT AND USER PROGRAMS
EXAM.INING STORAGE WORDS
Opening a Location
"Last-Opened-Register Pointer"
Closing/Reopening Locations
TYPE-OUT MODES
Address Modes
Instruction Modes
RETYPE COMMANDS
MODIFYING STORAGE WORDS
INPUT MODES
SEQUENCING
BREAKPOINTS
Setting Breakpoints
Breakpoint Restrictions
Breakpoint Type-Out
ReaSSigning and Removing Breakpoints
Proceeding After a Break
STARTING A PROGRAM
STOPPING A PROGRAM
ERRORS

DDT LANGUAGE AND SYNTAX

3.1
3.2
3.2.1
3.2.2
3.2.3
3.2.4
3.2.5
3.2.6
3.2.7

COMMAND STRUCTURE
ARGUMENTS
Syllables
The Symbol Table
Expressions
Symbolic Instruction Mode
Octal Number Mode
Transfer Vector Mode
ASCII Text (Output Only)

DEBUGGING WITH DDT

4.1
4.2
4.3
4.3.1
4.3.2
4.3.3
4.3.4

LOADING A PROGRAM
STARTING A PROGRAM
REGISTER EXN1INATION AND MODIFICATION
Type-Out Mode Commands
Special Symbols and Concepts
Register Examination Commands
Expression Retype Commands

iii

1-1
1-1
1-2

2-1
2-2
2-2
2-3
2-3
2-3
2-4
2-4
2-5
2-6
2-6
2-6
2-7
2-7
2-8
2-8
2-9
2-9
2-9
2-9
2-9

3-1
3-2
3-2
3-3
3-!'i

3-6
3-8
3-8
3-9

4-1
4-1
4-2
4-2
4-2
4-6
4-7

contents (Cont. )

Page

4.3.5
4.4
4.5
4.6
4.6.1
4.6.2
4.6.3
4.6.4
4.6.5
4.6.6
4.7
4.7.1
4.7.2
4.7.3
4.7.4
4.7.5
4.7.6
4.7.7
4.8

Register Modification Commands
DEFINING A SYMBOL
SEARCH OPERATIONS
BREAKPOINTS
Definition
Setting Breakpoints
Breakpoint Restrictions
Flow of Control at Breakpoints
What Happens on a Break
The Execute Command
MISCELLANEOUS FEATURES
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

4-7
4-9
4-10
4-13
4-13
4-13
4-15
4-16
4-19
4-21
4-23
4-23
4-23
4-23
4-23
4-24
4-24
4-24
4-24

APPENDIX A

SUMMARY OF DDT COMMAtmS

A-l

APPENDIX B

MNEMONIC INSTRUCTION TABLE

B-1

APPENDIX C

DDT MEMORY LOAD MAPS

C-l

Index

Index-l
TABLES

TABLE

1-1

Symbols Used in Text and Examples

1-2

-,
- ,

LIST OF ALL XVM MANUALS

The following is a list of all XVM manuals and their DEC numbers, including the latest version available.

Within this manual, other XVM

manuals are referenced by title only.

Refer to this list for the

DEC numbers of these referenced manuals.
BOSS XVM USER'S MANUAL

DEC-XV-OBUAA-A-D

CHAIN XVM/EXECUTE XVM UTILITY MANUAL

DEC-XV-UCHNA-A-D

DDT XVM UTILITY MANUAL

DEC-XV-UDDTA-A-D

EDIT/EDITVP/EDITVT XVM UTILITY MANUAL

DEC-XV-UETUA-A-D

8TRAN XVM UTILITY MANUAL

DEC-XV-UTRNA-A-D

FOCAL XVM LANGUAGE MANUAL

DEC-XV-LFLGA-A-D

FORTRAN IV XVM LANGUAGE MANUAL

DEC-XV-LF4MA-A-D

FORTRAN IV XVM OPERATING ENVIRONMENT MANUAL

DEC-XV-LF4EA-A-D

LINKING LOADER XVM UTILITY MANUAL

DEC-XV-ULLUA-A-D

MACII XVM ASSEMBLER LANGUAGE MANUAL

DEC-XV-LMLAA-A-D

MACRO XVM ASSEMBLER LANGUAGE MANUAL

DEC-XV-LMALA-A-D

MTDUMP XVM UTILITY MANUAL

DEC-XV-UMTUA-A-D

PATCH XVM UTILITY MANUAL

DEC-XV-UPUMA-A-D

PIP XVM UTILITY MANUAL

DEC-XV-UPPUA-A-D

SGEN XVM UTILITY MANUAL

DEC-XV-USUTA-A-D

SRCCOM XVM UTILITY MANUAL
UPDATE XVM UTILITY MANUAL
VP15A XVM GRAPHICS SOFTWARE MANUAL

DEC-XV-USRCA-A-D
DEC-XV-UUPDA-A-D
DEC-XV-GVPAA-A-D

VTI5 XVM GRAPHICS SOFTWARE MANUAL

DEC-XV-GVTAA-A-D

XVM/DOS KEYBOARD COMMAND GUIDE
XVM/DOS READER'S GUIDE AND MASTER INDEX

DEC-XV-ODKBA-A-D
DEC-XV-ODGIA-A-D

XVM/DOS SYSTEM MANUAL

DEC-XV-ODSAA-A-D

XVM/DOS USERS MANUAL

DEC-XV-ODMAA-A-D

XVM/DOS VIA SYSTEM INSTALLATION GUIDE

DEC-XV-ODSIA-A-D

XVM/RSX SYSTEM ~illNUAL

DEC-XV-IRSMA-A-D

XVM UNICHANNEL SOFTWARE MANUAL

DEC-XV-XUSMA-A-D

PREFACE
DDT XVM (DDT), which stands for Dynamic Debugging Technique , is an online interactive debugging program in the XVM!DOS software system.

the preparation of this manual, it was assumed that the reader is familiar with XVM!DOS e.g., its Monitor and Utility Programs.
Useful manuals to read in conjunction with this one are:
XVM!DOS Users Manual
FORTRAN IV XVM Language Manual
FORTRAN IV XVM Operating Environment Manual
MACRO XVM Assembler Language Manual
Linking Loader XVM Utility Manual

vii

CHAPTER 1
INTRODUCTION
1.1

GENERAL INFORMATION

DDT XVM (Dynamic Debugging Technique)

(DDT) is a conversational system

program which is available in XVM/DOS.

It provides both MACRO and

FORTRAN programmers (see Appendix D) with a convenient means for debugging and closely monitoring the operation of their programs.

DDT

commands entered via the console terminal permit the user to:

1) start

a program, 2) suspend its execution at predetermined points, 3) examine
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 "runaway" 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 infor-

mation required by DDT or output (printed) from DDT requires the user
to be familiar with machine language programming.

The format of DDT

console terminal 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 below the bootstrap by the Linking Loader.

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 Automatic Priority Interrupt are enabled, as well as
XVM addressing mode, if requested.

1-1

Introduction

The user converses with DDT via the console terminal.

Console terminal

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.

1.3

CONVENTIONS AND SPECIAL SYHBOLS

Table 1-1 lists special symbols which are used throughout this
manual to represent console terminal 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
SYHBOL

TELETYPE 1
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

CTRL and
I/TAB

Hoves carriage to next tab location (normally 8 spaces)

1\ or
SHIFT and

Control entry defined within programs

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

Char

---

OPERATION
INITIATED

N/
CTRL and
Character
Key
CTRL & T

Initiates a program or system
control operation defined within
the system

ALTHODE

Terminator whose use is defined
within the system program

[

Use defined within program

]

[ or
SHIFT and
K/VT
] or
SHIFT and

Use defined within program

RUBOUT or
SHIFT and
L/FORM

Character Rubout

ITeletype is a registered trademark of the Teletype Corporation.

1-2

....."

CFAPTER 2
BASIC DDT
This section introduces the main features of DDT to the beginning 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

LOADING DDT AND USER PROGRAMS

In XVM/DOS, the Keyboard 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 terminal is:
BANK MODE ON:

PAGE MODE ON:
LOADER XVM Vnxnnn

BLOADER XVM Vnxnnn
>

Typing a command string to the Loader, e.g.,
>P+MAIN,SUBR1,SUBR2(V
then loads the user program.
For detailed instructions, read the Linking Loader XVM
Utility Manual.

2-1

Basic DDT

When loading is complete, DDT takes control and types:
PAGE MODE ON:

BANK MODE ON:

DDT XVM Vnxnnn

BDDT XVM Vnxnnn
>

to indicate its readiness to accept DDT commands.
NOTE
NS is printed before DDT if the symbol table could
not be loaded into available core.
In XVM/DOS,

.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.
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,
2.2.1

"close" the location.

Openinq a Location

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

To open location ADR+l, type:
ADR+1/

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+1/

-lLAC~TEMP-5 -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

Basic DDT

2.2.2

"Last-Opened-Register Pointer"

Once a location is opened, DDT sets its address into a special pointer
register termed the "Last-Opened-Register-pointer".

The pointer is

represented in DDT by a period (.) 7 thus when location ADR+l is opened
the pointer (.) is set to its address (Le.,
2.2.3

.=ADR+l).

Closing/Reopening Locations

Opened registers are closed by entering a carriage-return (.-J).
EXAMPLE:
>ADR+l/

-/ LACL....ITEMP-S-/ ..J

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

(.).

The pointer symbol is typed fol-

lowed immediately by a slash (/).
EY.AMPLE:

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

2.3

EXAMPLE:

If ADR+l was the last opened register, the reopen
procedure would be:

Entry

Response

-I LACL....ITEMP-S-I

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).
by DDT.

This is the type-out mode initially assumed

All numeric quantities are printed in octal.

The symbolic mode is useful if the user expects the opened register to
contain a machine instruction.

However, that register might be inter-

preted instead, as octal data, as a transfer vector 1 , or as symbolic
text.

Additional commands to DDT are available which specify the form

IThe term transfer vector means a word which contains an address pointing to some other location in memory. Transfer vectors are used with
indirect address machine instructions because memory reference instructions cannot directly access more than 4K (in page mode) or 8K (in
bank mode) of memory.

2-3

Basic DDT

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 (Default).
Print addresses as absolute octal numbers
(18-bi t value).
Print addresses as octal numbers relative
to the lowest location 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,

2.3.2

Instruction Modes

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

Meaning
Print symbolic instructions; if the instruction is a
memory reference instruction, the address field will
be printed according to the current address mode (Default).
Interpret words as transfer vectors. Print the
15-bit value as an address in the form dictated
by the current address mode.
If the higher order
three bits are non-zero, precede the address by
the six digit octal value of these three bits
concatenated with a plus sign.
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.
Interpret data as SIXBIT Text.
$$R

Interpret data as RAD50 Text.
2-4

Basic DDT
EXAMPLE:
Assume that registers ADR and ADR+l
_____ contain, respectively, 206613
and 155102 octal.

>~
>
R/
LAC TEMP
> ADR/
LAC #+42
> ADR/ -+I LAC 6613
> ADR/ ~ LAC TEMP
> ADR/
206613
> ADR/ ~ 200000+TEMP
> ADR/ ~ #+42
> ADR/
! XYZ!

--I

-+I
-+I
~

-.j

--l

$F J

$A3
$R
$0)
$V)
$F)
$T)

Since #+42 is equivalent to the 15-bit value 6613, the relocation factor (#) is 6613-42
2.4

= 6551.

RETYPE COMMANDS

Often, while examininq reqisters in the prevailinq tvpe-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.

$?
$$?

Retype the value in SIXBIT.
Retype the value in RAD50.

EXAMPLE
Assume that TEMP is absolute location 6613.
Entry

Response

>$R)
>$0 J
>ADR/

-.j 226613

Entry/Response

-I ~LAC* TEMP

or
>ADR/

~226613

-I .l.220000+XYZ

or
-1226613
or

-.j ~ %XYZ%

>~=OOOOOO

2-5

Basic DDT

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/ -+j-JMP

BEG

-.o\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,
the expression should follow the same format as is recognized by the
MACRO assembler.

However, instruction and address fields cannot be

separated by a tab (-l) 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,
~,

are evaluated by DDT and taken as 1S-bit values.

There is no

provision for typing 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.
commands are listed below:

2-6

(b) close

A few of these sequencing

Basic DDT

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.

EXAMPLE:
The following illustrates the usefulness of these commands.
Opens registers ADR, ADR+l
and ADR+2

ADR/ ~
ADR+l ->i
ADR+2 -+f
~

TV+ 1
TV

->t

-+t

LACL-ITEMP+l
DAC L-I rEMP 2
JMS*·L-I TV
CAL * L-I ARG-l 0
DACL-INTX
CAL* ~ARG-lO

!
!

l 7

Opens register addressed

i·

~by contents of ADR+2

$Y/

:i: S UBR1-{i.

Retype the value of the lastopened register as a Transfer
Vector and open the next
register
.<-_ _~

2.8

Open the register preceding the
last opened register

BREAKPOINTS

One useful function 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 calleo "breakpoints".
ized by the command:
2.8.1

Breakpoints are symbol-

~B.

Setting Breakpoints

DDT allows up to four breakpoints, which are entered as numbers 1, 2,
3, and 4.

I The indirect bit and the index bit (if in page mode) are ignored.

2-7

Basic DDT

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.
2.8.2

Breakpoint Restrictions

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 all registers

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

-I 777777 -I
..., 000001 -+j

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

Basic DDT

2.8.4

Reassigning and Removing Breakpoints

Breakpoints may be reassigned or removed as follows:
a)

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:
O$B

To remove a specific breakpoint, for instance,
breakpoint 3, type:
3;0$B

2.8.5

Proceeding After a Break

After a break occurs, program execution may be resumed by typing:
$p
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.
2.9

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 break

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

ERRORS

If a typinq mistake is made by the user and DDT has not taken
action, typing either RUB OUT or CTRL U will erase the entire input
and allow it to be retyped.

2-9

Basic DDT

If control is returned to DDT, causinq 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 (?).

2-10

CHAPTER 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

COMMAND 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 t, =, or I), or the
character ALTMODE (echoed as $) followed by another character, such
as, $A, $T, or $S.
The following examples illustrate the forms which DDT commands may
take:

Isingle argument commands

Ino argument
$B

/both args missing [two required for B]

arg$B

/arg 1 missing

arg; $B

larg 2 missing

arg;arg$B

/both 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.

3-1

DDT Language and Syntax

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.
3.2.1

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 further

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

INTERPRETATION

TYPE

8 is not an octal digit.

A symbol

%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.

3-2

DDT Language and Syntax

INTERPRETATION

3.2.2

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 C).

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
(e.g., A# [the saved accumulator], L# [the saved Link], etc.), and any
definitions of symbols created by the user in commands to DDT.

This

part of the symbol table resides in the area of memory 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

memory 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.

3-3

DDT Language and Syntax

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
5YM, 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 ('), 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 ("), 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
will bypass the instruction mnemonic table when searchil'tg 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 secone JMP 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 being symbol constitu-

ents) much as the character # in MACRO may be located anywhere within
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 ex-

ample, in
JMP

3+ADR

the search for symbol ADR bypasses the instruction- mnemonic table.
The special characters ' and" are ignored when they appear within
octal numbers.

3-4

DDT Language and Syntax

NOTE
Symbols defined in a MACRO program by the use of
(as in SYM=lOO)
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 ad-

dresses 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 pre-

vailing 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
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+.

This implies that the value of the first argument

in a two-argument command is zero when the argument separator is
typed with no preceding argument, e.g.,
jADR$B is equivalent to OiADR$B

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

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

3-5

DDT Language and Syntax

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 ''inclusive 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.
3.2.4

Symbolic 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:
l.

JMPL-IJMP

(JMP is an address in the current program)

DACJ+AD

(Although AD evaluates to 18 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)

3-6

DDT Language and syntax

XCT *L-I AD

LAC L-I C, X

CAL '-A 77 5

Indexed instruction

(Once AAC, AXS, or AXR is encountered,
the remainder of the expression will not
alter the value of bit 0 through 8.)
On output if DDT is in symbolic mode, the following prc.cedure 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.,
775) •

If the OP code is 74 and bit 4 is off, the instruction
is output as an inclusive ORed microcoded operate instruction (740000 types out as NOP, 754000 as CLA!CLL).

If the OP code is 76 (74 and bit 4 is on), the instruction is output as LAW L-rN, where 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.

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 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
number representing bits 4-17 of the instruction, (e.g.,
IOT+314 for IORS and AXS~l for 725%%1).

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 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
"point" (the address of the last opened register), unless an
auto-index register is referred to indirectly (e.g., LAC*L-I1%).
If the index bit is set, the address is followed by ,X.
The
following are examples of memory reference instructions printed
by DDT:

3-7

DDT Language and Syntax

3.2.5

JMPL...IJMP

(JMP is an address in the current program)

DACL...IAD+1

(Relative address mode)

XCT*L...IAD

LACL...IAB,X

(Indexed address)

DACL...I21253

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

DACL...I#+2

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

Octal Number Mode

The conventions for numerical input were explained in 3.2.1.

On output,

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

(e.g., 060¢1¢ for DAC*L...I1¢).

Transfer Vector Mode

In this mode, bits ~-2 of the word to be output are ignored if they
are zero.

If bits

~-2

are non-zero, they are printed as an 18-bit

octal number preceding the address, e.g.,
LABELl

-+\W~~~il+ALPHA-+\

If 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.
within

1il~

If its value is

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 cur-

rent subprogram always take precedence over globals.
address mode, the 18-bit address is output in octal.

3-8

In absolute

DDT Language and Syntax

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"

(the last opened register) .

In symbolic address mode, the address may be printed as in floating
mode, if a symbol within
symbol table.

1~~

octal of the address is not found in the

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

3-9

4~6~5~,

342212).

CHAPTER 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:
K$G
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.
there, type:

If K is missing,

To first change the value of SA# and then go

(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 "proceed count" and "proceeding after breakpoint":

4-1

Debugging with DDT

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 ( » .
4.3

4.3.1

Type-Out Mode Commands

'I'he type-out mode commands (see section 2.3 and also Appendix A, 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:

-I>

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

Debugging with DDT

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

Immediately after a breakpoint is reached and control
is returned to DDT, the value of "pointer" 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 "pointer" 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
which are two forms of retype commands.
The user may
restart the program at the pointer where it was interrupted by typing:
$p

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 XVM Vnxnnn
>

At this time the value of "pointer" 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 "pointer" is set to
the address of the last register typed out during
the search.
If no register was typed out, the
value of "pointer" is not changed.

The "current location", which is represented by the symbol.
in MACRO programs, differs at times from the value of "pointer"
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 "pointer" are the aadress just opened. The
values of "pointer" and the current location will differ when
the sequencing command 1! is used (see below).

The areas of memory 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 memory space "'Thich exists immediately above the Loader-built

4-3

Debugging with DDT
symbol table (see memory map in Appendix C) is treated as a pseudoprogram to which DDT has assigned the file name P#' (recall that
the ' declares the symbol to be a file name). ~his symbol represents the first location (relocation factor) of the patch file area
(free core). This area is used to insert additional code at runtime.

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:

Example
1
2
3

5
6
7
8
9

>g)

>PRGA'=003000
~ )
>PRGB'=004000
~ ~
>PRGB'/ -to\LAC
#+100-1 -#=004000
>.=004000--1 ~BEGIN
~
>PRGA' /
--I DAC #+3 ~ #=003000
>.=003000-\.......:...START
>BEGIN/ ....jLAC
#+100
#=004000

T
7"

>.=004000~

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
starts at location 4000).

(program PRGB

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 BEGLJ which refers to the
first location in that program. Assume for the moment that
PRGA and PRGB do not use any identical symbols.

4-4

Debugging with DDT
On line 6, PRGA' is opened. This changes the value of .
to the value of PRGA' 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.
E.

DDT will not allow core outside a specified range to
be modified by command 1 . The area that may be modified includes the auto-index registers, the core area
between the symbols P#' and e# and the core area
between TP# and the top of core. TP# is the address
of the first location above the bootstrap. P#' is the
address of the lowest register in the patch area. e#
is an address within DDT which contains the two's
complement value of the "proceed count".
Immediately
below e# 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 modifiec.
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 anyvvhere
in a user program or in the patch area, but is may not
be placed at an auto-index register nor within DDT.
In systems with XVM mode enabled, uninitialized COMMON
blocks are loaded starting at TP# and going up in
memory, if there is room.

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 P#'. The user may, however,
define a file name with a value less than P#' 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.

ISee section 4.7.7 for exceptions.

4-5

Debugging with DDT
Example:
>LOCI ~
~

>./

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

The basic register examination command is
where

Typing the command KI,

is an expression, will open register K and print its contents

in the current modes.
equivalent to

The default argument is "point"

(i.e.,

L is

L) .

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

(If 1 is preceded by ALTMODE

(1), then octal becomes the

permanent instruction mode.)
If

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

mode.

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

manent mode.)
If

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 nonexistent register.
Examples:
>~PR-l!

JMP* SUBR

>. [

623115

>~L

JMP* SUBR

>. $ [

>~:T

623115
623115

>.:.1.
>. <

JMP* SUBR
i-

ADR
ADR+l
>

(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)

4-6

Debugging with DDT

4.3.4

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 retype the contents of "point" and the following register as 5/7 ASCII text (!5. 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 !<) 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 (!5. becomes the value of Q#).
ister is closed without modification.

If!5. 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-

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
)

-2 there are "sequencing commands", which behave like

in that, if given an argument !5., !5. replaces the old contents of

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

Then a new register is opened.

low, with the exception of
current register.

For every command listed be-

the newly opened register becomes the

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.

4-7

Debugging with DDT

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 instruction.
The address part (12 or 13 bits,
page or bank mode, respectively) is taken, and that
address (in the same memory page or 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 g.
This command is similar to iY.
It modifies the open
register, closes it and then, treating K as a IS-bit
transfer vector, opens that 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
1
and P#' through C# and TP# through the top of core) .
In either case, the command is ignored.
Example

>LOC/
LOC+l
LOC+2

~ i~W - I

:\ ~AC LIT12$Z

DAC TMP
~+
JMS* TV
~ ~Y
DAC ARG- 77-.1 :- S UBRI -+j $J

~o
SUBRl+l~ CLL!CLA

-IT
-I T

>SUBRl+l~ CLL!CLA

~~TL!CLA)

SUBRl+2-.1 SAD XYZ-2 -I t

Note that the addresses opened by the commands $Y,
printed by DDT.

and $J are not

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

printed its contents as if i t were an instruction.

This did not look

right, so _ was typed to request reprint as a transfer vector.
The use of $Y or $Z following indexed and indirect instructions, such
as,

Isee Section 4.7.7.

4-8

Debugging with DDT
will not perform the indexing nor indirection to determine the address
of the next register to be examined.
would be opened.

In this example, register TAB

If the user wishes to effectively iDsert 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 point to patch
space in free core (which might not be in the same core bank or page
as the user's program).

Patch space begins at the register

P~'

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

and
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 C) .
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 Kill (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).
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 command and type an 0

(Overflow).

4-9

Debugging with DDT
4.5

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#

-..j

--I
-..j

be individually modified, e.g.,

777777
012252
025611

7700004l4000 414400 )

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.
free core patch area)

This sets the value in LO# to A and

The default arguments are P#'
and L#

(beginning of

(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 l's
in the corresponding bits in the mask argument ~.
For example, to search for all words between locations 100 and 200,
equal in value to JMP BEG, type:

4-10

Debugging with DDT
100;200$L
777177 ;JMP BEG$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 $W command does not cause the contents of M#
to be modified.
To search for all words between locations 100 and 200 whose Op code
parts contain JMP, type:
100;200$L
740000iJMP$W
Note that the mask is set so that only the high order 4 bits (the
Op-code bits) are tested for a match.
To print out the contents of registers 100 through 120, type:
100;120$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:

MiK$N
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:
100;200$L
$N

4-11

Debugging with DDT

(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:
MiK$E

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 l's in the mask.

(Bits

0-2 are always 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:
1000;2000$L
777770; 10$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
The contents of the hardware index register are saved in and restorec
from XR#, and 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).
if no printout occurred.

4-12

"Point" remains unchanged

Debugging with DDT
4. 6

BREAKPOINTS

4.6.1

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.
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.
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:
NiA$B
where N is a breakpoint number (1 to 4) and ~ is an expression evaluated to a lS-bit address.
N

This causes DDT to assign (set) breakpoint

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

For example, to set breakpoint 2 at location ADR, type:
2iADR$B
It is possible to reset a breakpoint to some other address without
first deassigning that breakpoint, e.g., if breakpoint 2 had been set
at location XYZ, the preceding command (2iADR$B) would supersede the
earlier (XYZ) assignment.
lA command exists to tell DDT to use some other auto-index register
for breakpoints (discussed later).
4-13

Debugging with DDT

If the argument A is missing, the default argument is "point".
example:

For

ADR/ ~ LAC TMP ~ 3; $B
This sets breakpoint 1. at location ADR because "point" has the value
of register ADR.

(3;$B is equivalent to 3;.$B.)

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.

For instance, as-

suming all four breakpoints are available, 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 a question mark:
X$B?
and ignores the command.
If register X is outside the legal range of registers which may have
breakpoints (p#' to C#),l DDT types:
X$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.
1

See Section 4.7.7.
4-14

Debugging with DDT

When a breakpoint is set at a given location, the contents of that
location are not changed, e.g.,

>&. -I LAC
>X/

-I LAC

TMP
TMP

-I $B

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

lSee section 4.7.7.

4-15

Debuggina with DDT

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" .
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., NOP), 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 executed.

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 com4-16

Debugging with DDT

mand, the contents of the associated conditional break instruction
register, 1#+N, is set to NOP.

NOP does not skip, so the break will

be determined by decrementing the proceed count.

After the $B com-

mand, one may modify 1#+N to contain any instruction one likes (e.g.,
CLA!SNL or LAC* 10 or JMS* V#+N).

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 ~ set aside by DDT
specifically to be used to store the transfer vector for a conditional
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<

JMS*

V#+2$Z)

~ TEST~)

(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.
$N

(with argument

missing) sets 1#+N to NOP (no skip)

O;$N

sets 1#+N to SKP (skip 1)

l;$N

sets 1#+N to skip 2 locations

4-17

Debugging with DDT

Only the last bit in argument X is used to determine what is placed
in I#+N.
The effective flow chart for the break/no break decision is as
follows:

EXECUTE CONDITIONAL BREAK
INSTRUCTION: XCT I#+N
1
SINGLE
SKIP

NOSKIP

DOUBLE
SKIP

$p COMMAND (PROCEED)
CONTINUE
USER
PROGRAM

Flow Chart, Break/No Break Decision

4-18

Debugging with DDT

4.6.5

What Happens on a Break

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 XVM Vnxnnn
>
It is not possible to proceed from such a breakpoint.

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, DDT
will type the contents of the API status register:
AP I

-to! 4 XXX XX

One may not proceed from such a breakpoint.
Any
other command is valid, including a command to
delete this breakpoint.
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

R#+N (N

-to! 7 7 6 4 0 3)

= 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 A#

(the stored AC).

There-

fore, if the user does not alter R#+N, the contents of the AC will be
printed ~hen 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.,

4-19

Debugging with DDT

the second is by using the command:
A;B$N
The first argument ~, changes the contents of I#+N (N

= 1 to 4) as

Argument ~ specifies the address whose

discussed in section 4.6.4.

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#, X#, and LR#, respectively.
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 command places the value of the expression !i 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

4-20

Debugging with DDT
gives control to the user program at the location immediately following the breakpoint.
If control returns to DDT because the user types
tT (CTRL T),

may be used to continue program execution from where

it 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 XVM Vnxnnn
>

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
(If breakpoint ~ is not set, BN#

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

DDT never uses these symbols on output, but

the user may use them on input to DDT.

If breakpoint 1 is set at

Bl#/ -I CLA
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:
11$0
will change the breakpoint instruction to JMS* 11.
of the argument A is 17
4.6.6

The default value

(iQ is 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:

4-21

Debugging with DDT
I$X
the user may execute the instruction I

(an expression).

For example,

CLA!CLL$X
LACL-ISUM$X
The first example will clear registers A# and L# (the saved AC and
DDT will type ? and ignore the command if an attempt

Link in DDT).

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 npointn executed as if it were located at
npoint".

The reason for this is that if it 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, it 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:
LAC (1
DAC TMP (breakpoint 1 set here)
TAD ADR

A
B

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

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.

4-22

Debugging with DDT

4.7

MISCELLANEOUS FEATURES

4.7.1

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
are ~, so that $U will clear the Link and AC (equivalent to CLA!CLL$X) .
4.7.2

Make Subprogram Current (Header Command)

The header command is used primarily to make a particular subprogram
current, thus giving preference to its local symbols when DDT performs
a symbol table search.

The argument

The form is:

is usually a filename (e.g., PRG1'), but it may be any

unique address symbol.

This command makes address

tion, sets the value of "point" equal to

the current loca-

and makes the program con-

taining that address the current program.
4.7.3

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
argument is
4.7.4

The default

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 XVM Vnxnnn

BLOADER XVM Vnxnnn
>

4-23

Debugging with DDT

When DDT is started, core has been cleared between P#' and C#, and no
load-time nor DDT-time symbols exist.
4.7.5

Restarting nDT

The use of tT (CTRL T) to interrupt a user program and return control
to DDT has been previously explained.

tT may also be used to abort a

search operation which is in progress.
4.7.6

Typing Mistakes

If the user discovers that he has made a typing error while inputting
a command, he may type tv (CTRL V) or rubout, both of which are echoed
as @, to delete the entire command.
4.7.7

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:
(~)

Proceed

following a iT

All register modifying commands

Breakpoint

(~)

Except for references to the autoindex registers in register modification commands and addresses within the range TP# and the top of core,
addresses used by these protected commands must fall within the range
P#' to C#.

This restriction prevents the user from altering the Moni-

tor, DDT's symbol table, DDT itself, or the bootstrap.
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. DDT
will not check for nonexistent memory references.
~

4.8

Reenables the protection feature.

ERROR RECOVERY

sections 2.11, 4.7.5 and 4.7.6 explain how to correct typing errors
and how to stop a runaway program.
4-24

Debugging with DDT

The following DDT-generated error messages are not fatal:
U

(undefined symbol)

(illegal symbol definition)

(DDT-time symbol table overflow)

(general error message)

API

(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) mayor may not be
fatal.

After unrecoverable lOPS errors, control can then return to

DDT if the user types CTRL T (tT).
If an error is trapped by the FORTRAN Object Time System (OTS) , which
prints .OTS XX, it will exit to the Monitor.
at that point.

4-25

No recovery is possible

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
to value.

(20000)

INSTRUCTION MODE COMMANDS
$S

(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.
If bits 0-2 are nonzero, precede the address printout with these
bits as a 6-digit octal number whose rightmost
5 digits are zero.

(Text)

$$R

(Text)

Consider registers "point" and "point-pIus-one"
as 5/7 packed ASCII and print the corresponding
five characters.
Consider the register "point" to be RAD50 and
display the 3 characters.

$$S

(Text)

Consider the register "point" to be SIXBIT and
display the 3 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 18-bit octal
numbers.

(Floating)

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

SPECIAL ADDRESSES IN AND ABOVE DDT
A#

Where the AC is stored on breaks.
A-I

Summary of DDT Commands

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

MQ#

Where the MQ is stored on breaks.

LR#

Where the limit register is stored on breaks.

Where the index register is stored on breaks.

M#
LO#

Where the default mask for searches is stored.
Where the lower limit address of search operations is stored.

HI#

Where the upper limit address of search operations is stored.

R#+N

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

I#+N

Contains the instruction to be executed on reacing breakpoint N.

V#+N

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

Contains the 2's complement of the proceed count.

TP#

Address of the first word of core memory above the bootstrap.
Contents of TP# are undefined.

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

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

P#'

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

SA#

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

B1#

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).

Symbol S defined as ~; same as ~).

$K
O$K

Kill DDT-time user defined symbols.
Kill load-time user symbols.

A-2

Summary of DDT Commands

STARTING A PROGRAM
AD$G
$G
;AD$G

Start at address AD (expression) •
start at address SA#.
Set SA#=AD and start there.

BREAKPOINT COMMANDS
K$B

Put lowest available breakpoint at a(:1.dl~ess I< (must be
non-zero; default:
.)
DDT types breakpoint number
after the B.

O$B

Remove all breakpoints.

N;K$B

Set breakpoint N (default:
non-zero; default:
.).

N;O$B

Remove breakpoint N (Default:

M;K$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 ML is missing, normal procedure will be
followed at breakpoint.
If M bit 17 is ~, breakpoint
will never break.
If M bit 17 i~ 1, breakpoint will
always break.

4) at address K (must be
4).

E$O

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

17) for

M$P

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

1) in

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 18-bit address of the
next register to be opened and made current.
If not,
the register "point" is opened.

LINE
FEED
t

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 (page
mode) or 13 (bank mode) 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.

A-3

Summary of DDT Commands

Behaves like $Y except that the new open register is not
made current.

[

Behaves like /, but forces the printout to be in
numeric mode.

]

Behaves like /, but forces the printout to be in
symbolic mode.

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

Print argument (default:

Q#) as an octal number.

Behaves like

= but causes symbolic printouts.

Behaves like
vectors.

= but retypes expressions as transfer

Prints out contents of
packed ASCII text.

Prints out contents of . as SIXBIT ASCII.

$$?

Prints out contents of

and .+1 interpreted as 5/7

as RAD5.0.

WORD SEARCHES

MiN$L

Set the contents of LO# to M (default:
N (default: L#).

MiD$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 bits that are 1 in M (default:
contents of M#) .

MiD$N

Search as in $W, but print unequal words.

MiD$E

Search as in $W and $N for memory reference instructions
effectively addressing (directly or indirectly) an address which is identical to D (default; .0) in every
bit which is 1 in M (default: contents of M#).

p#') and HI# to

OTHER DDT COMMANDS
I$X
tu

RUBOUT
C$H
MiN$U
N$M

Execute instructions I (default:
if located at address "point").

contents of "point" as

Wipe out current command.
Same as {U.
Make the address C (e.g., FILE') current, but do not
open it.
(Default: SA#).
Update contents of L# to M (default:
of A# to N (default: .0).

.0) and the contents

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.
A-4

Summary of nDT Commands

Enable Protect mode.

r-~.

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

ERROR MRSSAGES
?

General error indication.

Command ignored.

symbol definition command attempted to redefine a nonDDT-time symbol. Conmand 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.

Command ignored.

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

ALTr-10DE

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

Indicates that the command has been performed by DDT.
Same as

».

Incticates that the preceding symbol is a file name (must
be used with file names for correct operation) .
Indicates that the preceding symbol is an ado.ress (omit
searching the instruction mnemonic symbol table).

Sets the indirect bit of an argument (20000).

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

A-5

APPENDIX B
MNEMONIC INSTRUCTION TABLE

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

OPERATE CLASS

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

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

EAE GROUP
EAE

640000

INPUT/OUTPUT
IOT

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

INDEX CLASS
* DDT interprets 740000 as NOP.
PAX
PAL
AAC
PXA
AXS
PXL
PLA
PLX
CLX
CLLR
AXR

721000
722000
723000
724000
725000
726000
730000
731000
735000
736000
737000

B-1

APPENDIX C
DDT MEMORY LOAD MAPS
Areas Which
the User May
Modify
Order For
Symbol Table
Search

Top of Core

f
Free *

. SCOM+ 7 7;if

Core
.SCOM+76"
I
Uninitialized*

Common Block

·
·
·

Common
Blocks

DDT

TP#~

Common Block

• SCOM+{J

Bootstrap
DDT

D~T

Run-time Symbols
DDT Special Symbols

continue
here

Instruction Mnemonics

Starts here,
unless instructions bypassed

Symbol
Tabte

C#~

DDT Special Registers
f"'-..

Hain Program

subprogram 1

User
Programs

·
·
·

Subprogram N

1
j

Free
Core

!
I

Load-Time
symbols

.SCOM+3 J
Patch Area
.SCOM+~~

Eli '

Subprogram N Symbols

·
· Symbols

subprogram 1
Main Program

The area above the
bootstrap is loaded
with uninitialized
COMMON blocks if XVM
mode is ON.
Free core
above the bootstrap
should not be used as
a patch area, although
it can be used to store
additional data.
The
area above the bootstrap
should not be used for
any purpose if XVM mode
is OFF.

Symbols

Final Search
Here
Main Program
Has Initial
Precedence
(Current Program)

Resident
Monitor

Auto I ndex
Registers

Auto Index
Registers
C-l

INDEX
AC operation, 4-23
Addition operator, 3-6
Addresses, A-I
Address modes, 2-4
commands, A-I
search, 4-12
Address tag, 3-4
Arguments, 3-1, 3-2
missing separator, 3-5
ASCII text, 2-4, 3-9

Bootstrap, 4-24
Breakpoint, 2-7, 4-13
commands, A-3
control, 4-16
number, 2-8
reassignment or removal, 2-9
restrictions, 2-8, 4-15
resume program execution, 2-9
typeout, 2-8
Break procedure, 4-19

Defining a symbol, 4-9, A-2
Double quote (n) character, 3-4

EAE group instructions, B-1
Effective address search, 4-12
Error messages, A-5
Error recovery, 4-24
Errors, typing, 2-9
Exclusive OR operator, 3-6
Execute command, 4-21
Expression operators, A-I
Expression retype commands, 4-7
Expressions, 3-5

Floating address mode, 3-9
Format, I/O, 1-1
FORTRAN IV local symbols, 3-5

Global symbols, 3-5
Carriage return (-' 1, 2-3
Changing storage words, 2-6
Closing and reopening locations,
2-3
Commands,
execute, 4-21
expression retype, 4-7
header, 4-23
protect mode, 4-24
register examination, 4-6
register modification, 4-7
search, 4-10
sequencing, 4-7
summary, A-I
typeout mode, 4-2
Command structure, 3-1
Concepts, 4-2
CTRL U, 2-9
Current location, 4-3

DAT slot assignments, 2-2

Header, 3-3
command, 4-23

Image alphanumeric mode, 1-2
Inclusive OR operator, 3-6
Index instructions, B-1
Initialize memory, 4-23
Input mode commands, 2-6
Input/output
format, 1-1
instructions, B-1
Instruction modes, 2-4
commands, A-I
Internal symbols, 3-3

Language, 3-1
Last-opened-register-pointer, 2-3
INDEX-l

Linking Loader, 1-1, 3-3
Link operation, 4-23
Loading DDT without a program,
4-23
Loading instructions, 2-1

Machine instruction mnemonics, 3-3
Mask, 4-10
Memory initialization, 4-23
Memory load maps, C-l
Memory reference instruction, 2-4,
4-22, B-1
Mnemonic instruction table, B-1
Modifying storage words, 2-6

Not-Word search, 4-11
Number (#) symbol, 4-4
Numerical input, 3-2
Numerical output, 3-8

Octal mode, 3-8, 4-6
Opening a location, 2-2
Operate instructions, B-1
Operators, 3-6
Operators, expression, A-l

Parenthesis usage, 3-1
Period or point (.) usage, 2-3,
4-3
Pound (#) symbol, 4-4
Proceed count, 4-2, 4-16
Proceeding after a break, 2-9
Protect mode commands, 4-24

Searches, word, A-4
Semicolon (i) usage, 3-1
Sequencing, 2-6, 4-7
Setting breakpoints, 2-7, 4-13
Single instructions, 4-21
Single quote (') character, 3-4
Special symbols, 4-2
Starting a program, 2-9, 4-1, A-3
Stopping a program, 2-9
Storage word modification, 2-6
Subtraction operator, 3-6
Summary of commands, A-I
Syllables, 3-2
Symbol definition, 4-9, A-2
Symbolic instruction mode, 2-3,
3-6, 4-2, 4-6
Symbols, 3-2
DDT, A-5
internal, 3-3
used in text, 1-2
Symbol table, 3-3, 4-5
Symbol, undefined, 4-2
Syntax, 3-1

Tabbing, 2-2
Transfer vectors, 2-4, 3-5, 3-8
2's complement arithmetic, 3-6
Typing mistake, 2-9, 4-24
Type-in mode, 4-6
Type-out modes, 2-3
commands, 4-2

Unary minus operator, 3-6
Undefined symbol, 4-2
User locations, A-2

Word search, 4-10, A-4
Reassign a breakpoint, 2-9
Register examination commands,
4-6, A-3
Register modification commands,
4-7
Relative symbolic address mode,
4-2
Relocation factor, 3-3, 3-4
Remove breakpoints, 2-9
Reopening registers, 2-3
Restarting DDT, 4-24
Restrictions, breakpoint, 4-15
Retype commands, 2-5, 4-7
RUBOUT, 2-9

Search commands, 4-10
Search mask, 4-10

INDEX-2

DDT XV~1 Utili ty Nanual
DEC-XV-UDDTA-A-D
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