DDT44C
TOPS-20 DDT Manual
| Electronic distribution with Autopatch Tape 16
| August 1987
This manual describes the use
of TOPS-20 DDT, the Dynamic
Debugging Tool for MACRO-20
programs.
| This manual updates the
_______ ___ ______
| TOPS-20 DDT Manual printed for
| TOPS-20 6.1. This version of
| the manual is not printed and
| is not available from DIGITAL
| in printed form. It is
| distributed on the Autopatch
| tape #16 for TOPS-20 in .MEM
| file format only. You can
| print this file on any
| printer; page length has been
| set at 58 lines. Change bars
| indicate changes and bullets
| indicate data deletions since
| the previous version of this
| manual.
OPERATING SYSTEM: TOPS-20 V6.1
| SOFTWARE: DDT V44C(670)
digital equipment corporation marlboro, massachusetts
First Printing, May 1985
| Autopatch Revision, August 1987
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 only be used or copied in accordance with the terms of such
license.
No responsibility is assumed for the use or reliability of software on
equipment that is not supplied by DIGITAL or its affiliated companies.
| Copyright (C) 1985, 1987, Digital Equipment Corporation.
All Rights Reserved.
The following are trademarks of Digital Equipment Corporation:
DEC DECnet IAS
DECUS DECsystem-10 MASSBUS
Digital Logo DECSYSTEM-20 PDT
PDP DECwriter RSTS
UNIBUS DIBOL RSX
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VT
CONTENTS
PREFACE
CHAPTER 1 INTRODUCTION TO DDT
1.1 SYMBOLIC DEBUGGING . . . . . . . . . . . . . . . . 1-1
1.2 TOPS-20 VARIANTS OF DDT . . . . . . . . . . . . . 1-1
CHAPTER 2 GETTING STARTED WITH DDT
2.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . 2-1
2.2 LOADING DDT . . . . . . . . . . . . . . . . . . . 2-1
2.3 BASIC FUNCTIONS . . . . . . . . . . . . . . . . . 2-2
2.3.1 Error Conditions . . . . . . . . . . . . . . . . 2-3
2.3.2 Basic Concepts . . . . . . . . . . . . . . . . . 2-4
2.3.3 Starting and Stopping the Program . . . . . . . 2-5
2.3.4 Examining and Modifying Memory . . . . . . . . . 2-6
2.3.5 Executing Program Instructions . . . . . . . . . 2-9
2.4 A SAMPLE DEBUGGING SESSION USING DDT . . . . . . 2-10
2.5 PROGRAMMING WITH DDT IN MIND . . . . . . . . . . 2-21
CHAPTER 3 DDT COMMAND FORMAT
3.1 COMMAND SYNTAX . . . . . . . . . . . . . . . . . . 3-1
3.2 INPUT TO DDT . . . . . . . . . . . . . . . . . . . 3-2
3.2.1 Values in DDT Expressions . . . . . . . . . . . 3-2
3.2.2 Operators in DDT Expressions . . . . . . . . . . 3-7
CHAPTER 4 DISPLAYING AND MODIFYING MEMORY
4.1 DISPLAY MODES . . . . . . . . . . . . . . . . . . 4-1
4.1.1 Default Display Modes . . . . . . . . . . . . . 4-1
4.1.2 Selecting Display Modes . . . . . . . . . . . . 4-2
4.2 DISPLAYING EXPRESSIONS . . . . . . . . . . . . . . 4-6
4.3 DISPLAYING BYTE POINTERS . . . . . . . . . . . . . 4-6
4.4 DISPLAYING AND DEPOSITING IN MEMORY . . . . . . . 4-7
4.4.1 Commands that Use the Current Location . . . . 4-10
4.4.2 Commands that Use the Location Sequence Stack 4-11
4.4.3 Commands that Use an Address within the Command 4-12
4.5 DISPLAYING ASCIZ STRINGS . . . . . . . . . . . . 4-19
4.6 ZEROING MEMORY . . . . . . . . . . . . . . . . . 4-19
4.7 AUTOMATIC WRITE-ENABLE . . . . . . . . . . . . . 4-20
4.8 AUTOMATIC PAGE CREATION . . . . . . . . . . . . 4-21
4.9 DISPLAYING PAGE ACCESSIBILITY INFORMATION . . . 4-22
4.10 WATCHING A MEMORY LOCATION . . . . . . . . . . . 4-23
4.11 TTY CONTROL MASK . . . . . . . . . . . . . . . . 4-23
iii
CHAPTER 5 CONTROLLING PROGRAM EXECUTION
5.1 BEGINNING EXECUTION . . . . . . . . . . . . . . . 5-1
5.2 USING BREAKPOINTS . . . . . . . . . . . . . . . . 5-1
5.2.1 Setting Breakpoints . . . . . . . . . . . . . . 5-3
5.2.2 Proceeding from Breakpoints . . . . . . . . . . 5-6
5.2.3 Conditional Breakpoints . . . . . . . . . . . . 5-9
5.2.4 The "Unsolicited" Breakpoint . . . . . . . . . 5-10
5.3 EXECUTING EXPLICIT INSTRUCTIONS . . . . . . . . 5-11
5.4 SINGLE-STEPPING INSTRUCTIONS . . . . . . . . . . 5-11
5.5 EXECUTING SUBROUTINES AND RANGES OF INSTRUCTIONS 5-13
5.5.1 Single-Stepping "Dangerous" Instructions . . . 5-15
5.6 USER-PROGRAM CONTEXT . . . . . . . . . . . . . 5-16
CHAPTER 6 SEARCHING FOR DATA PATTERNS IN DDT
CHAPTER 7 MANIPULATING SYMBOLS IN DDT
7.1 OPENING AND CLOSING SYMBOL TABLES . . . . . . . . 7-1
7.2 DEFINING SYMBOLS . . . . . . . . . . . . . . . . . 7-2
7.3 SUPPRESSING SYMBOL TYPEOUT . . . . . . . . . . . . 7-3
7.4 KILLING SYMBOLS . . . . . . . . . . . . . . . . . 7-3
7.5 CREATING UNDEFINED SYMBOLS . . . . . . . . . . . . 7-4
7.6 FINDING WHERE A SYMBOL IS DEFINED . . . . . . . . 7-4
7.7 SEARCHING FOR SYMBOLS . . . . . . . . . . . . . . 7-5
7.8 LISTING UNDEFINED SYMBOLS . . . . . . . . . . . . 7-5
7.9 LISTING SYMBOLS . . . . . . . . . . . . . . . . . 7-5
7.10 LOCATING SYMBOL TABLES WITH PROGRAM DATA VECTORS . 7-6
CHAPTER 8 INSERTING PATCHES WITH DDT
CHAPTER 9 FILDDT
9.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . 9-1
9.2 USING FILDDT . . . . . . . . . . . . . . . . . . . 9-1
9.2.1 FILDDT Commands . . . . . . . . . . . . . . . . 9-3
9.2.2 Symbols . . . . . . . . . . . . . . . . . . . . 9-4
9.2.3 Commands to Establish Formats and Parameters . . 9-4
9.2.4 Commands to Access the Target and Enter DDT . . 9-5
9.2.5 Exiting FILDDT . . . . . . . . . . . . . . . . . 9-8
CHAPTER 10 PRIVILEGED MODES OF DDT
10.1 MDDT . . . . . . . . . . . . . . . . . . . . . . 10-2
10.2 KDDT . . . . . . . . . . . . . . . . . . . . . . 10-3
10.3 EDDT . . . . . . . . . . . . . . . . . . . . . . 10-4
iv
CHAPTER 11 PHYSICAL AND VIRTUAL ADDRESSING COMMANDS
CHAPTER 12 EXTENDED ADDRESSING
12.1 LOADING DDT INTO AN EXTENDED SECTION . . . . . . 12-1
12.2 EXAMINING AND CHANGING MEMORY . . . . . . . . . 12-2
12.3 BREAKPOINTS . . . . . . . . . . . . . . . . . . 12-2
12.3.1 The Breakpoint Block . . . . . . . . . . . . . 12-2
12.3.2 Enabling and Disabling Inter-section
Breakpoints . . . . . . . . . . . . . . . . . 12-3
12.4 DISPLAYING SYMBOLS IN NONZERO SECTIONS . . . . . 12-4
12.5 DEFAULT SECTION NUMBERS . . . . . . . . . . . . 12-5
12.5.1 Permanent Default Section . . . . . . . . . . 12-6
12.5.2 Floating Default Section . . . . . . . . . . . 12-6
12.6 EXECUTING SINGLE INSTRUCTIONS . . . . . . . . . 12-8
12.7 ENTERING PATCHES IN EXTENDED SECTIONS . . . . . 12-8
APPENDIX A ERROR MESSAGES
GLOSSARY
INDEX
FIGURES
2-1 Sample Program X.MAC . . . . . . . . . . . . . . 2-11
2-2 Annotated Debugging Session . . . . . . . . . . 2-12
2-3 Terminal Display of Debugging Session . . . . . 2-20
4-1 DDT Session Showing Columnar Output . . . . . . 4-25
8-1 Annotated Patching Session . . . . . . . . . . . . 8-4
8-2 Terminal Display of Patching After an Instruction 8-5
8-3 Terminal Display of Patching Before an Instruction 8-6
TABLES
3-1 Commands that Return Values . . . . . . . . . . . 3-3
3-2 Effects of Operators When Evaluating Expressions . 3-8
4-1 Evaluation of Symbolic Display Mode . . . . . . . 4-1
4-2 DDT Display Modes . . . . . . . . . . . . . . . . 4-4
4-3 Commands to Display Expressions . . . . . . . . . 4-6
4-4 DDT Commands to Display Memory . . . . . . . . . . 4-9
4-5 TTY Control Mask . . . . . . . . . . . . . . . . 4-24
5-1 Breakpoint Locations of Interest . . . . . . . . . 5-2
5-2 User-Program Context Values . . . . . . . . . . 5-16
vi
PREFACE
MANUAL OBJECTIVES AND AUDIENCE
This manual explains and illustrates the features of TOPS-20 DDT, the
debugger for MACRO-20 programs. Although TOPS-20 DDT can be used to
debug the compiled code of programs written in higher-level languages,
this manual illustrates the use of TOPS-20 DDT to debug programs
written in MACRO-20 only.
This manual is both an introduction to the basic functions of TOPS-20
DDT and a reference guide to all TOPS-20 DDT commands and functions.
This manual assumes that the reader is familiar with using TOPS-20,
has done some programming in MACRO-20, and is familiar with the format
of MACRO-20 instructions.
STRUCTURE OF THIS DOCUMENT
This manual has 12 chapters, one appendix, and one glossary.
o Chapter 1 introduces the concept of symbolic debugging and
describes the variants of TOPS-20 DDT.
o Chapter 2 describes loading TOPS-20 DDT with your program,
discusses basic TOPS-20 DDT commands, and illustrates a
sample debugging session.
o Chapter 3 explains the syntax of a DDT command. Chapter 3
also describes expressions to enter data and explains how
TOPS-20 DDT evaluates expressions.
o Chapter 4 discusses how to examine and modify a program using
TOPS-20 DDT.
o Chapter 5 describes the use of TOPS-20 DDT to control program
execution: how to start, stop, and monitor the running of a
program.
vii
o Chapter 6 explains how to perform searches of a program's
address space using TOPS-20 DDT.
o Chapter 7 discusses the manipulation of program symbols using
TOPS-20 DDT.
o Chapter 8 describes how to use the TOPS-20 DDT patching
function to insert and test a new series of instructions in
your program without reassembling the program.
o Chapter 9 describes the use of FILDDT.
o Chapter 10 describes the use of the privileged DDTs: KDDT
and MDDT.
o Chapter 11 describes special-use commands that control
physical and virtual addressing. These commands are useful
primarily when running EDDT and FILDDT.
o Chapter 12 describes the use of DDT in non-zero sections
(NZS).
o Appendix A explains DDT and FILDDT error messages.
o The glossary defines important TOPS-20 DDT terms.
OTHER DOCUMENTS
Other documents to which the reader should have access are:
_____ _________ _________ ______
o MACRO Assembler Reference Manual
_______ ____ _________ ______
o TOPS-20 LINK Reference Manual
_______ ________ _________ ______
o TOPS-20 Commands Reference Manual
_________________________ _________ _________ ______
o DECsystem-10/DECSYSTEM-20 Processor Reference Manual
_______________ _______ ______ _________ ______
o TOPS-10/TOPS-20 RSX-20F System Reference Manual
viii
CONVENTIONS
The following conventions are used in this manual in the description
of DDT commands and concepts.
{} Curly brackets (braces) indicate that the enclosed item
is optional.
. (period) The address contained in DDT's location counter; also
_______ ________
called the current location.
addr A symbolic location within a program, a symbolic or
absolute address in memory, an AC, or ".", the current
location.
c A single ASCII or SIXBIT character.
expr Any expression that is legal in DDT.
filnam One or more components of a file specification.
instr Any instruction in the PDP-10 machine instruction set.
location sequence stack
A circular stack of memory locations that is used to
store the addresses of certain previously referenced
locations.
n A numeric argument.
page A page in memory. A page equals 512 words of memory.
symbol A symbol name of up to 6 RADIX50 characters.
text Any string of ASCII or SIXBIT characters.
word Any 36-bit value occupying one word of memory.
Represents pressing the ESCAPE or ALTMODE key once.
Represents pressing the ESCAPE or ALTMODE key twice.
Represents pressing a key (represented by X) at the
same time as you press the key labeled CTRL.
represents pressing the BACKSPACE key or .
Represents pressing the LINE FEED key.
Represents pressing the RETURN key.
Represents pressing the TAB key or .
ix
Numbers are in octal radix unless otherwise specified.
Examples of interaction between the user and DDT show user input in
lowercase and DDT output in uppercase.
The symbols , , , , , and always
represent user input.
NOTE
The descriptions of many DDT commands list the actions
and effects of those commands. The actions and
effects may not occur in precisely the order
specified, but this has no effect on the user.
x
1-1
CHAPTER 1
CHAPTER 1
INTRODUCTION TO DDT
INTRODUCTION TO DDT
DDT is a utility program you can use to help you debug your MACRO-20
programs. This manual describes how to use the DDT utility.
1.1 SYMBOLIC DEBUGGING
1.1 SYMBOLIC DEBUGGING
It is sometimes difficult to understand precisely the operation of a
program by reading the source code. DDT is a tool for interactively
examining the operation of a MACRO-20 program while it is running.
DDT is useful for finding programming errors (bugs) in programs that
do not run correctly. You can also use DDT to analyze the flow of
control in a program that is to be revised or rewritten.
With DDT, you can interrupt the execution of your program at locations
(breakpoints) you choose, and then examine and modify the program's
address space as required. You can execute instructions one-by-one to
check whether the effect of each instruction is what is intended. You
can then set other breakpoints in your program before continuing
execution.
When you refer to program locations and values, DDT allows you to use
the symbols that are defined in the program rather than absolute
values and addresses. This makes it much easier to refer to the
source listing and to find specific locations in memory.
After modifying the program's instructions or data, you can exit DDT
and save (with the monitor-level SAVE command) the changed version of
the program for further testing.
1.2 TOPS-20 VARIANTS OF DDT
1.2 TOPS-20 VARIANTS OF DDT
There are several variants of DDT, each useful under specific
circumstances or for specific tasks.
1-1
INTRODUCTION TO DDT
INTRODUCTION TO DDT
The variants of TOPS-20 DDT are:
o EDDT
o FILDDT
o KDDT
o MDDT
o RDDT
o SDDT
o UDDT
o XDDT
EDDT is used to debug programs that run in executive mode (such as
BOOT), and is described in Chapter 10.
FILDDT is used to examine and patch disk files and structures. You
can also use FILDDT to examine the running monitor. FILDDT is
described in Chapter 9.
KDDT is used to debug and patch monitor .EXE files and the running
monitor, and is described in Chapter 10.
MDDT is used to debug and patch the running monitor, and is described
in Chapter 10.
RDDT is a relocatable variant of DDT that can be used to debug
programs in user mode. If your program is in memory (and has been
loaded with RDDT as below), you invoke RDDT by entering (at TOPS-20
command level):
START
You load RDDT with your program by running LINK as follows:
@LINK
*MYPROG,SYS:RDDT.REL/GO
where MYPROG is the name of your program. Loading RDDT.REL with your
program does not prevent you from using other LINK features. You must
load RDDT.REL last, or its start address is lost. RDDT.REL is useful
in situations where you do not wish to have DDT loaded at its default
location.
This example shows only the minimal steps required to load the
____ _________ ______
relocatable DDT with your program. See the LINK Reference Manual for
further information about using LINK.
1-2
INTRODUCTION TO DDT
INTRODUCTION TO DDT
SDDT is a "stub" that places XDDT in its own section, with system
symbols defined as in MONSYM and MACSYM. SDDT is the DDT variant
invoked when, at TOPS-20 command level, you enter:
SDDT
SDDT exists so that entering SDDT invokes DDT version 44 in the same
manner as previous versions.
UDDT is a "stub" that resides in your user program's section if the
program has a TOPS-10-style entry vector and the program entry vector
is in section zero. This is done for compatibility with programs that
use locations 770000, 770001 and 770002. If you load a program in
section zero and the program has a TOPS-10-style entry vector, when
you use the DDT command, the EXEC loads the UDDT stub into your
program's section at address 770000. UDDT then loads XDDT into the
highest-numbered free (nonexistent) section (if XDDT is not already
loaded), and starts XDDT.
XDDT is the DDT variant normally used to debug user programs. If you
load your program in a nonzero section or the program does not have a
TOPS-10-style entry vector, the DDT command causes the EXEC to load
XDDT directly into the highest-numbered free section. XDDT is also
invoked by the SDDT and UDDT stubs. If you type in XDDT while at
TOPS-20 command level, the EXEC loads XDDT into section zero, with
system symbols defined.
1-3
1-4
CHAPTER 2
CHAPTER 2
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
2.1 INTRODUCTION
2.1 INTRODUCTION
This chapter is an introduction to using DDT. It describes how to
load DDT with your program and shows how to perform basic DDT
functions. It then illustrates a sample session debugging a simple
MACRO-20 program, using basic DDT functions. You can use DDT to debug
programs, using only the commands described in this chapter. Once you
are familiar with using these commands, you may wish to learn how to
use the commands and functions that are described in the rest of the
manual, to perform more sophisticated debugging.
The commands used in this chapter are described only in sufficient
detail for the debugging task being performed; all commands are
thoroughly described in Chapters 3 through 11 of this document.
_____
The best way to learn is by doing. You will learn the commands and
techniques discussed in this manual if you use them as you read about
them. If you have a MACRO-20 program that you wish to debug, use it
to practice the commands discussed here. If not, type in the program
X.MAC listed in Figure 2-1.
2.2 LOADING DDT
2.2 LOADING DDT
| It is much easier to debug a program when you can use the symbols that
| are defined in the program. For you to be able to use program
| symbols, DDT must have access to your program's symbol table. One way
| to provide this access is to use the TOPS-20 DEBUG command to load DDT
| with your program and retain your program symbols. Load an existing
| MACRO-20 program with the TOPS-20 DEBUG command as follows:
|
| DEBUG filnam
|
2-1
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
______
| where filnam is the name of your MACRO-20 program. The following
| appears on your terminal (if your .REL file is older than your .MAC
| file, MACRO-20 reassembles your program, otherwise the second line
| does not appear):
|
| @DEBUG PROG
| MACRO: filnam
| LINK: Loading
| [LNKDEB DDT execution]
| DDT
|
______
| where filnam is the name of your MACRO-20 program (with default
| extension .MAC). The last line (DDT) indicates that DDT is loaded,
| and is ready to accept your commands.
2.3 BASIC FUNCTIONS
2.3 BASIC FUNCTIONS
You must be able to perform certain basic functions to interactively
debug a program. Basic DDT functions are:
o starting the program
o stopping the program at specified locations
o examining and modifying memory
o executing program instructions one-at-a-time
o continuing execution of the program
|
| o deleting input
o exiting DDT
You must give DDT commands to tell DDT what functions to perform. DDT
does not wait for a line terminator (such as a carriage return) to
indicate the end of your command. Instead, DDT reads your commands
character-by-character as you enter them. When you enter a DDT
command, you almost never have to press the RETURN key. This manual
explicitly indicates the occasions when a command requires you to
press the RETURN key.
NOTE
You must press the ESCAPE key as part of entering many
DDT commands. This manual uses the symbol to
indicate where you press the ESCAPE key. When you
press the ESCAPE key, DDT displays a dollar sign ($)
on the screen. DDT never displays when you
press the ESCAPE key.
2-2
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
NOTE
This manual uses the symbols , , ,
, and to indicate where you press the
BACKSPACE, ESCAPE, LINE FEED, RETURN, and TAB keys,
respectively. This manual also uses the symbol
to indicate where you simultaneously press
the CONTROL key and the key indicated by X. These
symbols ALWAYS indicate where you press the specific
keys noted here. You need NEVER enter the characters
______ _____ ____ _____ _____ ________
, , , , , or , to
enter a DDT command.
Your commands appear on the screen as you type them. Use the DELETE
key to delete partially entered commands character-by-character. If
you try to delete more characters than you have entered, DDT displays:
XXX
You can delete an entire command line with . When you do, DDT
displays:
XXX
To exit DDT, enter:
The other basic DDT functions are described in the rest of this
chapter.
2.3.1 Error Conditions
2.3.1 Error Conditions
If DDT cannot execute a command, it displays a message to let you
know. The message may be only a single character (such as M or U, for
_ _
Multiply-defined symbol or Undefined symbol), a question mark (?), or
a complete message string. For most errors, DDT also sets a pointer
to the error string, so that if DDT did not display it, you can enter
a command to display the error string. The error string is available
for display until another error occurs, when DDT changes the pointer.
To display the error string that describes the last DDT error, enter:
?
(press the ESCAPE key, followed by a question mark).
o
2-3
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
2.3.2 Basic Concepts
2.3.2 Basic Concepts
_______ ________
A very useful DDT concept is that of the current location. The
current location is a memory location that you have referenced, either
implicitly or explicitly, with your last command, and that is the
default point of reference of your next command. The current location
can be thought of as the location "where you are". The symbol "."
(period) refers to the address of the current location, and can be
used as an argument in DDT commands.
________ _______
The location counter is a DDT pointer that contains the address of the
current location. The location counter performs a function similar to
that of a bookmark. You can enter a command to display the contents
of a specific location but not change the address of the current
location, in order to maintain a specific point of reference for your
next command. Most DDT commands change the address of the current
location, and therefore also change the location counter. The
commands that do not change the current location are so indicated.
____ ________
The open location is a memory word that can be modified by your next
command. DDT "opens" the location as a result of a command you give
to examine or modify memory. There is never more than one location
open at any given time. The open location is usually also the current
location.
__ ____ ___ ________ _______ __ ___ _______ ________
To find the symbolic address of the current location, enter:
._ (a period followed by an underscore)
This causes DDT to display the following:
ADDR1+n
where ADDR1 is a label defined in your program, and n is the offset of
the current location from that label (if the current location is
__
ADDR1, DDT does not display +n).
_______ ________
Another useful DDT concept is that of the current quantity. This is a
value that is the contents of the last word that DDT displayed, or the
value that you last deposited in memory. The current quantity is the
most recent of those values. Many DDT commands use arguments that
default to the current quantity.
________ ________ _____
The location sequence stack is a DDT storage area used to store the
addresses of previous current locations. Certain DDT commands store
the address of the current location on the location sequence stack.
Other DDT commands change the address of the current location to an
address that has already been stored on the location sequence stack.
The location sequence stack functions in a fashion similar to
inserting place-markers in a source code listing, to be able to "get
back" to prior references.
2-4
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
2.3.3 Starting and Stopping the Program
2.3.3 Starting and Stopping the Program
When your program is loaded and DDT is ready to accept your commands
___
(as indicated by DDT appearing on the terminal display), you can begin
execution of your program at its start address by entering:
G
___________
Unless you set one or more breakpoints before you start the program,
your program runs either to completion or until it commits a fatal
error. A breakpoint is a location in a program's executable code that
__
has been modified so that if the program attempts to execute the
______
instruction at that location, control passes to DDT before the
instruction is executed.
The command to set a breakpoint is:
addrB
____
where addr is the address at which to stop execution. If the
user-program PC reaches addr, DDT interrupts execution of the program
before the program executes the instruction at the specified address.
When DDT interrupts program execution at a breakpoint, it changes the
current location to the breakpoint and opens the current location (the
breakpoint).
While program execution is stopped at a breakpoint, you can display
and change the contents of instruction and data words, remove
breakpoints, set new breakpoints, and execute instructions one at a
time (single-step). As you examine memory, you may find an
instruction that is incorrect, and modify it. You can also examine
and modify data words in memory. After modifying incorrect
instructions and data in memory, you can immediately execute the
instructions to check the effects of the modifications, without having
to reassemble the source code.
Once you have made your changes, you can continue program execution at
the place where execution was interrupted, restart the program at the
beginning, or start execution at any other location you choose. The
program will run to completion, until it reaches a breakpoint, or
until it gets a fatal error.
2-5
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
2.3.4 Examining and Modifying Memory
2.3.4 Examining and Modifying Memory
One command to examine memory is:
addr/
____
where addr is the address of the memory word you wish to examine
(display), and can be numeric or symbolic. DDT displays the contents
of the word located at addr. If the opcode field (bits 0-8) of the
memory word matches a recognized instruction or user-defined OPDEF,
DDT displays the contents of addr as an instruction (or OPDEF). If
DDT finds (in the symbol table) any of the values to be displayed, DDT
displays those symbols rather than the numeric values. For example,
either of the following display lines might appear on your terminal
(depending on the address and contents of the word):
ADDR1/ MOVE 2,SYM1
ADDR1+5/ SYM1,,SYM2
where ADDR1, SYM1, and SYM2 have been defined in the program.
If you enter a symbol that DDT does not find in the symbol table, DDT
_
sounds the terminal buzzer or bell, and displays U on the screen. If
you enter a symbol that is defined as a local symbol in more than one
_
module, DDT sounds the terminal buzzer or bell and displays M. You
can eliminate the multiply-defined symbol problem by "opening" the
symbol table of the module in which the correct symbol is defined.
See Chapter 7 (Manipulating Symbols in DDT) for more information.
When searching for a symbol to display, DDT uses global symbols in
preference to local symbols. However, DDT searches the "open" symbol
table first, and treats local symbols found in the open symbol table
as global symbols. If DDT finds only a local symbol that is not in
the open symbol table, DDT displays the symbol with a pound-sign (#)
appended to the symbol. For example, DDT might display:
ADDR#/ MOVE 2,SYM1#
See Chapter 7 (Manipulating Symbols in DDT) for more information on
symbols and symbol tables.
_____ ____
The command addr/ changes the current location to addr and opens the
____
word at addr.
____ _____
If you omit addr from an examine-memory command, such as addr/, DDT
uses the current quantity to determine the address of the location to
display. For example, after DDT displays the contents of ADDR1+5 as
above, if you enter "/", DDT displays the contents of the word located
at SYM2. The display line then appears:
ADDR1+5/ SYM1,,SYM2 / value
2-6
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
_____
where value is the contents of the word located at SYM2. By default,
_____
DDT displays value symbolically if it can.
____
The command / by itself (without addr) does not change the current
location. Both forms of the / command open the location displayed,
enabling you to modify the location with your next command.
Another very useful command for examining memory is . This
| command starts a new display line before displaying the contents of
____
| addr, making the display easier to read. For example, if you enter
after DDT displays the address and contents of ADDR1+5 (as
above) on your terminal, the terminal display appears:
ADDR1+5/ SYM1,,SYM2
SYM2/ value
_____
where value is the contents of the word located at SYM2. does
not appear on the screen, but is shown above to indicate where you
| press the key. changes the current location to SYM2 and
| opens the word at SYM2. In this example, the current quantity becomes
_____
| value.
also stores the address of the current location (ADDR1+5) on the
location sequence stack before changing the current location to the
location just displayed (SYM2). DDT uses the location sequence stack
to "remember" previous values of the location counter. To "get back"
to the previous current location, enter:
In the above example, after you press at ADDR1+5, DDT displays
the contents of SYM2 and changes the current location to SYM2. When
you enter , DDT changes the current location to ADDR1+5,
opens the location at ADDR1+5, and again displays the contents of
ADDR1+5.
o
_________ ____
If you use the command addr, DDT deposits addr in the open
____
location and closes the location before opening the location at addr
and displaying its contents. by itself does not deposit
anything, but does save the current location on the location sequence
stack, making more useful than / (slash by itself).
You can display and open the word after the current location by
entering:
2-7
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
DDT changes the current location to the next word in memory, starts a
new line, and displays the address of the (new) current location (as a
symbol or a symbol plus an offset, if it can find a corresponding
symbol in the symbol table), displays the contents of the current
location, and opens the current location. For example, to display the
next word in memory after ADDR1+5, enter:
DDT changes the current location to ADDR1+6, starts a new line, and
displays the address and contents of ADDR1+6. The screen display then
appears as follows:
ADDR1+5/ SYM1,,SYM2
ADDR1+6/ -1,,SYM3
____
Note that DDT does not display the characters . does not
affect the location sequence stack.
Entering another causes DDT to display and open the next word.
________
To display and open the word previous to the current location, enter:
DDT changes the current location to the previous word, starts a new
line, displays the address and contents of the (new) current location,
and opens the current location. does not affect the location
sequence stack. For example, if you enter to open and display
the location before ADDR1+5, the screen appears as follows:
ADDR1+5/ SYM1,,SYM2
ADDR1+4/ -3,,SYM2
______
Note that does not appear on the screen.
To change the contents of the open location, enter:
value
_____
where value can be an instruction, a symbol, or a numeric expression.
______
For example, if you enter the command LABL2/, DDT displays the
contents of the memory word at LABL2, and "opens" that word. If the
word at LABL2 contains:
MOVE 1,SYM1
and you wish to change SYM1 to SYM2, enter:
MOVE 1,SYM2
2-8
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
DDT stores the new instruction in the location at LABL2 and "closes"
the location. DDT does NOT display . The terminal display
appears as follows (your input is in lowercase):
labl2/ MOVE 1,SYM1 move 1,sym2
The current location is still LABL2, but there is no open location.
To check whether the instruction is now correct, you can enter:
./
to display the contents of the current location. The screen display
now appears (your input is in lowercase):
labl2/ MOVE 1,SYM1 move 1,sym2
./ MOVE 1,SYM2
After entering a command to display and open a location, if you enter:
value
DDT stores the new value, changes the current location to the next
location in memory, starts a new display line and opens and displays
the new current location. The example above would then appear as
follows (your input is in lowercase):
labl2/ MOVE 1,SYM1 move 1,sym2
LABL2+1/ CONTENTS
________
where CONTENTS is the value stored at LABL2+1.
o
2.3.5 Executing Program Instructions
2.3.5 Executing Program Instructions
When you have interrupted program execution at a breakpoint, you can
execute the next instruction (the one at the breakpoint), by entering:
X
DDT executes the instruction, displays the results of executing the
instruction, and displays the address and contents of the next
instruction to be executed. This command changes the current location
to the next instruction to be executed. For example, assume that the
next instruction to be executed is located at LABEL1, which contains:
MOVE 1,VARIBL
2-9
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
If the word at VARIBL contains SYM1, when you enter X, DDT starts
a new line and displays:
1/ SYM1 VARIBL/ SYM1
LABEL1+1/ instr
_____
where instr is the contents of LABEL1+1, and is the next instruction
to be executed. You can continue to execute instructions
one-at-a-time by entering successive X commands. This is known
_______________
as single-stepping.
_______ _ __________
To execute a subroutine, enter:
X
| DDT executes the subroutine and returns control to you if the
| subroutine returns to a location +1, +2, or +3 from the instruction
| that calls the subroutine. DDT changes the current location to the
| address of the next instruction to be executed.
o
________ _________ __ ___ _______
To continue execution of the program until the next breakpoint or
until program completion, enter:
P
DDT starts the program running again, beginning with the next
instruction to be executed. If you did not single-step any
instructions, the program begins by executing the instruction at the
breakpoint. If you have executed any instructions by single-stepping,
the program continues where you stopped. The effect is as if the
program were running without DDT in control.
o
2.4 A SAMPLE DEBUGGING SESSION USING DDT
2.4 A SAMPLE DEBUGGING SESSION USING DDT
This section describes a debugging session using DDT. The program
being debugged is X.MAC, shown in Figure 2-1. The program and the
sample session are for illustration only. There are many styles of
programming and debugging, and these examples are descriptive rather
than prescriptive in intent.
You will understand this section and learn the commands described more
easily if you type in the program listed in Figure 2-1 and use the
commands as they are described.
2-10
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
Figure 2-1: Sample Program X.MAC
Figure 2-1: Sample Program X.MAC
SEARCH MONSYM
TITLE X
R0=0 ;AC0
IDX=6 ;INDEX REGISTER
P=17 ;STACK COUNTER
START:: MOVE P,PWORD ;Set up stack counter
MOVEI IDX,TABLE1 ;Address of table with X & Y
PUSHJ P,ADDEM ;Do the addition
MOVEI IDX,TABLE1 ;Address of table
MOVE R0,ANSWER(IDX) ;Answer to R0
JFCL 0
HALTF% ;All done!
ADDEM: MOVE R0,X(IDX) ;Load X
ADD R0,Y(IDX) ;X + Y
MOVE R0,ANSWER(IDX) ;Store answer
POPJ P, ;Return
TABLE1: BLOCK 3 ;3 words
X==0 ;Offset for X
Y==1 ;Offset for Y
ANSWER==2 ;Offset for answer
STKSIZ==10 ;Stack size
PWORD: IOWD STKSIZ,STACK ;Stack pointer
STACK: BLOCK STKSIZ ;Stack
END START
Figure 2-2 is an annotated session debugging X.MAC, the program in
Figure 2-1. In the annotated session, the DDT terminal display is on
the left, user input is in the center in lowercase, and explanatory
comments about the session are on the right. This is not always the
way it appears on the terminal. Figure 2-3 shows the session as it
actually appears on the terminal.
The program is designed to pass the address of a table to a
subroutine. The table contains three elements. The subroutine is to
add the first two elements of the table and store the result in the
third element before returning to the main program. There are no
input or output routines in the program. The table is initialized
using DDT, and the result is checked while in DDT.
NOTE
DDT does not display , , or . These are
shown in the sample session to indicate user input.
2-11
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
NOTE
DDT does not display the AC field of an instruction if
it is zero. This means that if your program contains
the instruction MOVE R0,LABL1, where R0=0, DDT
displays the instruction as MOVE LABL1.
Figure 2-2: Annotated Debugging Session
Figure 2-2: Annotated Debugging Session
SCREEN DISPLAY USER INPUT EXPLANATION
@ TOPS-20 prompt.
debug x Begin the session by entering
"debug x", where x is the
name of your MACRO program.
MACRO: X MACRO reassembles your program
LINK: Loading (if needed), and LINK loads
[LNKDEB DDT execution] your program with DDT. DDT
DDT displays the "DDT" prompt.
start/ Begin examining code at
label "START".
MOVE P,PWORD# DDT displays the instruction
at START.
Press to display the next
instruction.
.JBDA+1/ MOVEI IDX,TABLE1# The first symbol in this
program happens to coincide
with .JBDA, a JOBDAT symbol.
When DDT scans the symbol
table, it finds .JBDA before
it finds START, and displays
.JBDA instead. DDT still
accepts START as an input
symbol.
Also note the pound-sign (#)
appended to TABLE1 and
PWORD. PWORD and TABLE1 are
local symbols that are not
in the open symbol table.
2-12
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
Figure 2-2: Annotated Debugging Session (Cont.)
Figure 2-2: Annotated Debugging Session (Cont.)
SCREEN DISPLAY USER INPUT EXPLANATION
.jbdak Enter .jbdak
to suppress DDT typeout of
symbol .JBDA. DDT will
display START rather than
.JBDA from now on.
x: Enter the module name (X)
followed by and a
colon to open the symbol
table associated with
X. DDT will not append any
more pound-signs.
Press to start a new
display line, evaluate the
current quantity as if it
were an instruction, and
display the contents of the
location addressed by the Y
field of the instruction.
(Entering / (slash) displays
the same word as , but
does not start a new line.)
also saves your place
(like a bookmark) on the
location sequence stack, so
you can get back here easily.
TABLE1/ 0 When you enter the
command, DDT displays the
address and the contents of
the location. The first
element of the table contains
zero. The command also
opens the location.
2 Enter "2" followed by to
deposit the value "2" in the
first element, and to open and
display the second element.
TABLE1+1/ 0 The second element contains
zero.
2-13
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
Figure 2-2: Annotated Debugging Session (Cont.)
Figure 2-2: Annotated Debugging Session (Cont.)
SCREEN DISPLAY USER INPUT EXPLANATION
3 Enter "3" followed by to
deposit the value "3" in the
second element and open and
display the third element.
The addition to be performed
by the program is 2+3.
TABLE1+2/ 0 The third element (the answer)
contains zero.
Press , then press
to return to the address you
saved on the location sequence
stack.
START+1/ MOVEI IDX,TABLE1 DDT displays the address and
contents of the last location
you displayed before you
entered .
Press to look at the
next location.
START+2/ PUSHJ P,ADDEM This is the call to the
subroutine that does the
computation.
.b Enter ".", press , and
enter "b" to set a
breakpoint at the current
location.
g Enter g to start
program execution.
$1B>>START+2/ PUSHJ P,ADDEM DDT displays the breakpoint
number, the address of the
breakpoint, and the
instruction at the breakpoint.
This instruction has not yet
been executed.
x Press twice, then
enter "x" to let DDT
execute the subroutine.
2-14
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
Figure 2-2: Annotated Debugging Session (Cont.)
Figure 2-2: Annotated Debugging Session (Cont.)
SCREEN DISPLAY USER INPUT EXPLANATION
START+3/ MOVEI IDX,TABLE1 DDT returns from the
subroutine at the next
instruction, and displays the
address and contents of the
instruction. If there is a
"skip return", DDT displays
"" if the program
skipped one instruction. If
the program skips 2 or 3
instructions, DDT displays
"", where n is the
number of instructions
skipped.
x Press and enter "x"
to execute the instruction.
IDX/ TABLE1 TABLE1 DDT displays the address and
contents of IDX (the result of
executing the instruction),
and also displays "TABLE1"
(the result of evaluating the
Y field of the instruction).
START+4/ MOVE 2(IDX) DDT then starts a new line and
displays the address and
contents of the next
instruction. Note that
DDT does not display the
zero in the AC field of
the instruction.
Press , then to
display the contents of the
location addressed by the
instruction, using any
indexing and indirection.
(If you omit , DDT uses
only the Y field, without
indexing and indirection.)
TABLE1+2/ 0 The location addressed by the
instruction is TABLE1+2, and
its contents is zero. This is
the table element that
contains the answer, which
should be 5.
2-15
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
Figure 2-2: Annotated Debugging Session (Cont.)
Figure 2-2: Annotated Debugging Session (Cont.)
SCREEN DISPLAY USER INPUT EXPLANATION
Press to see the
previous element in the table.
TABLE1+1/ 3 This element contains 3. That
is correct.
Press again to check
the previous element.
TABLE1/ 2 This element contains 2. That
is also correct. One way to
find the error is to
single-step through the
program.
startb Enter "start", press ,
and enter "b" to set a
breakpoint at the beginning of
the program.
g Press and enter "g" to
start the program again.
$2B>>START/ MOVE P,PWORD DDT displays the breakpoint
number, and the address and
contents of the instruction
at the breakpoint.
x Press , then enter "x" to
execute the instruction. This
instruction moves a memory
word to a register.
P/ -10,,PWORD PWORD/ -10,,PWORD
DDT displays the address and
new contents of the register,
and the address and contents
of the memory word.
START+1/ MOVEI IDX,TABLE1 DDT then displays the address
and contents of the next
instruction.
2-16
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
Figure 2-2: Annotated Debugging Session (Cont.)
Figure 2-2: Annotated Debugging Session (Cont.)
SCREEN DISPLAY USER INPUT EXPLANATION
x Press , then enter "x" to
execute this instruction,
which moves an immediate value
to a register.
IDX/ TABLE1 TABLE1 DDT displays the address and
new contents of the register,
and the immediate value.
START+2/ PUSHJ P,ADDEM DDT then displays the address
and contents of the next
instruction.
x Press , then enter "x"
to execute the instruction.
P/ -7,,STACK DDT displays the address and
new contents of the stack
pointer used by the PUSHJ.
DDT displays "" if the
change in PC is less than one
or greater than 4.
ADDEM/ MOVE 0(IDX) DDT displays the address and
contents of the next
instruction to be executed.
x Press and enter "x" to
execute the instruction.
0/ 2 TABLE1/ 2 The instruction moved the
contents of the word at
TABLE1 (which is 2) to AC0.
Looks OK so far.
ADDEM+1/ ADD 1(IDX) DDT displays the next
instruction.
x Press and enter "x"
to execute the instruction.
0/ 5 TABLE1+1/ 3 The instruction added the
contents of the word at
TABLE1+1 (which is 3) to AC0,
which now contains 5. OK.
2-17
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
Figure 2-2: Annotated Debugging Session (Cont.)
Figure 2-2: Annotated Debugging Session (Cont.)
SCREEN DISPLAY USER INPUT EXPLANATION
ADDEM+2/ MOVE 2(IDX) DDT displays the next
instruction.
x Press and enter "x"
to execute the instruction.
0/ 0 TABLE1+2/ 0 The instruction moved the
contents of the word at
TABLE1+2 to AC0. The MOVE
instruction at ADDEM+2 should
be MOVEM.
ADDEM+3/ POPJ P,0 DDT displays the next
instruction (as a result of
the x).
Press to display and
open the location with the
incorrect instruction.
ADDEM+2/ MOVE 2(IDX) DDT displays the previous
instruction. This is the
incorrect instruction.
movem r0,answer(idx)
Enter the new instruction
and press .
./ Check the current location
to see what you deposited.
MOVEM 2(IDX) Looks OK.
.b Set a breakpoint at
".", the current location.
g Restart the program at
the beginning.
$2B>>START/ MOVE P,PWORD DDT displays the breakpoint
information.
p Press and enter "p" to
proceed from breakpoint 2
to the next breakpoint.
2-18
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
Figure 2-2: Annotated Debugging Session (Cont.)
Figure 2-2: Annotated Debugging Session (Cont.)
SCREEN DISPLAY USER INPUT EXPLANATION
$1B>>START+2/ PUSHJ P,ADDEM DDT displays the breakpoint
information.
p Proceed from breakpoint 1.
$3B>>ADDEM+2/ MOVEM 2(IDX) DDT displays the breakpoint
information. This is the
instruction you changed.
x Single-step the instruction
to watch what it does.
0/ 5 TABLE1+2/ 5 The instruction moves the
contents of AC0 to the word
at TABLE1+2. OK!!
ADDEM+3/ POPJ P,0 DDT also displays the address
and contents of the next
instruction.
start+4b Set a breakpoint at
START+4 to check the results.
p Proceed from breakpoint 3.
$4B>>START+4/ MOVE 2(IDX) DDT displays the breakpoint
information.
x Single-step the instruction.
0/ 5 TABLE1+2/ 5 The instruction moves the
contents of the word at
TABLE1+2 to AC0. The new
value of AC0 is 5. OK!
START+5/ JFCL 0 DDT displays the address and
contents of the next
instruction.
Quit.
@ Back at TOPS-20 command level.
2-19
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
Figure 2-3 shows the session as it actually appears on the terminal
screen. Again, user input is in lowercase. Comments on the right
indicate where you enter characters that do not echo.
Figure 2-3: Terminal Display of Debugging Session
Figure 2-3: Terminal Display of Debugging Session
@debug x
MACRO: X
LINK: Loading
[LNKDEB DDT execution]
DDT
start/ MOVE P,PWORD# Enter .
.JBDA+1/ MOVEI IDX,TABLE1# .jbda$k x$: Enter .
TABLE1/ 0 2 Enter .
TABLE1+1/ 0 3 Enter .
TABLE1+2/ 0 $ Enter .
START+1/ MOVEI IDX,TABLE1 Enter .
START+2/ PUSHJ P,ADDEM .$b $g
$1B>>START+2/ PUSHJ P,ADDEM $$x
START+3/ MOVEI IDX,TABLE1 $x
IDX/ TABLE1 TABLE1
START+4/ MOVE 2(IDX) $ Enter .
TABLE1+2/ 0 Enter .
TABLE1+1/ 3 Enter .
TABLE1/ 2 start$b $g
$2B>>START/ MOVE P,PWORD $x
P/ -10,,PWORD PWORD/ -10,,PWORD
START+1/ MOVEI IDX,TABLE1 $x
IDX/ TABLE1 TABLE1
START+2/ PUSHJ P,ADDEM $x
P/ -7,,STACK
ADDEM/ MOVE 0(IDX) $x
0/ 2 TABLE1/ 2
ADDEM+1/ ADD 1(IDX) $x
0/ 5 TABLE1+1/ 3
ADDEM+2/ MOVE 2(IDX) $x
0/ 0 TABLE1+2/ 0
ADDEM+3/ POPJ P,0 Enter .
ADDEM+2/ MOVE 2(IDX) movem r0,answer(idx) Enter .
./ MOVEM 2(IDX) .$b $g
$2B>>START/ MOVE P,PWORD $p
$1B>>START+2/ PUSHJ P,ADDEM $p
$3B>>ADDEM+2/ MOVEM 2(IDX) $x
0/ 5 TABLE1+2/ 5
ADDEM+3/ POPJ P,0 start+4$b $p
$4B>>START+4/ MOVE 2(IDX) $x
0/ 5 TABLE1+2/ 5
START+5/ JFCL 0 ^Z
@
2-20
GETTING STARTED WITH DDT
GETTING STARTED WITH DDT
2.5 PROGRAMMING WITH DDT IN MIND
2.5 PROGRAMMING WITH DDT IN MIND
There are a few MACRO-20 programming techniques that make debugging
with DDT easier. These techniques primarily concern the use of labels
and symbols.
Labels that meaningfully describe (perhaps mnemonically) the function
of the code are more helpful when examining code and setting
breakpoints than labels that are alphanumerically coded (such as
A0001).
When using symbols as offsets into tables, you can prevent DDT from
displaying the offset symbol in place of the symbol's numeric value if
you define the symbol in this way:
symbol==expression
______
Symbol is still entered in the symbol table, and you can use symbol as
______
input to DDT, but DDT does not display symbol on output.
For example, if you have defined:
OFFSET==3
DDT displays the contents of a word that contains the value of 3 as:
addr/ 3
rather than:
addr/ OFFSET
_____ _________
where addr is the address of the word. See the MACRO Assembler
_________ ______
Reference Manual for more information about defining symbols.
2-21
2-22
CHAPTER 3
CHAPTER 3
DDT COMMAND FORMAT
DDT COMMAND FORMAT
3.1 COMMAND SYNTAX
3.1 COMMAND SYNTAX
The complete syntax of a DDT command is:
{arg1<}{arg2>}{arg3}{{}{arg4}}c{arg5}
_
where arg1, arg2, arg3, arg4, and arg5 are arguments to the command c.
Arg1, arg2, and arg3 can be any legal DDT expression. Arg1 must be
followed by a left angle bracket (<), and arg2 must be followed by a
right angle bracket (>). Arg4 can only be a number. Arg5 is a text
argument of the form:
/text/ or c
where text is a string of characters, the slashes (/) are delimiters
that can be any character not contained in text, and c is a single
character.
DDT commands never use all five arguments. Each argument is optional
or required according to the syntax of the specific command. Most DDT
commands are not more complicated than:
arg3c or arg3arg4c
You can enter alphabetic commands and text arguments in uppercase or
lowercase.
An argument to a command can be the result of executing another
command. For example, you can enter a command to evaluate a text
string, and then enter another command to deposit in memory the result
of evaluating the text string. The entire command line would be:
"/abcd/
______
where /abcd/ is the argument to the command " (quotation mark). The
function of the quotation mark command is to evaluate the string
(abcd) within the delimiters (/) as a left-justified ASCII string.
3-1
DDT COMMAND FORMAT
DDT COMMAND FORMAT
____
The left-justified ASCII string abcd is then the argument to the
command (entered by pressing the RETURN key). The function of
the command is to deposit an argument (in this case, the string
____
abcd) into the open location. The " command is described in this
chapter, and the command is described in Chapter 4 (Displaying
and Modifying Memory).
Most commands produce results that are immediately visible, such as
commands that display the contents of memory locations. However,
commands such as those that invoke search functions or those that
evaluate text expressions (as above) may not produce immediately
visible results. If you enter a question mark (?) while DDT is
performing a function invoked by one of these commands, DDT displays a
message that tells you what DDT is currently doing. For example, such
a message might be:
Searching: addr/ value
____
where addr is the address that DDT is to next test as part of a
_____ ____
search, and value is the contents of the memory location at addr.
Still other commands return values that DDT does not display, but can
use as arguments to other commands.
3.2 INPUT TO DDT
3.2 INPUT TO DDT
___________
You enter arguments to DDT as expressions. An expression can be a
single value, or a combination of two or more values with one or more
_________
operators.
3.2.1 Values in DDT Expressions
3.2.1 Values in DDT Expressions
Values in DDT expressions can be:
o octal or decimal integers
o floating point numbers
o symbols
o values that are returned by commands
o text
_____ _______ _____
To enter an octal integer value, simply enter the integer in octal
digits. For example:
70707065
3-2
DDT COMMAND FORMAT
DDT COMMAND FORMAT
_______ _______ _____
To enter a decimal integer value, enter the integer in decimal digits
and follow the value with a decimal point. For example:
9876.
________ _____ ______
To enter a floating point number, use regular or scientific notation.
For example, you can enter the value .034 as one of the following:
.034
3.4E-2
Note that 1. is a decimal integer, while 1.0 is a floating point
number.
______
To enter a symbol as a value in an expression, type in the symbol name
_________ ______
as defined in your program. To enter an undefined symbol that you can
define later, enter:
symbol#
______
where symbol is the symbol that you will later define. See Chapter 7
(Manipulating Symbols in DDT) for more information about using
undefined symbols.
_______ ____ _______ _ _____
You can enter a command that returns a value as a value in an
expression. DDT commands that return values and the values they
return are listed in Table 3-1.
Table 3-1: Commands that Return Values
Table 3-1: Commands that Return Values
COMMAND VALUE RETURNED VALUE ALSO
COMMAND VALUE RETURNED VALUE ALSO
KNOWN AS
KNOWN AS
. The address of the current location. .
. The address of the next user program $.
instruction to be executed.
. The previous value of ".". $$.
nB The address of the DDT location that $nB
contains the address of breakpoint n.
nI The address of the DDT location that
contains the saved machine state flags
(user-program context).
nM The address of DDT "mask" n.
Q The current quantity. $Q
3-3
DDT COMMAND FORMAT
DDT COMMAND FORMAT
Table 3-1: Commands that Return Values (Cont.)
Table 3-1: Commands that Return Values (Cont.)
COMMAND VALUE RETURNED VALUE ALSO
COMMAND VALUE RETURNED VALUE ALSO
KNOWN AS
KNOWN AS
Q The current quantity, with halves $$Q
swapped.
nU The address of the DDT location that
contains the argument (or default) that
was given in the virtual addressing
command: exprnU.
The commands nB, nI, nM, and nU, return values that
are the addresses of locations internal to DDT, which contain
information that you can use and modify. For brevity, these commands
are said to address those internal DDT locations.
For example, the command nB returns (but does not display) the
address of the DDT location that contains the address of breakpoint n,
and the command addr/ (address followed by slash) displays the contents
of the location at addr. To display the address of breakpoint n, enter:
nB/
where you enter the command nB as the expression for DDT to
____
evaluate as addr.
____
| You can enter text to be interpreted in the following ways:
|
| o left-justified ASCII strings
|
| o left-justified SIXBIT strings
|
| o single right-justified ASCII characters
|
| o single right-justified SIXBIT characters
|
| o RADIX50 words
|
You can enter text expressions in uppercase or lowercase. DDT
translates strings to uppercase for SIXBIT or RADIX50 text as required.
____ ____ ______
The term long text string refers to an expression in a DDT command that
is a string of text characters that requires more than one 36-bit
expression for full evaluation. You can enter long text strings in
SIXBIT and ASCII as DDT expressions. If you use a long text string as
an expression, DDT assumes that you will enter a command that deposits
the expression in memory.
3-4
DDT COMMAND FORMAT
DDT COMMAND FORMAT
DDT evaluates the string one 36-bit expression at a time. After
evaluating the first 36-bit expression, DDT deposits the expression in
the open location, closes the open location, and opens the next
location.
DDT then evaluates the next 36-bit expression contained in the string,
and deposits that expression in the (new) open location. This process
_
continues until you enter c, the command. If you enter a command that
does deposit to memory, DDT deposits the final 36-bit expression in the
open location, and updates the location counter according to the rules
of that particular command. The current quantity is the last 36-bit
expression that DDT evaluated.
If you do not enter a command that deposits to memory, DDT uses, as the
argument to the command, the 36-bit expression that was last evaluated.
All other 36-bit expressions that were evaluated as part of the string
have been deposited, and the current and open locations were updated
accordingly. The current quantity is then the last 36-bit expression
that DDT evaluated.
If there is no open location when you begin typing the long text string,
DDT evaluates only the first 36-bit expression, ignores the rest of the
string, and uses the first 36-bit expression as the argument to the
command. The current quantity is then the first 36-bit expression that
DDT evaluated in the string. If you enter a command that deposits to
memory, it has no effect because there was no open location.
o
_____ ______
The syntax to enter an ASCII string is:
"/text/
____
where text is the string, and the slashes (/) represent any printing
____
character that is not contained within text. DDT evaluates the string
as a series of 36-bit expressions, each in 7-bit ASCII format
(left-justified), with all unused bits reset.
For example, if you enter:
"+abc/def+
_____
DDT evaluates one 36-bit expression as the 7-bit ASCII string abc/d in
bits 0-34, and bit 35 reset. If there is no open location, DDT uses
that expression as the argument to the command, and that expression
becomes the current quantity.
_____
If there is an open location, DDT deposits abc/d in the open location,
closes it, and opens the next location in memory. DDT then evaluates a
__
second 36-bit expression as the 7-bit ASCII string ef in bits 0-13, and
bits 14-35 reset. The last 36-bit expression evaluated becomes the
current quantity.
3-5
DDT COMMAND FORMAT
DDT COMMAND FORMAT
NOTE
You cannot use this format to enter an ASCII string that
begins with the ESCAPE character, because
terminates the command that enters a single
right-justified ASCII character (in this case, your
intended delimiter).
______ ______
The syntax to enter a SIXBIT string is:
"/text/
where text is the string, and the slashes (/) represent any printing
____
character that is not contained within text. DDT evaluates the string
as a series of 36-bit expressions, each in SIXBIT format
(left-justified), with any unused bits in the last 36-bit expression
reset. DDT translates lowercase characters to uppercase; all other
non-SIXBIT characters cause DDT to sound your terminal buzzer or bell
and display a question mark.
For example, if you enter:
"/qwertyu/
______
DDT evaluates one 36-bit expression as the SIXBIT string QWERTY in bits
0-35. If there is no open location, DDT uses that expression as the
argument to the command, and that expression becomes the current
quantity.
______
If there is an open location, DDT deposits QWERTY in the open location,
closes it, and opens the next location in memory. DDT then evaluates a
_
second 36-bit expression as the SIXBIT character U in bits 0-5, with
bits 6-35 reset. The last 36-bit expression evaluated becomes the
current quantity.
_____ _________
The syntax to enter a right-justified ASCII character is:
"c
where c is the character. DDT evaluates this as one 36-bit expression
_
with the 7-bit ASCII character c in bits 29-35, and bits 0-28 reset.
3-6
DDT COMMAND FORMAT
DDT COMMAND FORMAT
______ _________
The syntax to enter a right-justified SIXBIT character is:
"c
where c is the character. DDT evaluates one 36-bit expression with the
_
SIXBIT character c in bits 30-35, and bits 0-29 reset. DDT translates
lowercase characters to uppercase; all other non-SIXBIT characters cause
DDT to sound your terminal buzzer or bell and display a question mark.
_______ ____
The syntax to enter a RADIX50 word is:
text5"
____
where text is any string of RADIX50 characters up to six characters
long. DDT evaluates one 36-bit expression with bits 0-3 reset and the
____ ____
RADIX50 string text in bits 4-35. DDT ignores any characters in text
after the sixth.
For example, if you enter:
poiuytr5"
DDT evaluates one 36-bit expression with bits 0-3 reset and the RADIX50
______ _
string POIUYT in bits 4-35. DDT ignores the character r. DDT
____
translates lowercase characters to uppercase. Characters in text not in
the RADIX50 character set that are DDT commands use, as an argument to
____
the command, any characters already entered. Characters in text not in
the RADIX50 character set that are not DDT commands cause DDT to sound
your terminal buzzer or bell and display a question mark.
3.2.2 Operators in DDT Expressions
3.2.2 Operators in DDT Expressions
When you enter an expression, DDT evaluates the expression to create a
36-bit quantity but does not necessarily use all 36 bits when it
executes the command. For example, you can enter a complete MACRO
instruction when giving an argument to a command that requires an
address, but DDT uses only the address specified by the instruction (and
ignores the rest of the evaluated expression) when it executes the
command.
Table 3-2 lists DDT's expression operators and the effects those
_____ __ ___
operators produce on the evaluation. The term value so far represents
the accumulated 36-bit value resulting from evaluation of the expression
to that point.
3-7
DDT COMMAND FORMAT
DDT COMMAND FORMAT
Table 3-2: Effects of Operators When Evaluating Expressions
Table 3-2: Effects of Operators When Evaluating Expressions
OPERATOR EFFECT ON EVALUATION
OPERATOR EFFECT ON EVALUATION
+ Add the 36-bit value on the left to the 36-bit
value on the right, using two's complement
addition.
- Subtract the 36-bit value on the right from the
36-bit value on the left, using two's complement
subtraction.
* Multiply the 36-bit value on the left by the
36-bit value on the right, using PDP-10
full-word integer multiplication. DDT uses only
the low-order 36 bits of the result.
' (apostrophe) Divide the 36-bit value on the left by the
36-bit value on the right, using PDP-10
full-word integer division. DDT ignores any
remainder.
NOTE
Apostrophe is DDT's division
operator. / (slash) is a DDT command
to examine memory, and is never used
in DDT to indicate division.
space Add the previous expression (normally an opcode)
to the value so far, and add the low-order 18
bits of the value at the right of the space to
the low-order 18 bits of the value so far. DDT
ignores carries resulting from the addition, and
does not change the left half of the value so
far.
3-8
DDT COMMAND FORMAT
DDT COMMAND FORMAT
Table 3-2: Effects of Operators When Evaluating Expressions (Cont.)
Table 3-2: Effects of Operators When Evaluating Expressions (Cont.)
OPERATOR EFFECT ON EVALUATION
OPERATOR EFFECT ON EVALUATION
, (comma) If you are entering an I/O instruction, shift
the low-order 18 bits of the expression at the
left of the comma 26 bits to the left (to the
device field of the instruction), otherwise
shift the low-order 18 bits of the expression at
the left of the comma 23 bits to the left (to
the A field of an instruction). Then logically
OR the result into the value so far.
NOTE
DDT does not check whether the value
at the left of the comma is a
legitimate device or AC address, and
may overwrite other parts of the
instruction.
() Swap the halves of the expression within the
parentheses and add the resulting expression to
the value so far. This makes it possible to
enter an instruction that uses an index
register.
NOTE
DDT does not check whether the value
within the parentheses is a
legitimate AC address, and may
overwrite other parts of the
instruction.
@ Assume the expression is an instruction and set
the indirect bit (bit 13) of the value so far.
,, (two commas) Move the low-order bits of the expression at the
left of the commas to bits 0-17 and build a new
18-bit expression in the right half.
3-9
DDT COMMAND FORMAT
DDT COMMAND FORMAT
The nonarithmetic operators allow you to enter expressions in
___________ ______ ____ ______
instruction format as well as in data format.
___________
To enter an instruction, format the instruction as you would in a
MACRO-20 program. For example:
MOVE R4,@VAR1+OFFSET(R5)
NOTE
Follow an opcode (such as MOVE) with a space, not a
.
_________
To enter halfwords, enter the values (numbers or symbols) separated by
two commas (,,). The halfwords can be symbolic or absolute values. For
example:
-1,,SYM1
NOTE
DDT is not designed to evaluate complicated arithmetic
expressions. The nonarithmetic operators are
implemented to enable DDT to evaluate expressions you
enter as MACRO-20 instructions and halfwords. Using
values and operators for other purposes may not produce
the results you intend.
3-10
CHAPTER 4
CHAPTER 4
DISPLAYING AND MODIFYING MEMORY
DISPLAYING AND MODIFYING MEMORY
4.1 DISPLAY MODES
4.1 DISPLAY MODES
A major function of DDT is displaying the contents of memory words, both
data and instructions. You can choose whether to display the contents
of memory words as symbols or as numeric values. You can also select
the radix in which DDT displays numeric values.
DDT displays symbols, labels, and most messages in uppercase.
4.1.1 Default Display Modes
4.1.1 Default Display Modes
There is no sure way for DDT to distinguish between instruction and data
words, or between data words of different formats.
DDT displays memory words in symbolic mode by default. Symbolic mode is
described in Table 4-1. DDT tests for the condition on the left, and if
the condition is met, displays the word in the format described on the
right. DDT performs the tests in descending order.
Table 4-1: Evaluation of Symbolic Display Mode
Table 4-1: Evaluation of Symbolic Display Mode
CONDITION DDT DISPLAYS EXAMPLE
CONDITION DDT DISPLAYS EXAMPLE
Bits 0-18 are all set. A negative number -45
in the current
radix.
The 36-bit value is defined The symbol. SYMBL1
in the user program symbol HALT
table.
The opcode field is zero. Halfwords. 345,,-27
4-1
DISPLAYING AND MODIFYING MEMORY
DISPLAYING AND MODIFYING MEMORY
Table 4-1: Evaluation of Symbolic Display Mode (Cont.)
Table 4-1: Evaluation of Symbolic Display Mode (Cont.)
CONDITION DDT DISPLAYS EXAMPLE
CONDITION DDT DISPLAYS EXAMPLE
The opcode and I, X, and Y The OPDEF. CORE 6,
fields, or the opcode and A
fields match an OPDEF in the
user program symbol table.
The opcode matches a The instruction. MOVE 3,SYMBL
definition in DDT's internal
hardware instruction table.
No match. Halfwords. 3445,,-23
By default, DDT displays numeric values in radix 8. Leading zeros are
always suppressed.
4.1.2 Selecting Display Modes
4.1.2 Selecting Display Modes
You can select display modes to control:
o the format in which DDT tries to interpret the contents of memory
locations; for example, as instructions, or as floating-point
numbers.
o whether addresses are displayed as symbolic or numeric values.
o the radix in which numeric values are displayed.
In addition, you can specify these modes on a short-term (temporary mode)
or long-term (prevailing mode) basis.
A prevailing display mode remains in effect until you select another
prevailing mode, but may be overridden by a temporary mode until you enter
a command that restores the prevailing display mode. DDT commands that
restore the prevailing display mode are:
o {expr} (deposit expr and close location)
o G (start program execution)
o P (proceed from a breakpoint)
o W, E, N (perform a search)
o Z (zero memory)
4-2
DISPLAYING AND MODIFYING MEMORY
DISPLAYING AND MODIFYING MEMORY
_____
o instrX (execute instr)
o V (watch a location)
The syntax of commands that set the prevailing mode is:
mode
where mode is one of the display modes shown in Table 4-2.
The syntax of commands that set a temporary mode is:
mode
where mode is one of the display modes shown in Table 4-2.
_______ _______ ____
The current display mode is the mode (prevailing or temporary) in which
DDT will display the next word (unless you enter a command to change the
display mode).
DDT has two "masks" that control the action of two of the display modes.
3M is a command that addresses a DDT location that contains the
______ ____ ____ ____
output byte size mask. When the current display mode is O, each bit that
is set in the mask indicates the position of a low order bit of a byte in
the word being displayed. In this mode, bit 35 is always assumed to be
set. For example, if the output byte size mask contains:
510410100400 (octal)
the byte sizes specified are, from left to right, 1, 2, 3, 4, 5, 6, 7, and
8. When displaying a word in O mode that contains 777777,,777777, and the
current radix is 8, DDT displays:
1,3,7,17,37,77,177,377
The default value of the output byte size mask is zero, specifying one
36-bit byte.
You can set the output byte size mask with the command:
expr3M
____
where expr evaluates to the bit pattern required.
You can also examine and change the output byte size mask with the examine
and deposit commands described later in this chapter.
4-3
DISPLAYING AND MODIFYING MEMORY
DISPLAYING AND MODIFYING MEMORY
2M is a command that addresses a DDT location that contains the
_______ ________ ______
maximum symbolic offset. When DDT displays an address in R(elative) mode,
it displays the address symbolically, that is, as a symbol, or as a symbol
+ the numeric offset of the address from that symbol. The maximum
symbolic offset (minus 1) determines the maximum offset address that DDT
displays symbolically, and defaults to 1000 (octal). DDT displays
addresses beyond that offset in A(bsolute) mode. For example, assume that
the maximum symbolic offset is 2, and that you are examining subroutine
ADDEM in program X.MAC (Fig 2-1), using to display instructions in
sequence. DDT displays:
ADDEM/ MOVE 0(6)
ADDEM+1/ ADD 1(6)
addr/ MOVE 2(6)
____
where addr is the absolute address (for example, 14414) of the location.
You can set the maximum symbolic offset with the command:
expr2M
____
where expr evaluates to the offset required.
You can also examine and change the maximum symbolic offset with the
examine and deposit commands described later in this chapter.
DDT display modes and the commands that select them are described in Table
4-2.
Table 4-2: DDT Display Modes
Table 4-2: DDT Display Modes
FORMAT MODES
FORMAT MODES
MODE EFFECT
C Display memory word as numbers in the current radix (see
Radix Modes).
F Display memory word as a floating point decimal number.
H Display memory word as two halfword addresses (see
Address Modes) separated by two commas (,,).
O Display memory word as numeric bytes of sizes that are
specified by the 3M mask.
n0 Display memory word as n-bit numeric bytes, (with
trailing remainder byte, as required).
4-4
DISPLAYING AND MODIFYING MEMORY
DISPLAYING AND MODIFYING MEMORY
Table 4-2: DDT Display Modes (Cont.)
Table 4-2: DDT Display Modes (Cont.)
FORMAT MODES
FORMAT MODES
MODE EFFECT
S Display memory word in symbolic mode (default).
1S Search DDT's internal hardware opcode table before
searching the user's symbol table, otherwise follow
rules for symbolic mode.
nT Display memory word as ASCII text, using n-bit bytes.
| n=1: Byte Pointer Format
n=5: RADIX50
n=6: SIXBIT
| n=7 through 36:
|
| Specifies the number of bits per byte. The
| default is 7-bit ASCII.
|
| n=0: ASCIZ
|
| (Stop ASCIZ typeout by typing any character.)
A Display addresses as absolute values in the current
radix.
R Display addresses as values relative to symbols
(default). DDT displays the offsets in the current
radix. The maximum offset is controlled by the value
stored in the 2M mask, and defaults to 1000
(octal).
RADIX MODES
RADIX MODES
MODE EFFECT
nR Display numeric values in radix n (default=8), where n
is a decimal number greater than 1. If n=8, DDT
displays the word as octal halfwords, otherwise DDT
displays the word as one number.
o
4-5
DISPLAYING AND MODIFYING MEMORY
DISPLAYING AND MODIFYING MEMORY
4.2 DISPLAYING EXPRESSIONS
4.2 DISPLAYING EXPRESSIONS
DDT has three commands you can use to display expressions in different
modes. They are:
; (semicolon)
= (equal sign)
_ (underscore)
The syntax of these commands is:
{expr}c
____ ____
where expr is the expression to display (expr defaults to the current
_
quantity), and c is one of the above commands. These commands are
useful for redisplaying the current quantity without affecting the
current display mode. Table 4-3 lists the commands to display
expressions and their effects.
Table 4-3: Commands to Display Expressions
Table 4-3: Commands to Display Expressions
COMMAND EFFECT
COMMAND EFFECT
; Display the current quantity in the current display
mode.
expr; Display expr in the current display mode.
= Display the current quantity as a number in the
current radix.
expr= Display expr as a number in the current radix.
- Display the current quantity in 1$ mode.
expr_ Display expr in 1$ mode.
4.3 DISPLAYING BYTE POINTERS
4.3 DISPLAYING BYTE POINTERS
If you set the display mode to 1T, DDT displays the contents of the
memory location as a byte pointer. DDT can display one-word local,
one-word global, and two-word byte pointers. DDT displays the P and S
fields, and the address as determined by the I, X, and Y fields of the
byte pointer.
In section zero, DDT displays only one-word byte pointers (local and
global).
4-6
DISPLAYING AND MODIFYING MEMORY
DISPLAYING AND MODIFYING MEMORY
For example, if the contents of the location at ADDR2 is 100702,,addr,
where addr is the value of symbol LABL2, the following illustrates
one-word local byte pointer display:
addr2/ 100702,,addr lT; 10 7 LABL2(2)
The following illustrates one-word global byte pointer display, where
addr is the value of symbol LABL2:
1,,addr2/ 610002,,LABL2 lT; 44&7 2,,LABL2
The following illustrates two-word global byte pointer display, where
addr is the value of symbol LABL2 (DDT echoes as ^H):
1,,addr2/ 440740,,0
1,,addr2+1/ 3,,addr lT^H
1,,addr2/ 44 7 3,,MAIN. <2>
4.4 DISPLAYING AND DEPOSITING IN MEMORY
4.4 DISPLAYING AND DEPOSITING IN MEMORY
DDT allows you to display the contents of memory locations and deposit
a new value in the open location. In performing these functions, you
must understand the concept of the open location, the current
location, the location sequence stack, and the current quantity.
____ ________
The open location is a memory location (or AC) that is "open" for
modification by the next command. There is never more than one
location open at a time. DDT always closes the open location before
opening another.
________ _______
The location counter contains the address of a word in memory that has
been referenced (implicitly or explicitly) by the previous command,
and that is the default point of reference for the next command. That
_______ ________
word is known as the current location. DDT uses the address of the
current location as the default address in most commands. The current
location is often, but not always, the open location.
Most DDT commands change the current location to a word specified by
an address given (explicitly or by default) in the command. Commands
that do not are so indicated.
"." (period) is a command that returns (but does not display) the
address of the current location.
When you first enter DDT, the current location is zero.
4-7
DISPLAYING AND MODIFYING MEMORY
DISPLAYING AND MODIFYING MEMORY
________ ________ _____
The location sequence stack is a "ring" of seventeen words, each
containing the address of a prior current location, or of a match
found during a search. The present value of the current location is
not placed in the ring.
Entries are made to and retrieved from the location sequence stack in
a last-in, first-out manner. Most commands that change the location
counter by values other than +1 and -1 cause DDT to place the address
of the current location (before the change) on the location sequence
stack. Addresses of matching locations found during searches are also
placed on the location sequence stack. When DDT enters a new value in
_______ ________
the next word on the stack, the new value becomes the current location
_____ _____
stack entry. This is similar to PUSHing entries on a stack. When the
current location stack entry is the last location on the location
sequence stack, DDT enters a new value on the stack by "wrapping
around" to the beginning of the stack and overwriting the value in the
first location on the stack. The first location on the stack then
contains the current location stack entry.
Certain DDT commands change the address of the current location to the
current location stack entry, and then change the current location
stack entry to the previous entry. This is similar to POPing entries
off a stack, and allows you to "return" to locations that have
previously been the current location. When the first location on the
location sequence stack contains the current location stack entry and
DDT changes the address of the current location to the current
location stack entry, DDT "wraps around" to the end of the stack, and
the value contained in the last word of the stack becomes the current
location stack entry (whether or not the stack was previously "full").
_______ ________
The current quantity is a value that is the most recent of:
o the last 36-bit quantity that DDT displayed (an expression or
the contents of a memory location)
o the last expression that you entered as an argument to a
command that deposits to memory
____ _____ _____
This value is also known as the last value typed. Q is a command
that returns (but does not display) the current quantity. DDT issues
an implicit Q to return this value for use as the default
argument for some commands.
You can give the current quantity as an argument to a command by
entering the command Q as the argument.
The command Q returns the current quantity with the right
and left halves swapped.
This manual uses the term $Q to refer to the value that is returned by
the command Q, and the term $$Q to refer to the value that is
returned by the command Q.
4-8
DISPLAYING AND MODIFYING MEMORY
DISPLAYING AND MODIFYING MEMORY
Some commands calculate the address of the location to be opened from
an expression given or defaulted in the command. Other commands use
the address of the current location or entries on the location
sequence stack.
The general syntax of these commands is:
{expr}{}c
____
where expr is any legal DDT expression, and c is the command.
NOTE
______ __ ___ ___________
See Values in DDT Expressions in Chapter 3 for a
discussion of long text strings as values in DDT
expressions.
Table 4-4 summarizes the commands and their effects. Complete
descriptions of the commands follow the table.
Table 4-4: DDT Commands to Display Memory
Table 4-4: DDT Commands to Display Memory
COMMAND DISPLAY MODE OPEN CHANGE DEPOSIT
COMMAND DISPLAY MODE OPEN CHANGE DEPOSIT
CONTENTS OF THE CURRENT EXPR
CONTENTS OF THE CURRENT EXPR
DISPLAY LOCATION LOCATION
DISPLAY LOCATION LOCATION
/ Yes Current Yes Yes(1) No
[ Yes Numeric Yes Yes(1) No
] Yes Symbolic Yes Yes(1) No
! No Suppress Yes Yes(1) No
\ Yes(2) Current Yes No Yes(1)
Yes(2) Current Yes Yes Yes(1)
No Restore No No Yes(1)
Yes(2) Current Yes Yes(.+1) Yes(1)
Yes(2) Current Yes Yes(.-1) Yes(1)
or ^
(1) If you enter expr.
(2) If not suppressed by !.
4-9
DISPLAYING AND MODIFYING MEMORY
DISPLAYING AND MODIFYING MEMORY
4.4.1 Commands that Use the Current Location
4.4.1 Commands that Use the Current Location
The commands , , and use the address of the current
location to determine the next address of the current location.
These commands do not make entries to the location sequence stack.
{expr} does the following:
o deposits expr (if given) in the open location
o closes the open location
o resets the current typeout mode to the prevailing typeout
mode
o does not change the address of the current location
{expr} does the following:
o deposits expr (if given) in the open location
o closes the open location
o increments the location counter
o opens the current location
o
o displays the open location (unless display has been
suppressed by !)
{expr} and {expr}^ do the following:
o deposits expr (if given) in the open location
o closes the open location
o decrements the location counter
o opens the current location
o
o displays the open location (unless display has been
suppressed by !)
4-10
DISPLAYING AND MODIFYING MEMORY
DISPLAYING AND MODIFYING MEMORY
4.4.2 Commands that Use the Location Sequence Stack
4.4.2 Commands that Use the Location Sequence Stack
The commands