












                             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
        VAX                 EduSystem           VMS
                                                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.

   <ESC>          Represents pressing the ESCAPE or ALTMODE key once.

   <ESC><ESC>     Represents pressing the ESCAPE or ALTMODE key twice.

   <CTRL/X>       Represents pressing a key (represented  by  X)  at  the
                  same time as you press the key labeled CTRL.

   <BKSP>         represents pressing the BACKSPACE key or <CTRL/H>.

   <LF>           Represents pressing the LINE FEED key.

   <RET>          Represents pressing the RETURN key.

   <TAB>          Represents pressing the TAB key or <CTRL/I>.


                                     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 <BKSP>, <CTRL/x>, <ESC>, <LF>,  <RET>,  and  <TAB>  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 <ESC> 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  <ESC>  when  you
           press the ESCAPE key.


                                    2-2
                          GETTING STARTED WITH DDT
                          GETTING STARTED WITH DDT


                                    NOTE

           This manual uses  the  symbols  <BKSP>,  <ESC>,  <LF>,
           <RET>,  and  <TAB>  to  indicate  where  you press the
           BACKSPACE, ESCAPE, LINE FEED, RETURN,  and  TAB  keys,
           respectively.    This  manual  also  uses  the  symbol
           <CTRL/X> 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
           ______  _____  ____  _____   _____       ________
           <BKSP>, <ESC>, <LF>, <RET>,  <TAB>,  or  <CTRL/X>,  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 <CTRL/U>.  When you do, DDT
   displays:

        XXX

   To exit DDT, enter:

        <CTRL/Z>

   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:

        <ESC>?

   (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:

        <ESC>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:

        addr<ESC>B

         ____
   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  <TAB>.   This
|  command  starts  a  new display line before displaying the contents of
   ____
|  addr, making the display easier to read.  For example,  if  you  enter
   <TAB>  after  DDT  displays  the  address  and contents of ADDR1+5 (as
   above) on your terminal, the terminal display appears:

        ADDR1+5/   SYM1,,SYM2   <TAB>
        SYM2/   value

         _____
   where value is the contents of the word located at SYM2.   <TAB>  does
   not  appear  on  the  screen, but is shown above to indicate where you
|  press the <TAB> key.  <TAB> changes the current location to  SYM2  and
|  opens the word at SYM2.  In this example, the current quantity becomes
   _____
|  value.

   <TAB> 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:

        <ESC><RET>

   In the above example, after you press <TAB> at ADDR1+5,  DDT  displays
   the  contents  of SYM2 and changes the current location to SYM2.  When
   you enter <ESC><RET>, 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<TAB>,  DDT  deposits  addr  in  the  open
                                                                     ____
   location  and  closes the location before opening the location at addr
   and displaying  its  contents.   <TAB>  by  itself  does  not  deposit
   anything,  but does save the current location on the location sequence
   stack, making <TAB> more useful than / (slash by itself).

   You can display and open  the  word  after  the  current  location  by
   entering:

        <LF>








                                    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:

        <LF>

   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   <LF>
        ADDR1+6/   -1,,SYM3

                                                  ____
   Note that DDT does not display the characters  <LF>.   <LF>  does  not
   affect the location sequence stack.

   Entering another <LF> causes DDT to display and open the next word.

                                ________
   To display and open the word previous to the current location, enter:

        <BKSP>

   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.  <BKSP> does not affect  the  location
   sequence  stack.  For example, if you enter <BKSP> to open and display
   the location before ADDR1+5, the screen appears as follows:

        ADDR1+5/   SYM1,,SYM2   <BKSP>
        ADDR1+4/   -3,,SYM2

             ______
   Note that <BKSP> does not appear on the screen.

   To change the contents of the open location, enter:

        value<RET>

         _____
   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<RET>



                                    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  <RET>.  The terminal display
   appears as follows (your input is in lowercase):

        labl2/   MOVE 1,SYM1   move 1,sym2<RET>

   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<RET>
        ./   MOVE 1,SYM2

   After entering a command to display and open a location, if you enter:

        value<LF>

   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<LF>
        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:

        <ESC>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 <ESC>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 <ESC>X commands.  This  is  known
      _______________
   as single-stepping.

      _______ _ __________
   To execute a subroutine, enter:

        <ESC><ESC>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:

        <ESC>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 <LF>, <RET>, or <TAB>.  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<RET>   Begin the session by entering
                                           "debug x<RET>", 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. 

                               <LF>        Press <LF> 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

                            .jbda<ESC>k    Enter .jbda<ESC>k
                                           to suppress DDT typeout of
                                           symbol .JBDA.  DDT will
                                           display START rather than
                                           .JBDA from now on.

                               x<ESC>:     Enter the module name (X)
                                           followed by <ESC> and a
                                           colon to open the symbol
                                           table associated with
                                           X. DDT will not append any
                                           more pound-signs.

                               <TAB>       Press <TAB> 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 <TAB>, but
                                           does not start a new line.)
                                           <TAB> 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 <TAB>
                                           command, DDT displays the
                                           address and the contents of
                                           the location.  The first
                                           element of the table contains
                                           zero. The <TAB> command also
                                           opens the location.

                               2<LF>       Enter "2" followed by <LF> 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<LF>       Enter "3" followed by <LF> 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.

                               <ESC><RET>  Press <ESC>, then press <RET>
                                           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 <TAB>.

                               <LF>        Press <LF> to look at the 
                                           next location.

   START+2/   PUSHJ P,ADDEM                This is the call to the
                                           subroutine that does the
                                           computation.

                               .<ESC>b     Enter ".", press <ESC>, and
                                           enter "b" to set a
                                           breakpoint at the current
                                           location.

                               <ESC>g      Enter <ESC>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.

                            <ESC><ESC>x    Press <ESC> 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
                                           "<SKIP>" if the program
                                           skipped one instruction.  If
                                           the program skips 2 or 3
                                           instructions, DDT displays
                                           "<SKIP n>", where n is the
                                           number of instructions
                                           skipped.

                               <ESC>x      Press <ESC> 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.

                              <ESC><TAB>   Press <ESC>, then <TAB> to
                                           display the contents of the
                                           location addressed by the
                                           instruction, using any
                                           indexing and indirection.
                                           (If you omit <ESC>, 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

                               <BKSP>      Press <BKSP> to see the 
                                           previous element in the table.

   TABLE1+1/   3                           This element contains 3.  That
                                           is correct.

                               <BKSP>      Press <BKSP> 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.

                             start<ESC>b   Enter "start", press <ESC>,
                                           and enter "b" to set a
                                           breakpoint at the beginning of
                                           the program.

                               <ESC>g      Press <ESC> 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.

                               <ESC>x      Press <ESC>, 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

                               <ESC>x      Press <ESC>, 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.

                               <ESC>x      Press <ESC>, 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.

   <JUMP>                                  DDT displays "<JUMP>" 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.

                               <ESC>x      Press <ESC> 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.  

                               <ESC>x      Press <ESC> 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.

                               <ESC>x      Press <ESC> 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 <ESC>x).

                               <BKSP>      Press <BKSP> 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)<RET>

                                           Enter the new instruction
                                           and press <RET>.

                               ./          Check the current location
                                           to see what you deposited.

   MOVEM 2(IDX)                            Looks OK.

                               .<ESC>b     Set a breakpoint at 
                                           ".", the current location.

                               <ESC>g      Restart the program at
                                           the beginning.

   $2B>>START/   MOVE P,PWORD              DDT displays the breakpoint
                                           information.

                               <ESC>p      Press <ESC> 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.

                               <ESC>p      Proceed from breakpoint 1.

   $3B>>ADDEM+2/   MOVEM 2(IDX)            DDT displays the breakpoint
                                           information.  This is the
                                           instruction you changed.

                               <ESC>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+4<ESC>b   Set a breakpoint at
                                           START+4 to check the results.


                               <ESC>p      Proceed from breakpoint 3.

   $4B>>START+4/   MOVE 2(IDX)             DDT displays the breakpoint
                                           information.

                               <ESC>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.

                               <CTRL/Z>    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 <LF>.
   .JBDA+1/   MOVEI IDX,TABLE1#   .jbda$k   x$:      Enter <TAB>.
   TABLE1/   0   2                                   Enter <LF>.
   TABLE1+1/   0   3                                 Enter <LF>.
   TABLE1+2/   0   $                                 Enter <ESC><RET>.
   START+1/   MOVEI IDX,TABLE1                       Enter <LF>.
   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 <ESC><TAB>.
   TABLE1+2/   0                                     Enter <BKSP>.
   TABLE1+1/   3                                     Enter <BKSP>.
   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
   <JUMP>
   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)   $x
      0/   0   TABLE1+2/   0
   ADDEM+3/   POPJ P,0                               Enter <BKSP>.
   ADDEM+2/   MOVE 2(IDX)   movem r0,answer(idx)     Enter <RET>.
   ./   MOVEM 2(IDX)   .$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}{<ESC>{<ESC>}{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<ESC>

   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:

        arg3<ESC>c     or     arg3<ESC>arg4c

   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/<RET>

         ______
   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 <RET> (entered by pressing the RETURN key).  The  function  of
   the  <RET> 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 <RET> 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.           .

      <ESC>.     The  address  of  the next user program       $.
                 instruction to be executed.

   <ESC><ESC>.   The previous value of "<ESC>.".               $$.

     <ESC>nB     The  address  of  the DDT location that       $nB
                 contains the address of breakpoint n.

     <ESC>nI     The  address  of  the DDT location that
                 contains the saved machine state  flags
                 (user-program context).

     <ESC>nM     The address of DDT "mask" n.

      <ESC>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

   <ESC><ESC>Q   The   current   quantity,  with  halves       $$Q
                 swapped.

     <ESC>nU     The  address  of  the DDT location that
                 contains the argument (or default) that
                 was given  in  the  virtual  addressing
                 command: expr<ESC>nU.


   The commands <ESC>nB, <ESC>nI, <ESC>nM, and <ESC>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 <ESC>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:

        <ESC>nB/

   where you enter the  command  <ESC>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  <ESC>
           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:

        <ESC>"/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:

        <ESC>"/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<ESC>

   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:

        <ESC>"c<ESC>

   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:

        text<ESC>5"

         ____
   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:

        poiuytr<ESC>5"

   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
           <TAB>.

            _________
   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}<RET>               (deposit expr and close location)

         o  <ESC>G                    (start program execution)

         o  <ESC>P                    (proceed from a breakpoint)

         o  <ESC>W, <ESC>E, <ESC>N    (perform a search)

         o  <ESC>Z                    (zero memory)



                                    4-2
                      DISPLAYING AND MODIFYING MEMORY
                      DISPLAYING AND MODIFYING MEMORY


                                               _____
         o  instr<ESC>X               (execute instr)

         o  <ESC>V                    (watch a location)

   The syntax of commands that set the prevailing mode is:

        <ESC><ESC>mode

   where mode is one of the display modes shown in Table 4-2.

   The syntax of commands that set a temporary mode is:

        <ESC>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.

   <ESC>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:

        expr<ESC>3M

         ____
   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


   <ESC>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 <LF> 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:

        expr<ESC>2M

         ____
   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 <ESC>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  <ESC>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   <ESC>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   <ESC>lT;   44&7 2,,LABL2

   The following illustrates two-word global byte pointer display,  where
   addr is the value of symbol LABL2 (DDT echoes <BKSP> as ^H):

        1,,addr2/   440740,,0   <LF>
        1,,addr2+1/   3,,addr   <ESC>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.  <ESC>Q is a command
   that  returns (but does not display) the current quantity.  DDT issues
   an implicit <ESC>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 <ESC>Q as the argument.

   The command <ESC><ESC>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  <ESC>Q,  and  the term $$Q to refer to the value that is
   returned by the command <ESC><ESC>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}{<ESC>}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)

       <TAB>     Yes(2)     Current      Yes        Yes      Yes(1)

       <RET>       No       Restore      No         No       Yes(1)

       <LF>      Yes(2)     Current      Yes     Yes(.+1)    Yes(1)

      <BKSP>     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 <RET>, <LF>, and <BKSP> 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}<RET> 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}<LF> 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}<BKSP>  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 <ESC><RET>, <ESC><LF>, and <ESC><BKSP>  use  the  current
   location  stack  entry  to  determine  the next address of the current
   location.

   Repetitions of these commands refer to successively earlier entries on
   the stack, until you again address the most recent entry.

   These commands do not make entries to the location sequence stack.

   {expr}<ESC><RET> does the following:

         o  deposits expr (if given) in the open location

         o  closes the open location

         o  changes the value contained in the location  counter  to  the
            current location stack entry

         o  opens the current location

         o  starts a new line and displays the address  and  contents  of
            the open location in the current display mode

         o  causes the previous entry on the location sequence  stack  to
            become the current location stack entry

                                    NOTE

           If display is suppressed as a result of  using  the  !
           command,  the  command  {expr}<ESC><RET>  restores the
           current display mode, which can be either a  temporary
           or prevailing display mode.

   {expr}<ESC><LF> does the following:

         o  deposits expr (if given) in the open location

         o  closes the open location

         o  changes the value contained in the location  counter  to  the
            current location stack entry

         o  increments the location counter

         o  opens the current location

         o  starts a new line  and  displays  the  address  of  the  open
            location




                                    4-11
                      DISPLAYING AND MODIFYING MEMORY
                      DISPLAYING AND MODIFYING MEMORY


         o  displays the contents of the open  location  (unless  display
            has been suppressed by !)

         o  causes the previous entry on the location sequence  stack  to
            become the current location stack entry


   {expr}<ESC><BKSP>  and  {expr}<ESC>^ do the following:

         o  deposits expr (if given) in the open location

         o  closes the open location

         o  changes the value contained in the location  counter  to  the
            current location stack entry

         o  decrements the location counter

         o  opens the current location

         o  displays the address of the open location
o  

         o  displays the contents of the open  location  (unless  display
            has been suppressed by !)

         o  causes the previous entry on the location sequence  stack  to
            become the current location stack entry



   4.4.3  Commands that Use an Address within the Command
   4.4.3  Commands that Use an Address within the Command

   The commands:

        /      (slash)
        [      (left square bracket)
        ]      (right square bracket)
        !      (exclamation point)
        \      (backslash)
      <TAB>

   use an expression given  in  the  command  (either  explicitly  or  by
   default)  to  determine  the addresses of the current location and the
   open location.









                                    4-12
                      DISPLAYING AND MODIFYING MEMORY
                      DISPLAYING AND MODIFYING MEMORY


   The complete syntax of these commands is:

        {expr}{<ESC>{<ESC>}}c

   where expr may be an address, ".", a symbol, or any expression that is
   legal in DDT, and c is the command.

   When you use the commands /, [, ], !, \, and <TAB>:

                        ____
         o  If you omit expr

             >  DDT uses the current quantity as a default.

             >  <TAB> enters the address of the current location  on  the
                location  sequence stack and changes the current location
                to the address determined from the current quantity.

                         ____
         o  If you enter expr, DDT enters  the  address  of  the  current
            location on the location sequence stack (except \).

                       ____
         o  DDT treats expr (whether given or defaulted) as if it were in
            instruction   format   and  performs  the  effective  address
            calculation as follows:

             >  If you omit <ESC>,  DDT  does  not  perform  indexing  or
                indirection.

             >  If you include one <ESC>, DDT  treats  expr  as  an  IFIW
                (instruction  format indirect word), and uses the I and Y
                fields of expr to perform indexing and  indirection  when
                appropriate.

             >  If you  use  <ESC><ESC>,  DDT  utilizes  EFIWs  (extended
                format  indirect  words), as appropriate, when performing
                effective address calculations, and can thereby calculate
                30-bit addresses.

             >  In section zero,  when  you  include  <ESC><ESC>,  it  is
                treated as one <ESC>.

   These commands always do the following:

         o  close the open location

         o  open the location at the address indicated by expr

         o  change the current  quantity  to  the  value  displayed  (all
            commands except !)






                                    4-13
                      DISPLAYING AND MODIFYING MEMORY
                      DISPLAYING AND MODIFYING MEMORY


   The following is a list that  gives  a  complete  description  of  the
   effects of each command.

   COMMAND                  EFFECTS

   /

         o  closes the open location

         o  opens the location at the address calculated from the current
            quantity

         o  displays the contents of the open  location  in  the  current
            display mode

         o  sets the current quantity to the value displayed

   expr/

         o  closes the open location

         o  opens the location at the address calculated from expr

         o  enters the address of the current location  on  the  location
            sequence stack

         o  changes the current location to the location at  the  address
            calculated from expr

         o  displays the contents of the open  location  in  the  current
            display mode

         o  sets the current quantity to the value displayed

   [

         o  closes the open location

         o  opens the location at the address calculated from the current
            quantity

         o  displays the contents of the open location in numeric mode in
            the current radix
o  

         o  sets the current quantity to the value displayed








                                    4-14
                      DISPLAYING AND MODIFYING MEMORY
                      DISPLAYING AND MODIFYING MEMORY


   expr[

         o  closes the open location

         o  opens the location at the address calculated from expr

         o  enters the address of the current location  on  the  location
            sequence stack

         o  changes the current location to the location at  the  address
            calculated from expr

         o  displays the contents of the open location in numeric mode in
            the current radix

         o  sets the current display mode to numeric mode in the  current
            radix

         o  sets the current quantity to the value displayed

   ]

         o  closes the open location

         o  opens the location at the address calculated from the current
            quantity

         o  displays the contents of the open location in symbolic mode

         o  sets the current display mode to symbolic mode

         o  sets the current quantity to the value displayed

   expr]

         o  closes the open location

         o  opens the location at the address calculated from expr

         o  enters the address of the current location  on  the  location
            sequence stack

         o  changes the current location to the location at  the  address
            calculated from expr

         o  displays the contents of the open location in symbolic mode

         o  sets the current display mode to symbolic mode

         o  sets the current quantity to the value displayed




                                    4-15
                      DISPLAYING AND MODIFYING MEMORY
                      DISPLAYING AND MODIFYING MEMORY


   !

         o  closes the open location

         o  opens the location at the address calculated from the current
            quantity

         o  does not display the contents of the open location

         o  suppresses display of the open  location  by  the  \,  <TAB>,
            <LF>, and <BKSP> commands (any other display command restores
            the current display mode)

         o  does not change the current quantity

   expr!

         o  closes the open location

         o  opens the location at the address calculated from expr

         o  enters the address of the current location  on  the  location
            sequence stack

         o  changes the current location to the location at  the  address
            calculated from expr

         o  does not display the contents of the open location

         o  suppresses display of the open  location  by  the  \,  <TAB>,
            <LF>, and <BKSP> commands (any other display command restores
            the current display mode)

         o  does not change the current quantity

   \

         o  closes the open location

         o  opens the location at the address calculated from the current
            quantity

         o  displays the contents of the open  location  in  the  current
            display mode (unless display has been suppressed by !)

         o  sets the current quantity to the value displayed








                                    4-16
                      DISPLAYING AND MODIFYING MEMORY
                      DISPLAYING AND MODIFYING MEMORY


   expr\

         o  deposits expr in the open location

         o  closes the open location

         o  opens the location at the address calculated from expr

         o  does not change the address of the current location (and does
            not enter the address of the current location on the location
            sequence stack)

         o  displays the contents of the open  location  in  the  current
            display mode (unless display has been suppressed by !)

         o  sets the current quantity to the value displayed

   <TAB>

         o  closes the open location

         o  opens the location at the address calculated from the current
            quantity

         o  enters the address of the current location  on  the  location
            sequence stack

         o  changes the current location to the location at  the  address
            calculated from the current quantity

         o  starts a new line  and  displays  the  address  of  the  open
            location (which is also the current location)

         o  displays the contents of the open  location  in  the  current
            display mode (unless display has been suppressed by !)

         o  sets the current quantity to the value displayed

   expr<TAB>

         o  deposits expr in the open location

         o  closes the open location

         o  opens the location at the address calculated from expr

         o  enters the address of the current location  on  the  location
            sequence stack

         o  changes the current location to the location at  the  address
            calculated from expr



                                    4-17
                      DISPLAYING AND MODIFYING MEMORY
                      DISPLAYING AND MODIFYING MEMORY


         o  starts a new line  and  displays  the  address  of  the  open
            location (which is also the current location)

         o  displays the contents of the open  location  in  the  current
            display mode (unless display has been suppressed by !)

         o  sets the current quantity to the value displayed

                 ____
   You can treat expr as an IFIW (instruction format indirect word),  and
                                                      ____
   use  any  indexing  and  indirection  specified by expr to compute the
   effective address of the location to be opened.  Use the command form:

        {expr}<ESC>c

   where c is /, [, ], !, \, or <TAB>.

   For example, assume the  following  conditions  as  indicated  by  the
   display commands:

   COMMAND           DISPLAY            EXPLANATION

   LABL1/             SYM1      Display contents of LABL1.
   LABL1+1/           SYM2      Display contents of LABL1+1.
   SYM2/              SYM3      Display contents of SYM2.
   2/                 1         Display contents of AC 2.
   @LABL1(2)/         SYM1      DDT uses Y field only.
   @LABL1(2)<ESC>/    SYM3      <ESC> causes indexing and indirection.

   Note that DDT does not start a new line unless you enter <TAB>, <RET>,
   <LF> or <BKSP>, or until the display wraps around the end of the line.
   DDT also displays three spaces (or a tab, depending on the TTY control
   mask)  before  and  after  its  output.   Thus, an actual DDT terminal
   display might be the following (user  input  is  lowercase;  <LF>  and
   <TAB> do not appear on the screen, but are shown to indicate where you
   pressed the corresponding keys):

        2/   1   labl1/   SYM1   <LF>
        LABL1+1/   SYM2   <TAB>
        SYM2/   SYM3   sym4/   MOVE 1,@LABL1(2)   <ESC><TAB>
        SYM2/   SYM3

                 ____
   You can treat expr as an EFIW (extended format indirect word) and  use
                                             ____
   any indexing and indirection specified by expr to compute the (global)
   effective address of the location to be opened.  Use the command form:

        {expr}<ESC><ESC>c

   where c is /, [, ], !, \, or <TAB>.






                                    4-18
                      DISPLAYING AND MODIFYING MEMORY
                      DISPLAYING AND MODIFYING MEMORY


   4.5  DISPLAYING ASCIZ STRINGS
   4.5  DISPLAYING ASCIZ STRINGS

   You can display memory as an ASCIZ string.  The command

        addr<ESC>0T

   where addr defaults to the open location (if there is  one,  otherwise
   addr  defaults  to  the  current location), displays memory, beginning
   with addr, as an ASCIZ string.  The display stops  when  DDT  finds  a
   zero  byte, or when you type in any character, which DDT displays, but
   otherwise ignores.  The current location remains unchanged.



   4.6  ZEROING MEMORY
   4.6  ZEROING MEMORY

   To deposit the same value in each of a string of memory words  (useful
   for initializing memory to zero), enter:

        addr1<addr2>{expr}<ESC>Z

   where expr is any legal DDT expression, addr1 is  the  first  word  to
   receive  expr,  and addr2 is the last.  Follow addr1 with a left angle
   bracket (<) and addr2 with a right angle bracket (>).  Both addr1  and
   addr2  are required.  If you omit expr, it defaults to zero.  Prior to
   execution, DDT enters the address  of  the  current  location  on  the
   location  sequence  stack  and closes the open location.  When DDT has
|  completed execution of the command, the current location is  the  word
|  at  addr2  + 1.  There is no open location.  This command restores the
   prevailing display mode.

   If you enter:

        ?

   while DDT is executing the <ESC>Z command, DDT displays:

        Depositing:  addr/   value

         ____
   where addr is the location where DDT will make the next  deposit,  and
   _____                    ____
   value is the contents of addr before the deposit.

   If you enter any other  character,  DDT  stops  executing  the  <ESC>Z
|  command,  and  waits  for  your  next command.  The character that you
|  enter to terminate the <ESC>Z command is otherwise ignored.









                                    4-19
                      DISPLAYING AND MODIFYING MEMORY
                      DISPLAYING AND MODIFYING MEMORY


   4.7  AUTOMATIC WRITE-ENABLE
   4.7  AUTOMATIC WRITE-ENABLE

   If  you  attempt  to  deposit  a  value  in   a   location   that   is
   write-protected, DDT returns the message

        ?NOT WRITABLE

   This is the TOPS-20 default condition.

   To allow DDT to modify write-protected memory, type in

        <ESC>{0}W

   If you  now  attempt  to  deposit  a  value  in  a  location  that  is
   write-protected,  DDT  removes the protection, deposits the value, and
   then reinvokes the protection.

   Note that you cannot use this command to enable patching in FILDDT.

   To prevent DDT from modifying write-protected memory, type in

        <ESC><ESC>{0}W

   The zero in the above commands is optional and has no  effect  on  the
   operation of the commands.  DDT allows the zero for compatibility with
   prior versions of DDT.




























                                    4-20
                      DISPLAYING AND MODIFYING MEMORY
                      DISPLAYING AND MODIFYING MEMORY


   4.8  AUTOMATIC PAGE CREATION
   4.8  AUTOMATIC PAGE CREATION

   If you attempt to deposit a value in a location within  a  nonexistent
   page,  DDT creates the page and deposits the value.  If you attempt to
   deposit a value within a nonexistent section, DDT creates the  section
   as well as the page.  This is the default condition.

   To prevent DDT from creating a page when  you  attempt  to  deposit  a
   value within a nonexistent page, type in

        <ESC><ESC>1W

   If you now  attempt  to  deposit  a  value  in  a  location  within  a
   nonexistent page, DDT returns the error message

        CAN'T CREATE PAGE

   To allow DDT to create the page (and the section,  as  required)  when
   you attempt to deposit a value within a nonexistent page, type in

        <ESC>1W

































                                    4-21
                      DISPLAYING AND MODIFYING MEMORY
                      DISPLAYING AND MODIFYING MEMORY


   4.9  DISPLAYING PAGE ACCESSIBILITY INFORMATION
|  4.9  DISPLAYING PAGE ACCESSIBILITY INFORMATION
|  
|  You can get information about the access requirements of the pages and
|  sections  in  the  program  you  are  debugging,  using the $L and $$L
|  commands.  The complete format for this command is:

        {{arg1<}arg2}{<ESC>}<ESC>L

   where arg1 and arg2 are section numbers.  Using one <ESC>  causes  DDT
   to  display  access information about the section and about individual
   pages.  Using <ESC> twice causes DDT  to  display  access  information
   only  about  the  section(s).   If you include both arg1 and arg2, DDT
   displays the information for all sections that your  program  and  DDT
   are  using, in the range arg1 to arg2, inclusive.  If you include only
   arg2, DDT displays access information for that section only.   If  you
   omit  both arguments, DDT displays access information for all sections
   that your program and DDT are using.

   The page and section accessibility bits and their meanings are:

        Read            Page can be read.
        Write           Page can be written.
        Copy-on-write   Page is copy-on-write
        Execute         page can be executed      
        Private         page is private
        Zero            Page is allocated but zero.(FILDDT only)

   For example, the command <ESC>L might produce the following display:

|        Section 0   Read, Write, Execute, Private
|       000       Read, Write, Execute, Private
|       770       Read, Execute
|       771       Read, Write, Execute, Private
|        Section 37   Read, Write, Execute, Private
|       700-701   Read, Copy-on-write, Execute
|       703-730   Read, Copy-on-write, Execute
|       735-736   Read, Write, Execute, Private
|       740-753   Read, Execute

   And  the  command  <ESC><ESC>L  might  produce  a  display  like   the
   following:

         Section 0   Read, Write, Execute, Private
         Section 37   Read, Write, Execute, Private










                                    4-22
                      DISPLAYING AND MODIFYING MEMORY
                      DISPLAYING AND MODIFYING MEMORY


   4.10  WATCHING A MEMORY LOCATION
   4.10  WATCHING A MEMORY LOCATION

   If you wish to have DDT monitor or "watch"  a  memory  location  while
   your  program  is  running,  and  display  the  location  whenever its
   contents change, enter:

        addr<ESC>V

         ____
   where addr is the address of the location to be watched, and  defaults
   to the current location.  When you enter the command, DDT starts a new
   line and displays:

        addr/   value

         ____                                                   _____
   where addr is the address of the location being watched, and value  is
   the  contents  of  the  location.   This  command  also  restores  the
   prevailing display mode.

              ____
   DDT checks addr every "jiffy" (about 20  milliseconds),  and  displays
                                    ____
   the  address  and  contents  of  addr  whenever those contents change.
                                ____
   (Executive mode EDDT watches addr continuously.)

   If you enter a question mark (?) while DDT is watching, DDT displays:

        Watching: addr/   value

         ____                                                   _____
   where addr is the address of the location being watched, and value  is
                   ____
   the contents of addr.

   To  terminate  the  watch,  enter  any  other  character.   DDT  stops
   monitoring  the  word, starts a new display line, echoes the character
   you enter, starts  another  line,  and  waits  for  more  input.   The
   character that you enter to terminate the watch is otherwise ignored.

   Because any input character terminates the watch, you cannot  continue
   execution  and  watch  your  own  user program.  The <ESC>V command is
   useful to watch activity in a separate process (such  as  the  running
   monitor  or  other  job,  for which you must be using EDDT or FILDDT).
   The page that contains the word you wish to watch must be mapped  into
   your own process (the one that contains DDT and your program).



   4.11  TTY CONTROL MASK
   4.11  TTY CONTROL MASK

                                                                      ___
   You can control certain aspects of DDT's display by setting DDT's  TTY
   _______ ____
   control mask.  The command <ESC>1M returns a value that is the address
   of the DDT location that contains this mask.  Table 4-5 summarizes the
   features controlled by the bits in the TTY control mask.





                                    4-23
                      DISPLAYING AND MODIFYING MEMORY
                      DISPLAYING AND MODIFYING MEMORY


   Table 4-5:  TTY Control Mask
   Table 4-5:  TTY Control Mask


       BIT    VALUE                      EXPLANATION
       BIT    VALUE                      EXPLANATION

       15       0     Display the commands (and results) from the  file
                      executed by the <ESC>Y command (default).

                1     Do not display the commands (or results) from the
                      file executed by the <ESC>Y command.

       16       0     When  interrupting   program   execution   at   a
                      breakpoint,  display  the address and contents of
                      the breakpoint (default).

                1     When  interrupting   program   execution   at   a
                      breakpoint,  display  only  the  address  of  the
                      breakpoint.

       17       0     Display 3 spaces when spacing DDT output (1).

                1     Display DDT output fields at tab stops (1).

       34       0     The terminal does not have a tab mechanism (2).

                1     The terminal has a tab mechanism (2).

       35       0     Echo deleted characters (3).

                1     Backspace over deleted characters (3).


      (1) If bit 17 is reset (default), DDT displays 3 spaces  between
      output fields (such as between the address of a location and the
      contents of the location), and at the end of display lines.   If
      bit  17  is  set,  DDT  lines  up  the  output fields in columns
      beginning at tab stops (see bit 34).  Figure 4-1 illustrates the
      two different modes.

      (2) If bit 34 is set, DDT displays a  tab  character  (<CTRL/I>)
      between  fields.  If bit 34 is reset, DDT displays enough spaces
      to start the field at the next tab stop.  When starting up,  DDT
      checks   whether   your   terminal  can  handle  TAB  characters
      (<CTRL/I>), and sets this bit accordingly.

      (3) When starting up,  DDT  checks  whether  your  terminal  can
      backspace to delete characters, and sets this bit accordingly.







                                    4-24
                      DISPLAYING AND MODIFYING MEMORY
                      DISPLAYING AND MODIFYING MEMORY


   To change the settings of the TTY control mask, use the command:

        expr<ESC>1M

   where expr evaluates to the required bit pattern.

   You can also open the location addressed by <ESC>1M with  one  of  the
   DDT  display commands, and deposit an expression that contains the new
   bit settings.

   Figure 4-1 is an illustration of the effects of  bit  17  in  the  TTY
   control  mask.   The  code  being  examined  is the first few lines of
   X.MAC, listed in Figure 2-1.  The example is not a complete  debugging
   session;  only  enough is shown to illustrate the effects of bit 17 of
   the TTY control mask.  The numbers at the  left  of  the  DDT  display
   lines  are  to assist you in following the commentary that follows the
   display.  User input is in lowercase.
|  
|  
   Figure 4-1:  DDT Session Showing Columnar Output
|  Figure 4-1:  DDT Session Showing Columnar Output
|  
|  
                               SCREEN DISPLAY

   1.       DDT
   2.       start/   MOVE P,PWORD   x$:   .$b   $g
   3.       $1B>>START/   MOVE P,PWORD   $x
   4.          P/   -10,,STACK   PWORD/   -10,,STACK
   5.       START+1/   MOVEI IDX,TABLE1   $x
   6.          IDX/   TABLE1   TABLE1   $1m/   2   1,,2
   7.       start$g
   8.       $1B>>START/     MOVE P,PWORD    $x
   9.               P/      -10,,STACK      PWORD/  -10,,STACK
   10.      START+1/        MOVEI IDX,TABLE1        $x
   11.              IDX/    TABLE1  TABLE1
                                 COMMENTARY


        Line 1:

         o  DDT is loaded and waiting for a command.

        Line 2:

                  ______
         o  Enter start/ to examine location start.

                  _______
         o  Enter x<ESC>:  to open the symbol table for module X.

                  _______                               _____
         o  Enter .<ESC>b to set breakpoint at location START.

                  ______
         o  Enter <ESC>g to begin execution.



                                    4-25
                      DISPLAYING AND MODIFYING MEMORY
                      DISPLAYING AND MODIFYING MEMORY


        Line 3:

         o  DDT displays breakpoint information.

                  ______
         o  Enter <ESC>x to execute the next instruction.

        Line 4:

         o  DDT displays results of executing the instruction.

        Line 5:

         o  DDT displays the next instruction.

                  ______
         o  Enter <ESC>x to execute the instruction.

        Line 6:

         o  DDT displays the results of executing the instruction.

                  ________
         o  Enter <ESC>1m/ to display and open the TTY control mask.

         o  DDT displays the mask.  Bit 34 is set.

                  _________
         o  Enter 1,,2<RET> to set bits 17 and 34.

        Line 7:

                  ___________
         o  Enter start<ESC>g to restart the program.

        Line 8:

         o  DDT displays the breakpoint information.

                  ______
         o  Enter <ESC>x to execute the instruction.

        Line 9:

         o  DDT displays the results of executing the instruction.

        Line 10:

         o  DDT displays the next instruction.

                  ______
         o  Enter <ESC>x to execute the next instruction.

        Line 11:

         o  DDT displays the results of executing the instruction.





                                    4-26












                                 CHAPTER 5
                                 CHAPTER 5

                       CONTROLLING PROGRAM EXECUTION
                       CONTROLLING PROGRAM EXECUTION



   5.1  BEGINNING EXECUTION
   5.1  BEGINNING EXECUTION

   To begin execution of your program, enter:

        <ESC>G

   Your program will run, beginning at its start address.   If  you  have
   not  set any breakpoints, your program runs to completion, or until it
   makes a fatal error.  At TOPS-20 command level, you can then  use  the
   ___
   DDT command to reenter DDT and examine your program.

   You can start or continue program execution at any  address  with  the
   command:

        addr<ESC>G



   5.2  USING BREAKPOINTS
   5.2  USING BREAKPOINTS

     __________
   A breakpoint is a program location that has been altered such that  if
   your  program  PC  reaches the address of the breakpoint, your program
   transfers control to DDT.

   When you set a breakpoint with DDT, DDT  stores  the  address  of  the
   breakpoint  in  an  internal  table.  When you command DDT to begin or
   continue program execution,  DDT  stores  the  instructions  from  all
   breakpoints in the table, and replaces them with JSRs into a DDT entry
   table.

   While program execution is suspended at a breakpoint, you can  examine
   and modify memory, remove breakpoints, insert new breakpoints, execute
   individual instructions, and continue program execution.






                                    5-1
                       CONTROLLING PROGRAM EXECUTION
                       CONTROLLING PROGRAM EXECUTION


   During this time, the command "<ESC>." returns the value that  is  the
   address   of  the  next  instruction  to  be  executed.   The  command
   "<ESC><ESC>." returns a value that is the previous value  returned  by
   "<ESC>.".   When you first receive control at the breakpoint, "<ESC>."
   returns the address of the breakpoint and "<ESC><ESC>." returns  zero.
   Before you start execution with <ESC>G, "<ESC>." and "<ESC><ESC>." are
   illegal commands (if you try to execute them, DDT sounds the  terminal
   buzzer or bell and displays a question mark).

                                    NOTE

           This manual uses the term "$." to represent the  value
           returned  by  the command "<ESC>.", and the term "$$."
           to  represent  the  value  returned  by  the   command
           "<ESC><ESC>.".

   You can set up to 12 breakpoints at a time (this  is  a  DDT  assembly
   parameter)  in your program.  These breakpoints are numbered 1 through
                                                 ___________  __________
   12.   There  is  also  one  breakpoint  (the  unsolicited  breakpoint,
   numbered zero) that can be used by your MACRO program to "call" DDT.

   Each breakpoint has several internal DDT locations associated with it,
   which  contain  information  to control DDT action with respect to the
   breakpoint.  You can examine and modify these DDT locations  with  the
   same DDT commands that you use to examine and modify locations in your
   user program.  <ESC>nB is a command that returns the value that is the
   address  of  the  first  DDT  word  associated with breakpoint n.  The
   symbol $nB is used here to represent that address.

   Table 5-1 contains a list of the breakpoint locations of  interest  to
   the user, and their contents.


   Table 5-1:  Breakpoint Locations of Interest
   Table 5-1:  Breakpoint Locations of Interest


     LOCATION                    CONTENTS
     LOCATION                    CONTENTS

       $nB        Address of breakpoint n.

       $nB+1      Instruction for conditional breakpoint n.

       $nB+2      Proceed count for conditional breakpoint n.

       $nB+3      Address of a location to be opened and displayed when
                  the breakpoint is reached.

|      $nB+4      Address of an ASCIZ DDT command string to be executed
|                 when the breakpoint is reached.





                                    5-2
                       CONTROLLING PROGRAM EXECUTION
                       CONTROLLING PROGRAM EXECUTION


   When your user-program PC reaches a breakpoint, your program  executes
   the JSR into DDT.  When this occurs, DDT does the following:

         o  saves your user-program context

         o  replaces the JSR instructions at  all  breakpoints  with  the
            original program instructions

         o  displays the breakpoint number, breakpoint address,  and  the
            contents  of  the  breakpoint (depending on bit 16 of the TTY
            control mask)

         o  sets "$." to the breakpoint address

         o  sets "$$." to zero

         o  enters the address of the current location  (set  before  you
            started  the  program  or proceeded from a breakpoint) on the
            location sequence stack

         o  changes the current location to the breakpoint

         o  waits for you to give a DDT command

   When you command DDT to restart or  continue  program  execution,  DDT
   does the following:

         o  saves the program instructions from all breakpoints

         o  replaces the program instructions at all breakpoints with JSR
            instructions to DDT

         o  if you have not executed the instruction  at  the  breakpoint
            with  <ESC>X,  DDT  simulates execution of the instruction at
            the breakpoint

         o  restores your user-program context

         o  DDT performs a JRSTF (if in section zero,  otherwise  XJRSTF)
            to the next instruction to be executed



   5.2.1  Setting Breakpoints
   5.2.1  Setting Breakpoints

      ___ _ __________
   To set a breakpoint, enter:

        addr<ESC>{n}B

   where addr is the address where you want to  suspend  execution  (addr
   can  be  ".",  the  command  that  returns  the address of the current
   location), and n is the number of the breakpoint (and defaults to  the
   lowest unused breakpoint number).

                                    5-3
                       CONTROLLING PROGRAM EXECUTION
                       CONTROLLING PROGRAM EXECUTION


   If you do not specify n, it defaults to the lowest  available  (unset)
   breakpoint.   If you have already set twelve breakpoints, DDT displays
   "?" and sounds the terminal buzzer or bell.

   If you specify n, it must be greater than zero and less than 13.   DDT
   restores  the  original  contents  of  any (previously set) breakpoint
   designated as breakpoint n before setting new breakpoint n.

   You cannot set more than one breakpoint  at  the  same  address.   DDT
   simply  sets the same breakpoint again, even if you explicitly specify
   a breakpoint number the second time.

   You cannot set a breakpoint at AC zero.

   Assume the following conditions:

         o  location LABL1+3 contains the instruction MOVE 1,LABL2

         o  breakpoint 2 is set at LABL1+3

   If your program reaches LABL1+3 it executes the JSR to  DDT,  and  DDT
   does the following:

         o  saves your user-program context

         o  restores the original program instructions to the breakpoints

         o  sets "$." to LABL1+3

         o  sets "$$." to zero

         o  enters the address of the current location  on  the  location
            sequence stack

         o  changes the current location to LABL1+3 (the breakpoint)

         o  opens location LABL1+3

         o  displays:   $2B>>LABL1+3/   MOVE 1,LABL2

      ___ _ __________ ___ ____ ___ _______ __ __________  ________
   To set a breakpoint and have DDT display an additional  location  when
   your program reaches the breakpoint, enter:

        addr1<addr2<ESC>{n}B

   where addr1 is the location to be displayed, and addr2 is the location
   of the breakpoint.  Follow addr1 with a left angle bracket (<).







                                    5-4
                       CONTROLLING PROGRAM EXECUTION
                       CONTROLLING PROGRAM EXECUTION


   Assume the following conditions:

         o  location LABL1+3 contains the instruction MOVE 1,LABL2

         o  location LABL3 contains value SYMBL1

         o  breakpoint 2 was set by the command:

               LABL3<LABL1+3<ESC>B

   If your program reaches LABL1+3 it executes the JSR to  DDT,  and  DDT
   does the following:

         o  saves your user-program context

         o  restores the original program instructions to the breakpoints

         o  sets "$." to LABL1+3

         o  sets "$$." to zero

         o  enters the address of the current location  on  the  location
            sequence stack

         o  changes the current location to LABL1+3 (the breakpoint)

         o  enters the address of the current location  (the  breakpoint)
            on the location sequence stack

         o  changes the current location to LABL3

         o  opens location LABL3

         o  displays:   $2B>>LABL1+3/   MOVE 1,LABL2   LABL3/   SYMBL1

   Note that, because DDT placed the breakpoint address on  the  location
   sequence  stack,  you  can  enter  <ESC><RET>  to  change  the current
   location back to the breakpoint.
















                                    5-5
                       CONTROLLING PROGRAM EXECUTION
                       CONTROLLING PROGRAM EXECUTION


      _______ ___ _______
   To display the address of any breakpoint, enter:

        <ESC>nB/

   where n is the address of the breakpoint.  DDT displays the address of
   breakpoint n, and you can use the examine commands to open and display
   the instruction at breakpoint n.  If breakpoint  n  is  not  set,  DDT
   displays zero.

      ______ __________
   To remove breakpoint n, enter:

        0<ESC>nB

   To remove all breakpoints, enter:

        <ESC>B



   5.2.2  Proceeding from Breakpoints
   5.2.2  Proceeding from Breakpoints

   After  your  program  has  reached  a  breakpoint,  you  can  continue
   execution at "$." by entering:

        <ESC>P

   DDT saves the program instructions from all breakpoints, replaces  the
   program  instructions  with  JSRs  to  DDT, restores your user-program
   context, and if you have not executed any  program  instructions  with
   the  <ESC>X  command,  simulates  execution  of the instruction at the
   breakpoint.  DDT then executes a JRSTF (in section zero, otherwise DDT
   executes an XJRSTF) to the next instruction to be executed.

   You can cause the program to start execution at a  different  location
                                           ____
   with  the  {addr}<ESC>G  command, where addr defaults to the program's
   start address.


















                                    5-6
                       CONTROLLING PROGRAM EXECUTION
                       CONTROLLING PROGRAM EXECUTION


   Once your program has reached a breakpoint  and  DDT  has  interrupted
   execution,  you  can  cause  DDT to continue execution but NOT stop at
   that breakpoint until your  program  has  reached  that  breakpoint  a
   specified number of times.  To do this, enter:

        expr<ESC>P

         ____
   where expr is the proceed count.  DDT places expr at  location  $nB+2,
   where  n  is  the  number  of the breakpoint at which your program has
   stopped.  DDT resumes execution  of  your  program.   Each  time  your
   program  reaches breakpoint n, DDT decrements the proceed count stored
   at $nB+2.  Your program continues execution until:

         o  it reaches a different breakpoint

         o  it terminates normally

         o  it commits a fatal error

         o  the proceed count reaches zero

                                     _________  _______  ____
   Each breakpoint has an associated automatic  proceed  flag.   If  this
   flag is set and the program reaches the breakpoint, DDT decrements the
   proceed count at $nB+2 (where n is the number of the  breakpoint)  and
   displays  the breakpoint information if the proceed count is less than
   one.  DDT then automatically continues program execution.

                         ______
   The  <ESC>P  command  resets  (clears)  the  automatic  proceed   flag
   associated  with  the  breakpoint  at  which DDT has suspended program
   execution.

      ___ _ __________ ___ ___ ___  __________  _________  _______  ____
   To set a breakpoint and set the  associated  automatic  proceed  flag,
   enter:

        {addr1<}addr2<ESC><ESC>{n}B

   where addr2 is the address of the breakpoint and may be ".", addr1  is
   an  (optional)  additional location to be displayed, and n is optional
   and defaults to the lowest unused breakpoint.

   Each time your  program  reaches  breakpoint  n,  DDT  decrements  the
   associated  proceed  count,  and  if  the  result  is  less  than one,
   displays:

        $nB>>addr2/   instr

   where n is  the  breakpoint  number,  addr2  is  the  address  of  the
   breakpoint, and instr is the contents of the word at addr2.






                                    5-7
                       CONTROLLING PROGRAM EXECUTION
                       CONTROLLING PROGRAM EXECUTION


                  ______
   If you entered addr1< when you gave the command, DDT displays:

        $nB>>addr2/   instr   addr1/   contents

   where n is  the  breakpoint  number,  addr2  is  the  address  of  the
   breakpoint,  instr  is the contents of the word at addr2, addr1 is the
                                            ________
   additional location to be displayed, and contents is the  contents  of
   the word at addr1.

   DDT then automatically continues program execution until:

         o  your program reaches a different breakpoint

         o  your program terminates normally

         o  your program commits a fatal error

         o  you enter any character while your program is at breakpoint n

   You can interrupt the  automatic  proceed  function  if  you  enter  a
   character  while your program is at breakpoint n.  DDT then resets the
   automatic  proceed  flag  and  suspends  program  execution   at   the
   breakpoint.   DDT  echoes  the  character  that  you entered, which is
   otherwise ignored.

      _______ ____ _ __________ ___ ___ ___ __________ _________  _______
   To proceed from a breakpoint and set the associated automatic  proceed
   ____
   flag, give the command:

        {expr}<ESC><ESC>P

   where expr is the proceed count.  DDT  stores  the  proceed  count  at
   $nB+2.






















                                    5-8
                       CONTROLLING PROGRAM EXECUTION
                       CONTROLLING PROGRAM EXECUTION


   5.2.3  Conditional Breakpoints
   5.2.3  Conditional Breakpoints

   To cause DDT to interrupt program execution at a breakpoint only if  a
   specific  condition  is  satisfied,  you  must  store  a  single  test
   instruction or a call to a test routine  in  DDT's  breakpoint  table.
   You  can  use a test routine in your program, or one that you enter in
   DDT's patching area.  See Chapter 8 (Inserting Patches with  DDT)  for
   more   information  about  the  patching  area.   To  enter  the  test
   instruction (or the call to the test routine), open the  DDT  location
   addressed by the command <ESC>nB+1 by entering:

        <ESC>nB+1/

   where n is the number of the breakpoint.  You must  enter  n,  or  DDT
   interprets  the  command  as  <ESC>B,  and  removes  all  breakpoints.
|  Deposit the test instruction or the call to the test  subroutine.   If
|  your  program  reaches  breakpoint  n, DDT executes the instruction at
   $nB+1.  DDT then proceeds as follows:

         o  If the instruction does not cause a program counter skip, DDT
            decrements the proceed count at $nB+2.  If the result is zero
            or less, DDT interrupts execution at breakpoint n.

         o  If a program counter skip of 1  does  occur,  DDT  interrupts
            execution at breakpoint n.

         o  If the conditional instruction is a call to a subroutine that
            returns  by  skipping over two or more instructions, DDT does
            not interrupt program execution.

   If DDT interrupts execution because the test instruction resulted in a
   program  counter  skip,  DDT displays only one angle bracket after the
   breakpoint identification, as:

        $3B>LABL1/   MOVE 1,LABL2



















                                    5-9
                       CONTROLLING PROGRAM EXECUTION
                       CONTROLLING PROGRAM EXECUTION


   5.2.4  The "Unsolicited" Breakpoint
   5.2.4  The "Unsolicited" Breakpoint

   You can cause your MACRO  program  to  "call"  DDT  by  inserting  the
   following instruction in your program:

        JSR $0BPT##

   The two pound-signs (##) appended  to  $0BPT  in  your  MACRO  program
   declare the symbol as EXTERNAL.

                                    NOTE

           "$" represents the dollar  sign  character,  which  is
           part  of  the  symbol,  and is not the DDT echo of the
           ESCAPE key.

   You must load RDDT.REL with your program or you will get a LINK  error
   (?LNKUGS  undefined global symbol)  when  you load your program.  Load
   RDDT.REL with your program as follows (your input is in lowercase; the
   last  line  indicates  that  DDT  is  loaded  and ready to accept your
   commands):

|       @link
|       */debug filnam/go
|       DDT
|  
         ______
|  where filnam is the name of your MACRO-20 program.  You can start your
|  program running with the <ESC>G command.  If your program executes the
|  JSR instruction, DDT interrupts program execution and displays:

        $0B>>addr+1/   instr

         ______                       _____       ___ _____
   where addr+1 is the first location after  the  JSR $0BPT  instruction,
       _____
   and instr is the contents of that location.




















                                    5-10
                       CONTROLLING PROGRAM EXECUTION
                       CONTROLLING PROGRAM EXECUTION


   5.3  EXECUTING EXPLICIT INSTRUCTIONS
   5.3  EXECUTING EXPLICIT INSTRUCTIONS

   To execute a specific instruction, enter the instruction  followed  by
   <ESC>X:

        instr<ESC>X

   For example:

        MOVE 1,@LABL1(3)<ESC>X

   After executing the instruction, DDT starts a new line and displays:

        o  <>         if in-line execution of instr would result in
                      skipping no instructions.

        o  <SKIP>     if in-line execution of instr would result in
                      skipping 1 instruction.

        o  <SKIP 2>   if in-line execution of instr would result in
                      skipping 2 instructions.

        o  <SKIP 3>   if in-line execution of instr would result in
                      skipping 3 instructions.

                                    NOTE

           "In-line execution" means execution of the instruction
           as  part  of  normal  program  flow.  The execution of
           instructions with this command has no effect  on  your
           user-program PC.

   This command restores the prevailing display mode.



   5.4  SINGLE-STEPPING INSTRUCTIONS
   5.4  SINGLE-STEPPING INSTRUCTIONS

   After your program has transferred control to DDT from  a  breakpoint,
   you  can  execute  program instructions one at a time.  This is called
   "single-stepping."

   "<ESC>." is a command that returns the address of the next instruction
   to be executed.

   To execute the instruction whose  address  is  returned  by  "<ESC>.",
   enter:

        <ESC>X





                                    5-11
                       CONTROLLING PROGRAM EXECUTION
                       CONTROLLING PROGRAM EXECUTION


   For example, breakpoint 3 is set  at  LABL1+3.   If  your  program  PC
   reaches LABL1+3, control passes to DDT, which displays:

        $3B>>LABL1+3/   ADD 1,LABL2(2)

   Examining the environment, you learn the following:

         o  AC 1 contains 1

         o  AC 2 contains 3

         o  LABL1+4 contains MOVEM 1,@LABL2(3)

         o  LABL2+3 contains SYM3

   as shown by the following terminal display (DDT does not display  <LF>
   or <ESC>):

        $3B>>LABL1+3/   ADD 1,LABL2(2)   <ESC>\   SYM3   <LF>
        LABL1+4/   MOVEM 1,@LABL2(3)   1/   1   <LF>
        2/   3         

   If you now enter the command <ESC>X, DDT does the following:

         o  changes "$$." to LABL1+3

         o  executes the instruction at LABL1+3

         o  changes "$." to LABL1+4

         o  changes the current location to LABL1+4

         o  opens LABL1+4

         o  displays:

                    1/   SYM3+1   LABL2+3/   SYM3
                 LABL1+4/   MOVEM 1,@LABL2(3)

   If single-stepping an instruction results in a value of ($. minus $$.)
   not equal to 1, DDT also begins a new line and displays:

         o  <SKIP>    if ($. minus $$.) = 2

         o  <SKIP 2>  if ($. minus $$.) = 3

         o  <SKIP 3>  if ($. minus $$.) = 4

         o  <JUMP>    if ($. minus $$.) is greater than 4 or less than 1





                                    5-12
                       CONTROLLING PROGRAM EXECUTION
                       CONTROLLING PROGRAM EXECUTION


   before displaying the address and contents of the next instruction  to
   be  executed.   For  example,  the  following shows a typical terminal
   display where you enter <ESC>X to single-step the first instruction at
   a breakpoint (DDT echoes <ESC> as $):

        $4B>>LABL1+5/   AOSN 3   /   0    <ESC>x
           3/   1

        <SKIP>
        LABL1+7/   MOVEM 1,LABL2
o  



   5.5  EXECUTING SUBROUTINES AND RANGES OF INSTRUCTIONS
   5.5  EXECUTING SUBROUTINES AND RANGES OF INSTRUCTIONS

   To execute a series of n instructions beginning with  the  instruction
   whose address is returned by the command "<ESC>.", enter:

        n<ESC>X

   where n is the number of instructions to execute.

   DDT then does the following for each instruction:

         o  starts a new display line

         o  executes the instruction

         o  displays the address  of  any  register  or  memory  location
            referenced  by  the  execution  of  the  instruction, and the
            contents  of  those  locations   after   execution   of   the
            instruction

         o  changes the current location to the next  instruction  to  be
            executed

         o  opens the current location

         o  displays the address and contents of the next instruction  to
            be executed

         o  changes "$." to the address of the  next  instruction  to  be
            executed

         o  changes "$$." to the address of the instruction just executed








                                    5-13
                       CONTROLLING PROGRAM EXECUTION
                       CONTROLLING PROGRAM EXECUTION


   To suppress typeout of all but the last instruction executed, use  the
   command:

        n<ESC><ESC>X

   where n is the number of instructions to execute.

   To continue program execution until the PC (program counter) enters  a
   range of instructions, enter:

        {addr1<}{addr2>}<ESC><ESC>X

   where addr1 is the lower end of the range, and addr2 is the upper end.
   Addr1  defaults  to  1 + "$." and addr2 defaults to addr1 + 3.  Follow
   addr1 with a left angle bracket (<)  and  addr2  with  a  right  angle
   bracket (>).

   This command also indicates skips and jumps.

   This command is useful for executing  a  loop  or  a  subroutine  call
   quickly and without typeout.

   For example, breakpoint 3 is at location LABL1.

         $3B>>LABL1/   PUSHJ 17,SUBRTN   <ESC><ESC>X  ;Enter <ESC><ESC>X
         <SKIP>                                       ;SUBRTN returns + 2
         LABL1+2/   ADD 1,2

   If you enter a question mark (?) while DDT is executing an <ESC><ESC>X
   command, DDT displays:

        Executing: addr/   instr

         ____
   where addr is the address of the next instruction to be executed,  and
   _____
   instr is the instruction.

      _________ ___ _________ __ ___ ______ __  ____________
   To terminate the execution of the series of  instructions,  enter  any
   character other than ?  (question mark).  DDT does the following:

         o  echoes the character

         o  displays  <SKIP>,  <SKIP  2>,  <SKIP  3>,   or   <JUMP>,   as
            appropriate

         o  starts a new display line

         o  changes the current location  to  the  address  of  the  next
            instruction to be executed

         o  displays the address and contents of the current location




                                    5-14
                       CONTROLLING PROGRAM EXECUTION
                       CONTROLLING PROGRAM EXECUTION


         o  opens the current location

         o  waits for your next command



   5.5.1  Single-Stepping "Dangerous" Instructions
   5.5.1  Single-Stepping "Dangerous" Instructions

   DDT classifies the following as "dangerous" instructions:

         o  instructions that can modify memory

         o  instructions that can cause an arithmetic trap

         o  instructions that can cause a stack overflow

         o  a monitor call or I/O instruction

   Before single-stepping  one  of  these  instructions,  DDT  saves  and
   replaces  the  original  instructions  at the breakpoints with JSRs to
   DDT, and restores the full user-program context  (including  interrupt
   system and terminal characteristics) before executing the instruction.
   After  executing  the  instruction,  DDT  replaces  the  JSRs  at  the
   breakpoints with the original program instructions, and saves the full
   user-program context.

   DDT does not check whether the instruction actually results in one  of
   these  conditions,  only  whether  the  opcode  is  in  the  class  of
   instructions that can cause these effects.  This  can  make  executing
   subroutines  and  ranges  of  instructions under DDT control extremely
   time-consuming.

|  To execute a subroutine or series of instructions without checking for
|  dangerous instructions, use the command:

        {addr1<}{addr2>}<ESC><ESC>1X

   where addr1 is the lower end of the range, and addr2 is the upper end.
   Addr1  defaults  to  1 + "$." and addr2 defaults to 3 + addr1.  Follow
   addr1 with a left angle bracket (<), and  addr2  with  a  right  angle
   bracket (>).

                                  CAUTION

           This command executes much  faster  than  <ESC><ESC>X,
           but  if  the  execution  of  an  instruction  causes a
           software  interrupt,  the  error  and  trap   handling
           mechanism  may  not  function correctly.  In addition,
           program instructions that change or rely  on  terminal
           or  job  characteristics that are also used by DDT can
           cause unpredictable results.



                                    5-15
                       CONTROLLING PROGRAM EXECUTION
                       CONTROLLING PROGRAM EXECUTION


   5.6  USER-PROGRAM CONTEXT
   5.6  USER-PROGRAM CONTEXT

   When DDT interrupts your program's  execution  at  a  breakpoint,  and
   after  it  has  executed  a  dangerous instruction during an <ESC>X or
   <ESC><ESC>X command, it saves the user-program context.   The  command
   <ESC>nI, where 0<=n<=8 (decimal), returns the address of the word that
   contains the information for "function" n.  You can use  this  address
   to  display  and modify these values.  Most of these values are useful
   only in  executive  mode.   DDT  displays  the  address  of  the  word
   containing the information for function n as:

        $I+n

   where 1<=n<=10 (octal).  If n = 0, DDT displays only $I.

|  Table 5-2 lists the functions.


   Table 5-2:  User-Program Context Values
   Table 5-2:  User-Program Context Values


      FUNCTION                            VALUE
      FUNCTION                            VALUE

          0       Executive mode CONI PI.

          1       Executive mode PI channels turned off.

          2       Executive mode CONI APR.

          3       User PC flags.

          4       User PC address.

          5       EPT page address.

          6       UPT page address.

          7       CST base virtual address.

         10       SPT base virtual address.


   DDT restores the user-program context  whenever  you  execute  <ESC>G,
   <ESC>P,  and  when  you  execute  <ESC>X,  or <ESC><ESC>X of dangerous
   instructions.









                                    5-16
                       CONTROLLING PROGRAM EXECUTION
                       CONTROLLING PROGRAM EXECUTION


   Functions 5 through 10 (octal) affect  DDT's  interpretation  of  your
   program's  virtual  address space.  You can alter DDT's interpretation
   of your program's virtual address space with the physical and  virtual
   addressing  (<ESC>nU)  commands  described in Chapter 11 (Physical and
   Virtual Addressing Commands).  However, any alterations that you  make
   do  not  become  part  of your user-program context, and do not affect
   TOPS-20's interpretation of your program's virtual address space.

   DDT also saves and restores  the  user-program  ACs  as  part  of  the
   user-program  context.   DDT  stores  the  contents  of  the ACs in an
   internal "register" block.  Any references you make to addresses  0-17
   refer  to  the  relative  locations  in DDT's internal register block.
   These actions are totally transparent to you.









































                                    5-17

























































                                    5-18












                                 CHAPTER 6
                                 CHAPTER 6

                     SEARCHING FOR DATA PATTERNS IN DDT
                     SEARCHING FOR DATA PATTERNS IN DDT



   With DDT you can search for memory locations that contain  a  specific
   value, and conversely, for words that do not contain a specific value.
   You can also set a mask to indicate to DDT that  only  specified  bits
   are to be considered when performing the search.  In addition, you can
   search for words that reference a specific address.  You can specify a
   range  within which to perform the search, or default the range to all
   of your program's address space.  In either  case,  DDT  compares  the
   contents of each location within the range with the specified value.

      ______ ___ _____ ____ _____ _ ________ _____
   To search for words that match a specific value, enter:

        {addr1<}{addr2>}expr<ESC>W

   where expr is the value for which DDT is  to  search,  and  addr1  and
   addr2  delimit  the  range  in  which  the  search is to be conducted.
   Follow addr1 with a left angle bracket (<)  and  addr2  with  a  right
   angle  bracket  (>).   Addr1  defaults  to  zero and addr2 defaults to
   777777 in the current section.  Expr can be any legal DDT expression.

   DDT does the following:

         o  compares each location (after ANDing it with the search mask)
            within  the search range with the 36-bit value resulting from
            evaluating expr

         o  starts the search by comparing the  contents  of  addr1  with
            ____
            expr

         o  stops the search after comparing the contents of  addr2  with
            ____
            expr

         o  displays (on a new line) the address  and  contents  of  each
            location that matches expr

         o  enters the address of each matching location on the  location
            sequence stack



                                    6-1
                     SEARCHING FOR DATA PATTERNS IN DDT
                     SEARCHING FOR DATA PATTERNS IN DDT


         o  sets the current location to addr2

         o  displays a blank line to indicate the search is over

         o  restores the prevailing display mode

                                    NOTE

           If DDT finds more matching locations  than  there  are
           words  on  the  location  sequence  stack, the earlier
           entries are overwritten.

o  
      ______ ___ _____ ____ __ ___ _____ _ _________ _____
   To search for words that do NOT match a specified value, enter:

        {addr1<}{addr2>}expr<ESC>N

   where expr is the value which is not to  be  matched,  and  addr1  and
   addr2  delimit  the range within which DDT is to search.  Follow addr1
   with a left angle bracket (<) and addr2 with  a  right  angle  bracket
   (>).   Addr1  defaults  to  zero  and  addr2 defaults to 777777 in the
   current section.  Expr is any legal DDT expression.

                                    NOTE

           When you use the DDT search  functions  while  running
           FILDDT,  addr2  defaults  to  777777  (in  the current
           section) unless:
|  
|           o  the target is the running monitor job and you  are
|              using physical addressing

            o  the target is an  .EXE  file  and  you  are  using
               normal virtual addressing

            o  the target is a disk structure or data file

           In these cases, addr2 defaults to the last word of the
           target.   See  Chapter  9  (FILDDT),  and  Chapter  11
           (Physical and Virtual Addressing Commands),  for  more
           information.













                                    6-2
                     SEARCHING FOR DATA PATTERNS IN DDT
                     SEARCHING FOR DATA PATTERNS IN DDT


   DDT functions as for the <ESC>W command, except:

         o  DDT searches for and displays the address and contents of any
            word  within the address range that does NOT match the 36-bit
            value resulting from evaluating expr.

         o  DDT  enters  the  locations  of  non-matching  words  on  the
            location sequence stack.

o  
      ______ ___ __________ __ __ _______
   To search for references to an address, enter:

        {addr1<}{addr2>}expr<ESC>E

   where addr1 and addr2 delimit  the  range  of  the  search,  and  expr
   contains  the address for which DDT is to search.  Follow addr1 with a
   left angle bracket (<) and addr2  with  a  right  angle  bracket  (>).
   Addr1  defaults  to  zero  and addr2 defaults to 777777 in the current
   section.  Expr is any legal DDT  expression.   DDT  performs  an  IFIW
   effective address calculation on the expression contained in each word
   within the range, and uses the  18-bit  result  to  determine  whether
   there is a match.

   Thus, if bits 14-17 (the X field of an instruction) or bit 13  (the  I
   field  of  an  instruction)  are  nonzero, indexing or indirection may
   result in DDT finding different search results at different times.

   DDT does not check whether the expression is actually  an  instruction
   before performing the effective address calculation.
o  
   If you enter a question mark (?) while DDT is performing  any  of  the
   above searches, DDT displays:

        Searching: addr/   value

         ____
   where addr is the address of the location that will next compare,  and
   _____                    ____
   value is the contents of addr.

      _____ ___ ______
   To abort the search, enter any character other than question mark (?).
   DDT stops searching, and waits for more input.  The character that you
   enter to terminate the search is otherwise ignored.

|  Each of the above search  commands  restores  the  prevailing  display
|  mode.










                                    6-3
                     SEARCHING FOR DATA PATTERNS IN DDT
                     SEARCHING FOR DATA PATTERNS IN DDT


   <ESC>M is a command that addresses a  DDT  location  that  contains  a
   ______  ____
   search  mask  used  to  prevent specified bits in the memory word from
   being considered during the search.  This mask is used only by  <ESC>W
   and  <ESC>N,  not  by <ESC>E.  DDT logically ANDs the search mask with
   the memory word before making the comparison, but does not change  the
   memory word.  If DDT finds a match, it displays the entire word.

   DDT sets the search mask to 777777,,777777 (compare all  36  bits)  by
   default.

   To set the search mask, enter:

        expr<ESC>M

         ____
   where expr evaluates to the required bit pattern.

   For example, to search for all of  the  RADIX50  references  to  MAIN.
   (user input is in lowercase):

        <ESC><ESC>5t             ;Set typeout mode to RADIX50.
        37777,,777777<ESC>m      ;Ignore the left 4 bits.
        main.<ESC>5"<ESC>w       ;Enter RADIX50 symbol, start search.
        4112/   4 MAIN.          ;DDT displays match found.
        4775/   0 MAIN.          ;DDT displays match found.
                                 ;Search over, DDT displays blank line.

   You can also examine and modify the search mask with the  examine  and
   deposit  commands  described  in  Chapter  4 (Displaying and Modifying
   Memory).

























                                    6-4












                                 CHAPTER 7
                                 CHAPTER 7

                        MANIPULATING SYMBOLS IN DDT
                        MANIPULATING SYMBOLS IN DDT



   7.1  OPENING AND CLOSING SYMBOL TABLES
   7.1  OPENING AND CLOSING SYMBOL TABLES

   Each  separate  program  module  has  its  own  symbol  table.    When
   displaying  a  value  symbolically, if more than one symbol is defined
   with that value, DDT displays the first  global  symbol  found.   When
   searching  for  a  symbol, DDT searches the "open" symbol table first.
   For display purposes, DDT treats  local  symbols  found  in  the  open
   symbol table as global symbols.  DDT appends a pound-sign (#) to local
   symbol names that it finds in a symbol table that is  not  open.   For
   example:

        SYMBL1#

   where SYMBL1 is a local symbol that DDT found in a symbol  table  that
   is not open.

   If you enter an expression that contains a symbol that is  defined  in
   more  than  one  of  your  program  modules, DDT uses the value of the
   symbol that is contained in the open symbol table.  If the  symbol  is
   not  defined  in  the open symbol table, or if there is no open module
   and there is not a global definition of the symbol, DDT displays:

        M

      ____ ___ ______ _____
   To open the symbol table of a program module, enter:

        name<ESC>:

         ____
   where name is the name of the program module as specified by the TITLE
   pseudo-op  in  your MACRO-10 program (or the equivalent mechanism in a
   higher-level language program).  DDT closes any currently open  symbol
                                                           ____
   table and opens the symbol table associated with module name.







                                    7-1
                        MANIPULATING SYMBOLS IN DDT
                        MANIPULATING SYMBOLS IN DDT


      ____ ___ ____ __ ___ ______ __________ ____ ___ ____ ______  _____
   To find the name of the module associated with the open symbol  table,
   enter:

        <ESC>1:

   If there is an open symbol table, DDT displays the name of the  module
   associated  with  the  open  symbol table.  For example, if the symbol
   table for module X is open, the screen  display  is  as  follows  (DDT
   echoes <ESC> as $, and does not display any spaces between the command
   and the module name):

        $1:/X

   If there is no open symbol table, DDT displays three spaces (or a tab,
   depending on the TTY control mask), and waits for your next command.


      _____ ___ ____ ______ _____
   To close the open symbol table, enter:

        <ESC>:



   7.2  DEFINING SYMBOLS
   7.2  DEFINING SYMBOLS

   To redefine a symbol or to create a new symbol in the  current  symbol
   table, enter:

        expr<symbol:

         ____                                  ______
   where expr is any legal DDT expression, and symbol is the symbol name.

             ______
   To define symbol as the  address  of  the  open  location,  enter  the
   command:

        symbol:

|  If there is no open  location,  DDT  uses  the  address  of  the  last
                                           ______
|  location  that  was  open.  DDT defines symbol as a global symbol.  If
                       ______
|  you previously used symbol as an undefined  symbol,  DDT  inserts  the
                                                      ______
|  correct  value  in  all  the places you referenced symbol, and removes
   ______
|  symbol from the undefined symbol table.












                                    7-2
                        MANIPULATING SYMBOLS IN DDT
                        MANIPULATING SYMBOLS IN DDT


   7.3  SUPPRESSING SYMBOL TYPEOUT
   7.3  SUPPRESSING SYMBOL TYPEOUT

   To prevent a symbol from being displayed, enter:

        symbol<ESC>K

         ______                                                    ______
   where symbol is the symbol to be suppressed.  DDT still accepts symbol
                                    ______
   as input, but no longer displays symbol as output.

   To suppress the last symbol that DDT displayed (in an address, in  the
   contents  of  a  memory  word, or in the evaluation of an expression),
   enter:

        <ESC>D

   DDT suppresses the last symbol  displayed,  and  then  redisplays  the
   current quantity.  DDT does not display its usual three spaces between
   the command and the displayed value.

   In the following example, assume that symbol SIZE  is  defined  as  3.
   User  typein  is  lowercase  (<LF>  does  not  appear  on the terminal
   screen).

        start/   JFCL 0   <LF>
        LOOP/   AOS 1   <LF>
        LOOP+1/   MOVE 2,1   <ESC>dMOVE 2,1   <LF>
        START+3/   MULI 2,SIZE   <ESC>dMULI 2,3

      __________ _ ______ ___ _______
   To reactivate a symbol for typeout, redefine the symbol.  For example,
   to reactivate the display of symbol SIZE, above, enter:

        size<size:

   Note that SIZE is now defined as a  global  symbol,  even  if  it  was
   previously a local symbol.



   7.4  KILLING SYMBOLS
   7.4  KILLING SYMBOLS

   To remove a symbol from the symbol table, enter:

        symbol<ESC><ESC>K

               ______
   DDT removes symbol from the  symbol  table,  and  no  longer  displays
   ______            ______
   symbol or accepts symbol as input.








                                    7-3
                        MANIPULATING SYMBOLS IN DDT
                        MANIPULATING SYMBOLS IN DDT


   7.5  CREATING UNDEFINED SYMBOLS
   7.5  CREATING UNDEFINED SYMBOLS

   It is sometimes convenient to use  symbols  that  have  not  yet  been
   defined.  To create an undefined symbol, enter:

        symbol#

         ______                                           ______
   where symbol is the undefined symbol name.  DDT enters symbol  in  the
   undefined  symbol table.  When you later define the symbol, DDT enters
   it into the defined symbol table, removes it from the undefined symbol
   table,  and  enters  the  correct  value  in  all  locations where you
   referenced the symbol.

   You can use undefined symbols only as parts of  expressions  that  you
   are depositing to memory.  Undefined symbols can be either fullword or
   right-halfword values; they cannot be used as the A or X fields of  an
   instruction, or as the left-halfword of an expression.
|  
|  
|  
   7.6  FINDING WHERE A SYMBOL IS DEFINED
|  7.6  FINDING WHERE A SYMBOL IS DEFINED
|  
   To determine the modules in which a symbol is defined, enter:

        symbol?

         ______
   where symbol is the name of the symbol.  DDT displays the name of each
                            ______
   program  module in which symbol is defined.  If the symbol is a global
   symbol, DDT displays a "G", as:

        sym?
        MAIN.  G

   DDT does not display G following a local  symbol  found  in  the  open
   symbol  table.   When  DDT  has  searched  the entire symbol table, it
   displays a blank line.


















                                    7-4
                        MANIPULATING SYMBOLS IN DDT
                        MANIPULATING SYMBOLS IN DDT


   7.7  SEARCHING FOR SYMBOLS
   7.7  SEARCHING FOR SYMBOLS

   To search for all the symbols that begin  with  a  specific  character
   pattern, use the command

        sym<ESC>?

   where sym is the character pattern for which you  are  searching,  and
   may  be  one  to six characters long.  DDT searches your symbol tables
   and displays all symbols that  begin  with  that  pattern.   DDT  also
   displays  all modules in which the symbol is found, whether the symbol
   is global, and the value of the symbol.  In addition,  if  the  symbol
   represents  a  value  in  which  only one bit is set, DDT displays the
   number of the bit.  For example, the command

        fdb<ESC>?

   might cause the following display:

        FDBIN    INOUT   G   3
        FDBOUT   INOUT   G   2   (1B34)

        FDB      MOD1        7



   7.8  LISTING UNDEFINED SYMBOLS
   7.8  LISTING UNDEFINED SYMBOLS

   To get a list of all currently undefined symbols, enter:

        ?

   DDT displays a list containing each undefined symbol.
|  
|  
|  
   7.9  LISTING SYMBOLS
|  7.9  LISTING SYMBOLS
|  
|  To get a list of all symbols starting with a certain character or  set
|  of characters, enter:
|  
|       {val1<{val2>}}{sym}<ESC>{n}?
|  
         ____     ____
|  where val1 and val2 restrict the values of symbols which DDT displays.
           ____
|  If only val1 is present, only symbols having that value are displayed.
           ____      ____
|  If both val1 and  val2  are  present,  symbols  with  (signed)  values
            ____      ____                               _
|  between  val1  and val2 inclusive are displayed.  The n argument is an
|  octal mask of flags.
|  
|  If bit 35  is  on,  only  symbols  defined  in  the  open  module  are
|  displayed.   If  any  of bits 33-30 are on, the corresponding bit from
|  0-3 must be present in the symbol's definition.  Any other bits are in
|  error.

                                    7-5
                        MANIPULATING SYMBOLS IN DDT
                        MANIPULATING SYMBOLS IN DDT


|  For example, entering "<ESC>4?" displays all global symbols, "<ESC>1?"
|  displays  all symbols defined in the open module, and "5<100>T<ESC>5?"
|  displays all global symbols defined in the open module, starting  with
|  "T", whose values are in the range of 5-100.



   7.10  LOCATING SYMBOL TABLES WITH PROGRAM DATA VECTORS
   7.10  LOCATING SYMBOL TABLES WITH PROGRAM DATA VECTORS

   DDT Version 44 can access symbol tables pointed to by JOBDAT, by  PDVs
   (program data vectors) and by values you store in DDT.

   The command <ESC>5M  returns  the  address  of  a  DDT  location  that
   contains  information  to direct DDT to the current symbol table.  The
   symbol $5M refers to the memory location at the  address  returned  by
   the <ESC>5M command.

   If the value contained  in  $5M  is  negative  (bit  0  is  set),  the
   right-halfword  contains  the  number of the section that contains the
   JOBDAT area.

   If the value contained in $5M is positive (bit zero is clear, and  the
   value  in  $5M is nonzero), $5M contains the 30-bit address of the PDV
   currently in use by DDT.

   If $5M contains zero, DDT uses values (which  can  be  stored  by  the
   user)  pointed to by locations 770001 and 770002 of UDDT, to determine
   which symbol  table(s)  to  use.   The  algorithm  that  DDT  uses  is
   described below.

   To set $5M to a PDV address, type in

        addr<ESC>5M

   where addr is the the PDV address.  If you know the PDV name, you  can
   type in

        <ESC><ESC>:/name/

         ____
   where name is the name of the PDV, and the slashes (/)  represent  any
                                     ____      ____
   characters  that do not appear in name.  If name is a null string, DDT
   searches for a PDV with no name or  a  null  name.   DDT  ignores  any
                 ____
   characters in name beyond a length of 39.

                                ____
   DDT searches for a PDV named name, and places its address in $5M.   If
   DDT  does  not  find  the  PDV, it displays ?  and sounds the terminal
   buzzer or bell.







                                    7-6
                        MANIPULATING SYMBOLS IN DDT
                        MANIPULATING SYMBOLS IN DDT


   You can learn the names of the PDVs associated with  your  program  by
   using the following sequence of TOPS-20 commands:

        @GET program-name
        @INFORMATION VERSION

   To display the name of the PDV addressed by $5M, type in

        <ESC><ESC>1:

   If $5M contains the address of a PDV, DDT displays  the  name  of  the
   PDV; otherwise, it does nothing.

   Whenever DDT is entered from its start address or from  a  breakpoint,
   if $5M is zero, DDT initializes $5M according to the following rules:

         o  If XDDT was started  by  the  UDDT  stub,  AND  the  location
            addressed by location 770001 in the stub has bit zero set:

             >  DDT uses the location addressed by  location  770001  (in
                the  stub)  as  an  IOWD pointer to a symbol table in the
                section that contains the stub.

             >  DDT uses the location addressed by  location  770002  (in
                the  stub)  as  as  IOWD  pointer to the undefined symbol
                table in the section that contains the stub.

         o  If XDDT was not started by the UDDT  stub,  OR  the  location
            addressed by location 770001 in the stub has bit zero clear:

             >  If no PDVs exist, DDT sets $5M to -1,,n, where n is:

                 *  the section that contains the UDDT stub (if the  stub
                    exists)  OR

                 *  the section that contains the  entry  vector  (if  an
                    entry vector exists)  OR

                 *  section zero.

             >  If there is one (only) PDV, DDT sets $5M to  the  address
                of the PDV.












                                    7-7
                        MANIPULATING SYMBOLS IN DDT
                        MANIPULATING SYMBOLS IN DDT


             >  If there is more than one PDV, DDT examines  word  .PVSYM
                of each PDV in ascending memory order (DDT first looks at
                the PDV closest to 0,,0).  DDT then sets $5M to:

                 *  the address of the first (lowest in memory) PDV  that
                    contains a .PVSYM word that contains a global address
                    (if there is one).

                 *  the address of the first (lowest in memory) PDV  that
                    exists  in  or above the section containing the entry
                    vector (if there is one).

                 *  the address of the first (lowest in memory) PDV.


                                    NOTE

                  DDT ignores its  own  PDV  when  setting
                  $5M.



































                                    7-8












                                 CHAPTER 8
                                 CHAPTER 8

                         INSERTING PATCHES WITH DDT
                         INSERTING PATCHES WITH DDT



   To replace the instruction at the  open  location  with  a  series  of
   instructions  and  test the new instructions without reassembling your
   program, you can use the DDT  patch  function.   DDT  deposits  (in  a
   patching   area)   the   replaced   instruction,  the  new  series  of
   instructions, and one or more JUMPA instructions back to the main line
   of your program.  DDT also deposits (in the location that contains the
   replaced instruction) a JUMPA instruction to the  first  word  of  the
   patch.

      ______ _ _____ ____ ____ __ ________ ______ ___ ___________
   To insert a patch that will be executed before the instruction at  the
   open location, enter:

        {expr}<ESC><

         ____
   where expr is the start of the patching location, and  defaults  first
   to  PAT..,  then  to  PATCH.   KDDT  and  MDDT default to FFF (an area
   created during the monitor build), PAT.., and PATCH,  in  that  order.
                         ____
   If  you  do not enter expr, and DDT finds none of the default symbols,
   DDT uses the value contained in JOBDAT location .JBFF as  the  address
                           ____
   to begin the patch.  If expr is a symbol (or the default), DDT updates
   the symbol table when you terminate the  patch,  so  that  the  symbol
   identifies the first word after the patch that you just terminated.
o  
   If there is no open location when you initiate the patch, DDT displays
   "?" and sounds the terminal buzzer or bell.

                                    NOTE

              ____
           If expr is an AC address, or resolves to a value  less
           than  0,,140, DDT displays "?" and sounds the terminal
           buzzer or bell.








                                    8-1
                         INSERTING PATCHES WITH DDT
                         INSERTING PATCHES WITH DDT


   When you issue a command to start a patch, DDT saves  the  address  of
   the  open  location,  closes  the  open  location, changes the current
   location to the first word in the patching area, and opens that  word.
   DDT  also  displays  the address and contents of the first word of the
   patching area.  For example:

        <ESC><
        PAT../   0

   You can now enter the patch, using deposit instructions (the  expr<LF>
   format  is  probably  most  useful).  DDT updates the current and open
   locations according to the rules for the command that you use.

      _________ ___ _____
   To terminate the patch, enter:

        {expr}<ESC>{n}>

         ____                                                       _
   where expr is the last word of the patch you are entering,  and  n  is
   the  number  of  returns  possible  from  execution of the patch.  The
   default for n is 2, allowing for a return to 1 + the  address  of  the
   instruction being replaced, and for a "skip return" to 2 + the address
   of the instruction being replaced.

   When you terminate the  patch,  DDT  deposits  the  instruction  being
   replaced  into  the  first  location  following  the current location,
   unless:

         o  display is not suppressed by !  AND

         o  the current location is zero  AND

                                                            ____
         o  the current location is closed  OR  you omitted expr

   in which case DDT deposits the instruction  being  replaced  into  the
   current  location.  This prevents the patch from containing unintended
   null words.

   DDT  deposits  n  JUMPA  instructions  in  the  locations  immediately
   following   the  one  in  which  it  deposited  the  original  program
   instruction.  The first JUMPA instruction has 1 in its  A  field,  and
   jumps to 1 + the address of the replaced instruction, the second JUMPA
   instruction has 2 in its A field and jumps to 2 + the address  of  the
   replaced  instruction,  and  so  on.   The  AC  numbers  are  used for
   identification  purposes  only.   Any  JUMPA  instruction  beyond  the
   sixteenth contains 17 in its A field.

   DDT then changes the current location to the location  that  was  open
   when you initiated the patch, deposits in the current location a JUMPA
   instruction to the first word of  the  patch  that  you  entered,  and
   displays  the  address,  original  contents,  and  new contents of the
   current location.  The current location is "open", and can be modified
   by your next command.


                                    8-2
                         INSERTING PATCHES WITH DDT
                         INSERTING PATCHES WITH DDT


                  ____
   If you default expr, or enter a symbol in  the  {expr}<ESC><  command,
   when you terminate the patch, DDT redefines the symbol that identifies
   the start of the patch.  If DDT used the  value  contained  in  JOBDAT
   location  .JBFF  as  the address of the patching area, DDT changes the
   values contained in .JBFF and the left half of JOBDAT location  .JBSA.
   In  all  cases,  the  new  value is the address of the memory location
   after the last word of the patch.

   By default, there are 100 (octal) words in  the  patching  area.   DDT
   does  not  check  whether your patch overflows the patching area.  You
   can control the size of the patching area with the  /PATCHSIZE  switch
   in LINK.

                                    NOTE

           DDT allows you to use other DDT commands while you are
           in  the  process  of  entering  a patch.  DDT does not
           check whether the current and open  locations  are  in
           the  patching  area, or whether you are entering patch
           instructions in  sequence.   When  you  terminate  the
           patch,  DDT deposits the instruction being replaced in
           the current location regardless of whether the current
           location is in the patching area.

      ______ _ _____ ____ ____ __ ________ _____ ___ ___________
   To insert a patch that will be executed after the instruction  at  the
   open location, enter:

        {expr}<ESC><ESC><

         ____
   where expr is the address of the  patching  location  (PAT..   is  the
   default).   The results are the same as inserting the patch before the
   instruction as above, except:

         o  When you open the patch DDT deposits the replaced instruction
            in the first word of the patch.

         o  When you terminate the patch, DDT deposits  the  first  JUMPA
            instruction  (rather  than the instruction being replaced) in
            the first location following the current location unless:

             >  display is not suppressed by !  AND

             >  the current location is zero  AND

                                                                ____
             >  the current location is closed  OR  you omitted expr

            in which case DDT deposits the first JUMPA instruction in the
            current   location.   This  is  to  prevent  the  patch  from
            containing unintended null words.





                                    8-3
                         INSERTING PATCHES WITH DDT
                         INSERTING PATCHES WITH DDT


                                    NOTE

              ____
           If expr is an AC address, or resolves to a value  less
           than  0,,140, DDT displays "?" and sounds the terminal
           buzzer or bell.

   Figure 8-1 illustrates  the  patching  function.   The  program  being
   patched  is  X.MAC  (see  Figure  2-1).   The  patch  inserts  a SKIPN
   instruction that is to be executed after the instruction at START+4.


   Figure 8-1:  Annotated Patching Session
   Figure 8-1:  Annotated Patching Session


   DDT OUTPUT                USER INPUT        EXPLANATION

   START+4/   MOVE 2(IDX)                  As a result of your last
                                           command, DDT displays the
                                           contents of START+4.

|                           <ESC><ESC><    Enter <ESC><ESC>< to start
|                                          the patch.
|  
|  PAT../   0   MOVE 2(IDX)                DDT displays the address
|                                          and contents of the first 
|                                          word of the patch area, and
|                                          deposits the instruction from
|                                          START+4 in the first word of
|                                          the patch.
|  
|  PAT..+1/   0                            DDT displays the address
|                                          and contents of the next word
|                                          of the patch area.
|  
|                              pat..=      Check the address of PAT.."
|                                          (the first word of the patch
|                                          area).

   14432                                   DDT displays the current
                                           address of "PAT..".

|                        skipn 1,0<ESC>2>  Enter the new instruction,
|                                          and terminate the patch with 
|                                          a normal return and one skip
|                                          return by entering <ESC>2>.
|  
|  PAT..+2/   0   JUMPA 1,START+5          DDT displays the next word
|                                          of the patch area, then
|                                          deposits a JUMPA instruction
|                                          to 1 + the address of the
|                                          replaced instruction.



                                    8-4
                         INSERTING PATCHES WITH DDT
                         INSERTING PATCHES WITH DDT


   Figure 8-1: Annotated Patching Session (Cont.)
|  Figure 8-1: Annotated Patching Session (Cont.)
|  
|  
|  DDT OUTPUT                USER INPUT        EXPLANATION
|  
|  PAT..+3/   0   JUMPA 2,START+6          DDT displays the address
|                                          and contents of the next word
|                                          of the patch area, then 
|                                          deposits a JUMPA instruction
|                                          to 2 + the address of the
|                                          replaced instruction.

   START+4/   MOVE 2(IDX)   JUMPA STACK+10

                                           DDT displays the address
                                           and original contents of the
                                           replaced instruction, then
                                           deposits and displays a
                                           JUMPA instruction to the
                                           first word of the patch.
                                           START+4 is the current
                                           location, and is "open".

                               pat..=      Check the address of the
                                           patch area.

   14436                                   DDT updated "PAT..".


   Figure 8-2 shows the terminal display as it actually appears when  you
   insert the patch described above.  Your input is in lowercase.


   Figure 8-2:  Terminal Display of Patching After an Instruction
   Figure 8-2:  Terminal Display of Patching After an Instruction


|       START+4/   MOVE 2(IDX)   $$<
|       PAT../   0   MOVE 2(IDX)
|       PAT..+1/   0   pat..=14432   skipn 1,0$2>
|       PAT..+2/   0   JUMPA 1,START+5
|       PAT..+3/   0   JUMPA 2,START+6
|       START+4/   MOVE 2(IDX)   JUMPA STACK+10   pat..=14436


|  Figure 8-3 shows the terminal display when inserting  the  same  patch
   ______
|  before  the  instruction at START+4.  You enter the instruction in the
        ________
|  form expr<LF> (user input is lowercase).  Note the use  of  the  patch
                               ____             _
   termination command without expr and without n.






                                    8-5
                         INSERTING PATCHES WITH DDT
                         INSERTING PATCHES WITH DDT


   Figure 8-3:  Terminal Display of Patching Before an Instruction
   Figure 8-3:  Terminal Display of Patching Before an Instruction


|       START+4/   MOVE 2(IDX)   $<
|       PAT../   0   .=14432   skipn 1,0
|       PAT..+1/   0   $>
|       PAT..+1/   0   MOVE  2(IDX)
|       PAT..+2/   0   JUMPA 1,START+5
|       PAT..+3/   0   JUMPA 2,START+6
|       START+4/   MOVE 2(IDX)   JUMPA STACK+10   pat..=14436


      _____ ___ _____ ___ ___ ________
   To abort the patch you are entering, enter:

        <ESC>0<

   DDT displays 3 spaces (or a tab, depending on the  TTY  control  mask)
   and  changes  the  current location to the location that was open when
   you initiated the patch.  The symbol that denotes  the  start  of  the
   patching area is unchanged.  Any deposits that you made as part of the
   patch remain in the patching area.  This allows  you  to  restart  the
   same patch, or to write over the patch with a new one.
































                                    8-6












                                 CHAPTER 9
                                 CHAPTER 9

                                   FILDDT
                                   FILDDT



   9.1  INTRODUCTION
   9.1  INTRODUCTION

   FILDDT is a utility used to examine and change disk files and physical
   disk  blocks.  You can also use FILDDT to examine monitor crash dumps,
   and to examine the running monitor.  With FILDDT, you can look at .EXE
   files  as  if they had been loaded with the monitor GET command, or as
   if they were binary data files.

   In selecting a disk file, a disk, or the monitor with FILDDT, you  are
                              _______ _______ _____
   really  establishing  the  virtual address space that FILDDT accesses.
   When discussing the contents of that virtual address space, where  the
   contents  can  be any of the above objects, this chapter uses the term
   ______
   target.

   Once you have accessed a target, you can examine and  modify  it  with
   the  DDT  examine  and  modify commands (you cannot modify the running
   monitor with FILDDT), and then save it with your  modifications.   You
   can  use all of DDT's commands for examining and modifying memory, but
   you cannot use any  commands  that  cause  the  execution  of  program
   instructions,  such  as  <ESC>X, <ESC>G, and so on.  If you attempt to
   execute a program instruction, FILDDT sounds the  terminal  buzzer  or
   bell.



   9.2  USING FILDDT
   9.2  USING FILDDT

   There are two command levels in FILDDT.  This document refers to these
                 ______ _______ _____     ___ _______ _____
   two levels as FILDDT command level and DDT command level.

   FILDDT command  level  accepts  FILDDT  commands  to  control  session
   parameters  and  to  select  the target.  FILDDT command level employs
   TOPS-20 command recognition and help.  When at FILDDT  command  level,
   FILDDT displays the prompt:

        FILDDT>



                                    9-1
                                   FILDDT
                                   FILDDT


   Once you access a target, FILDDT enters DDT  command  level.   At  DDT
   command level, use DDT commands to examine and modify the target.

   The syntax of a FILDDT command-level command is:

        command {file-spec{/switch...}}

         _______                                         _________
   where command is  a  FILDDT  command-level  command,  file-spec  is  a
                                    ______
   TOPS-20  file specification, and switch invokes a specific function or
   parameter about the function that you can  perform  (enable  patching,
   for  example).   You  can  use FILDDT commands to invoke functions and
   parameters that are invoked by analogous FILDDT switches.

   With a FILDDT command you can:

         o  request HELP on FILDDT

         o  specify the target to be examined

         o  invoke FILDDT functions

         o  establish certain parameters about the functions that you can
            perform

         o  enter DDT command level

         o  exit FILDDT

   A FILDDT command can have more than one of the above effects.

   The commands and switches are described in detail in the rest of  this
   chapter.

   To start FILDDT, enter the TOPS-20 command:

        FILDDT

   FILDDT enters FILDDT command level and prompts:

        FILDDT>

   You can now use the FILDDT commands described on the following pages.












                                    9-2
                                   FILDDT
                                   FILDDT


   9.2.1  FILDDT Commands
   9.2.1  FILDDT Commands

   There are two classes of FILDDT-level commands; those that select  the
   target that FILDDT is to access and those that establish what function
   FILDDT is to perform for the target (enable patching, extract symbols,
   and  treat  an  .EXE  file  as  data).   The following are the targets
   (virtual address spaces) that FILDDT can access, and the commands that
   select them:

          TARGET               COMMANDS

        Disk files              GET
        Disk structures         DRIVE, STRUCTURE
        The running monitor     PEEK

   To examine and patch disk structures, you must have  WHEEL,  OPERATOR,
   or  MAINTENANCE  privileges  enabled.  To examine the running monitor,
   you must have WHEEL or OPERATOR privileges enabled.

   Following are the parameters you can invoke for  the  target  and  the
   commands and switches that select them:

               FUNCTION                      COMMAND          SWITCH

        Treat file as pure binary data    ENABLE DATA-FILE    /DATA
        Enable patching                   ENABLE PATCHING     /PATCH
        Enable thawed access              ENABLE THAWED       /THAWED
        Load symbol table only from file  LOAD                /SYMBOL

   To get HELP, enter:

        HELP

   FILDDT displays a very brief description of the  FILDDT  commands  and
   redisplays the FILDDT> prompt.

   To return to TOPS-20 command level from FILDDT command level, enter:

        EXIT















                                    9-3
                                   FILDDT
                                   FILDDT


   9.2.2  Symbols
   9.2.2  Symbols

   To enhance performance, FILDDT uses a symbol table that it  builds  in
   its  own  address  space,  rather  than one which exists in the target
   address space.

   FILDDT automatically extracts symbols to  build  its  internal  symbol
   table from the first .EXE file it loads during a session.  Once FILDDT
   has an internal symbol table, it ignores any symbols  in  subsequently
   loaded  .EXE  files unless you use the LOAD command or the GET command
   with the SYMBOL switch.



   9.2.3  Commands to Establish Formats and Parameters
   9.2.3  Commands to Establish Formats and Parameters

   ENABLE DATA-FILE
   ENABLE DATA-FILE

        If you specify an .EXE file, DDT (by default) loads the  file  in
        virtual  memory  as  if  it were to be executed.  You can use the
        ENABLE DATA-FILE command to look at an .EXE file as if it were  a
        data file.  FILDDT then loads the entire file (including the .EXE
        directory block) as a binary file, starting at  virtual  location
        zero.   You  can accomplish the same thing by appending the /DATA
        switch to the file-spec when you use the GET command.

   ENABLE PATCHING
   ENABLE PATCHING

        The ENABLE PATCHING lets you modify the  target.   You  can  also
        enable  patching  by appending the /PATCH switch to the file-spec
        when you use the GET command.  If you do not enable patching, you
        can only examine the target.  If you attempt to modify the target
        but have not enabled patching, FILDDT displays:

             ? Patching is not enabled

        Note that you cannot enable patching in FILDDT  with  the  <ESC>W
        command.

   ENABLE THAWED
   ENABLE THAWED

        The ENABLE THAWED command lets you examine  and  modify  (if  you
        enable  patching) files that require thawed access.  You can also
        use the /THAWED  switch  when  loading  the  file  with  the  GET
        command.









                                    9-4
                                   FILDDT
                                   FILDDT


        LOAD
        LOAD

             The LOAD command tells FILDDT to copy the symbol table  from
             the  file  named  by  file-spec.   Once FILDDT has built its
             internal symbol table, FILDDT displays:

             [Extracting symbols from file file-spec]
             [n symbols loaded from file]

             where n is the number of symbols that FILDDT extracted  from
             the file.  FILDDT then again prompts you with FILDDT>.

             You can also load symbols from the file you specify  in  the
             GET   command   by  appending  the  /SYMBOL  switch  to  the
             file-spec.

             If the  file  you  specify  is  not  an  .EXE  file,  FILDDT
             displays:

             % Not in .EXE format -- Data file assumed.
             [Extracting symbols from file file-spec]
             ? Symbols cannot be extracted from a data file

             FILDDT then redisplays its prompt.



        9.2.4  Commands to Access the Target and Enter DDT
        9.2.4  Commands to Access the Target and Enter DDT

        DRIVE
        DRIVE

             The DRIVE command allows you to access a physical  structure
             directly.   This  may  be  useful if the home block has been
             damaged.  To access disk structures with  FILDDT,  you  must
             have WHEEL, OPERATOR, or MAINTENANCE privileges enabled.

             If you wish to be able to patch the disk structure, you must
             give  the  ENABLE  PATCHING  command  before using the DRIVE
             command.

             To access the disk structure, enter:

             DRIVE c k u

             where c is the channel, k is the controller, and  u  is  the
             unit, in decimal.








                                    9-5
                                   FILDDT
                                   FILDDT


             If the unit is part of a mounted structure, FILDDT displays:

             [Unit is part of structure name]

                   ____
             where name is the logical name of the disk structure.

             If FILDDT successfully access the unit,  FILDDT  enters  DDT
             command level and displays:

             [Looking at unit u on controller k on channel c]

             where c is the channel, k is the controller, and  u  is  the
             unit, in decimal.

        GET
        GET

             The GET command tells FILDDT to  load  the  file  you  name,
             invoke  any  parameters  for which you specify switches, and
             enter DDT command level.  Legal switches are /DATA,  /PATCH,
             /SYMBOL,   and   /THAWED,   and  correspond  to  the  ENABLE
             DATA-FILE,  ENABLE  PATCHING,  LOAD,   and   ENABLE   THAWED
             commands, respectively.

             If FILDDT extracts symbols and  builds  an  internal  symbol
             table, it displays:

             [Extracting symbols from file file-spec]
             [n symbols loaded from file]

             where n is the number of symbols loaded.

             When FILDDT has loaded the file, it displays:

             [Looking at file file-spec]

             where file-spec is the TOPS-20  file  specification  of  the
             file.

             If FILDDT does not find the file, it displays:

             ? Invalid file specification, message

             where message is a TOPS-20 error string.











                                    9-6
                                   FILDDT
                                   FILDDT


        PEEK
        PEEK

             Use the PEEK command to examine the running monitor.  To use
             FILDDT  to  examine the running monitor, you must have WHEEL
             or OPERATOR privileges enabled.

             Once you have invoked FILDDT, if you wish to be able to  use
             monitor  symbols  when  looking  at the running monitor, you
             must use the LOAD command first, as:

             LOAD SYSTEM:MONITR.EXE

             You cannot patch the running monitor with FILDDT.

             To examine the running monitor, enter:

             PEEK

             FILDDT displays:

             [Looking at running monitor]

             and enters DDT command level.


                                         NOTE

                            You cannot use FILDDT to  PEEK
                            at  the running monitor unless
                            you are using  normal  virtual
                            addressing.     If   you   are
                            PEEKing the monitor and change
                            memory mapping to a mode other
                            than normal virtual addressing
                            with  the  n<ESC>0U, n<ESC>1U,
                            n<ESC>2U,  or  $$U   commands,
                            FILDDT does not give an error.
                            However,  every  page  in  the
                            monitor then appears to DDT to
                            be  non-existent.    In   this
                            case,    most    attempts   to
                            reference memory causes DDT to
                            display  ?, sound the terminal
                            buzzer or bell,  and  set  the
                            error  string  to  "CAN'T PEEK
                            PHYSICAL".   Searches  do  not
                            cause    errors,   but   never
                            discover matches.






                                    9-7
                                   FILDDT
                                   FILDDT


        STRUCTURE
        STRUCTURE

             The STRUCTURE command allows you to access a disk  structure
             by its logical name.  To access disk structures with FILDDT,
             you must have WHEEL,  OPERATOR,  or  MAINTENANCE  privileges
             enabled.

             If you wish to be able to patch the disk structure, you must
             give  the ENABLE PATCHING command before using the STRUCTURE
             command.

             To examine a disk structure, enter:

             STRUCTURE name

             where name is the logical name of  the  structure.   If  the
             structure  contains  more  than  one  physical disk, you can
             access the entire logical structure.

             If FILDDT successfully accesses the structure, it enters DDT
             command level and displays:

             [Looking at file structure name]

                   ____
             where name is the logical name of the structure.



        9.2.5  Exiting FILDDT
        9.2.5  Exiting FILDDT

        When you are through examining and modifying the target, save the
        modified file by entering:

             <CTRL/E>

        FILDDT closes the file, saving any changes that  you  have  made,
        and returns to FILDDT command level.

        Any symbol table that you have loaded (explicitly or by  default)
        remains loaded until you specify another with the LOAD command or
        the /SYMBOL switch.

        If you have modified symbols, FILDDT  also  modifies  the  symbol
        table of the disk file, if one of the following occurred:

         o  FILDDT automatically loaded the symbol table.

         o  you loaded the symbol table and entered DDT command level  by
            entering:

            GET file-spec/SYMBOL



                                    9-8
                                   FILDDT
                                   FILDDT


|       FILDDT sometimes runs out of memory when  you  use  the  <CTRL/E>
|       command to save files without exiting FILDDT.  If FILDDT runs out
|       of memory while loading a file, it displays the message:
|  
|            ? Not enough memory for file pages
|  
|       If FILDDT runs out of space while building  a  symbol  table,  it
|       displays the message:
|  
|            ? Not enough memory for symbols
|  
|       To reclaim all of your available memory,  exit  FILDDT  with  the
|       <CTRL/Z>  command,  and  then  restart  FILDDT  with  the TOPS-20
|       command FILDDT.   Note  that  this  technique  restores  standard
|       virtual  addressing  conditions,  as  if  you had used the <ESC>U
|       command.   See  Chapter  11  (Physical  and  Virtual   Addressing
|       Commands)   for   more   information   about  virtual  addressing
|       conditions.

        To close the file, save  all  modifications  (as  with  <CTRL/E>,
        above) and exit from FILDDT, enter:

             <CTRL/Z>

|       If you exit FILDDT by entering <CTRL/C>, changes that you make to
|       a  disk  file can still be in FILDDT's output buffer; if so, they
|       will NOT be saved.

        When you exit FILDDT, you  can  save  FILDDT  with  its  internal
        symbol table.  This saves time if you often use FILDDT to debug a
        specific file (such as the monitor) that has a very large  symbol
        table.   Start FILDDT, load the symbol table, then exit.  Use the
        TOPS-20 SAVE command to create a copy of FILDDT to be  used  with
        that specific file.

        For example (your input is in lowercase):

             @filddt
             FILDDT>load system:monitr.exe
             [34472 symbols loaded from file]
             FILDDT>exit
             @save
              FILDDT.EXE.1 Saved











                                    9-9

























































                                    10-1












                                 CHAPTER 10
                                 CHAPTER 10

                          PRIVILEGED MODES OF DDT
                          PRIVILEGED MODES OF DDT





                                    NOTE

                  This chapter makes no attempt to explain
                  internal   monitor   mechanisms.    This
                  chapter assumes that you  are  aware  of
                  various   monitor   contexts.    Certain
                  monitor contexts may interfere  with  or
                  be   interfered   with  by  DDT  context
                  switching.  It is up to you to be  aware
                  of   these.   Note  also  that  internal
                  monitor  locations  that  are  used   in
                  examples  in this chapter are subject to
                  change in subsequent monitor releases.
























                                    10-1
                          PRIVILEGED MODES OF DDT
                          PRIVILEGED MODES OF DDT


   10.1  MDDT
   10.1  MDDT

   MDDT  is  used  to  debug  and  patch  the  running   monitor   during
   timesharing, and is an integral part of the swappable monitor.  To run
   MDDT, you must have WHEEL or OPERATOR privileges enabled.

   To invoke MDDT, start DDT  and  then  execute  the  MDDT%  JSYS.   For
   example (user input is lowercase):

        @enable
        $ddt
        DDT
        mddt%<ESC>x
        MDDT

   You can also invoke MDDT by running a MACRO-20 program  that  executes
   the MDDT% JSYS.

   MDDT runs in the executive virtual address space of the  process  that
   executed the MDDT% JSYS.  While in MDDT, you are still running in user
   context, you are  running  under  timesharing,  and  your  process  is
   subject to being swapped out, as is any other user process.

   If for some reason you cannot access system files, you can enter  MDDT
   through the MEXEC, as follows:

        @enable
        $<CTRL/E>QUIT
        MX>/

   where the / (slash) command to MEXEC enters MDDT.

   To exit MDDT, enter:

        mretn<ESC>g

   or enter:

        <CTRL/Z>.















                                    10-2
                          PRIVILEGED MODES OF DDT
                          PRIVILEGED MODES OF DDT


   10.2  KDDT
   10.2  KDDT

   You can run KDDT in executive mode or in user mode.  KDDT in executive
   mode  is  used to debug parts of the monitor which can not be debugged
   interactively such as those modules that deal with physical memory  or
   paging.  KDDT in user mode is used to debug and patch the monitor .EXE
   file, which you can then save for BOOTing at a later time.

   KDDT is part of the resident monitor.  When running KDDT in  executive
   mode,  you  can  exercise  any  normal DDT functions, such as changing
   memory and setting breakpoints.  When you stop  at  a  breakpoint  and
   control passes to KDDT, timesharing (if in effect) ceases.

   To run KDDT in executive mode use the  /E  command  when  BOOTing  the
   monitor.  For example (user input is in lowercase):

        BOOT>/e             ;Type in /e
        [BOOT: LOADING] [OK]
        EDDT                ;EDDT is loaded and waiting for your command

   Your debugging may be easier if you  lock  the  swappable  monitor  in
   core.  You can do this by executing the instruction that calls monitor
   routine SWPMLK.  For example:

        CALL SWPMLK<ESC>X

   To run KDDT in user mode:

        @get system:monitr.exe
        @start 140
        DDT

   You can use KDDT in user mode to patch the monitor (.EXE  file)  which
   will be booted the next time the system is BOOTed up.

   After you have started KDDT as above, use the DDT patching commands to
   insert  your  patch.   When  your  patch  is  complete, exit KDDT with
   <CTRL/Z> and use the TOPS-20 SAVE command to save the patched  version
   of the monitor.  For example:

        <ESC><                   ;ESCAPE key followed by a left angle 
                                 ;bracket.
         ....

         ....                    ;Type in the patch.

        <ESC>>                   ;ESCAPE key followed by right angle 
                                 ;bracket.
        <CTRL/Z>                 ;Exit KDDT
        @save system:monitr.exe  ;Save the new version.




                                    10-3
                          PRIVILEGED MODES OF DDT
                          PRIVILEGED MODES OF DDT


   10.3  EDDT
   10.3  EDDT

   You can use EDDT to debug user programs that run  in  executive  mode.
   You must load EDDT.REL with your program, as follows:

        @LINK
        S
        SYS:EDDT.REL,PROG/GO
        @SAVE PROG

   where PROG is the name of your MACRO-20 program.












































                                    10-4

























































                                    10-5

























































                                    11-1












                                 CHAPTER 11
                                 CHAPTER 11

                  PHYSICAL AND VIRTUAL ADDRESSING COMMANDS
                  PHYSICAL AND VIRTUAL ADDRESSING COMMANDS



|  All TOPS-20 DDTs (including FILDDT) can do  their  own  page  mapping.
|  The  commands described in this chapter allow you to set parameters to
   govern  the  interpretation  of  the  address  space  which  you   are
   examining.   You  can control the mapping of the address space you are
   examining by choosing to use or bypass the user process table (UPT) or
   the executive process table (EPT).  You can choose which special pages
   table (SPT) to use, and which hardware register block to  use.   Other
   commands  allow  you to emulate either KI-paging or KL-paging, control
   address relocation, and set memory protection limits.  In each of  the
   following commands, the argument (page, addr, n) defaults to zero.

                                    NOTE

           The DDT commands <ESC>G, <ESC>P, and <ESC>X have  side
           effects  that  affect  your  control over physical and
           virtual  addressing.   In  addition  to  their  normal
           functions, these commands also do the following:

            o  restore normal virtual addressing as if <ESC>U had
               been given (<ESC>X does NOT do this)

            o  set the FAKEAC flag (as if <ESC>U had been given)

            o  clear the relocation factor (as  if  0<ESC>8U  had
               been given)

            o  reset the address-protection address  to  infinity
               (377777,,777777)

            o  restore the active hardware register block to  the
               one in use before any <ESC>4U command was given








                                    11-1
                  PHYSICAL AND VIRTUAL ADDRESSING COMMANDS
                  PHYSICAL AND VIRTUAL ADDRESSING COMMANDS


   COMMAND                  EXPLANATION

   <ESC>U

        This command enables memory mapping by standard  TOPS-20  virtual
        addressing.  When you give this command, DDT restores the virtual
        addressing  conditions   that   were   in   effect   before   any
        {<ESC>}<ESC>nU  (where  0<=n<=2)  commands  were  given, and sets
        DDT's FAKEAC  flag,  thereby  forcing  DDT  to  interpret  memory
        addresses  0-17  as  DDT's own internal "registers", in which the
        user's registers were saved.

   <ESC><ESC>U

        This command enables DDT to use actual  physical  addresses  when
        accessing  memory,  and  clears DDT's FAKEAC flag, causing DDT to
        interpret memory addresses 0-17 as the hardware  registers  0-17.
        This  command  is  meaningful  only  when using KDDT in executive
        mode, or when using  FILDDT  to  look  at  the  running  monitor.
        Although  DDT  accepts  <ESC><ESC>U  at other times, this command
        then produces the same effect as <ESC>U.

   The general syntax of the following virtual addressing commands is:

        arg<ESC>nU

                                                      ___
   where n is the function number of the command, and arg is dependent on
   the function (see the function descriptions below).

   Functions 0, 1,  and  2  enable  you  to  control  memory  mapping  by
   selecting the executive process table (EPT), user process table (UPT),
   or the section map through which mapping occurs.   Setting  a  mapping
   condition  with  any one of these functions (0, 1, and 2) also has the
   effect of clearing the effects of  any  prior  use  of  one  of  these
   functions (0, 1, and 2).

   You can also specify the offset into the  special  pages  table  (SPT)
   with functions 0, 1, and 2 by using the following command:

        arg<ESC><ESC>nU

   where arg is the SPT offset, and 0<=n<=2.  This form is legal only  if
   KL-paging is in effect.

                                    NOTE

           All forms of <ESC>B and <ESC>X are illegal if you have
           used  the page mapping functions (0, 1, or 2) and have
           not restored standard mapping with the <ESC>U command.





                                    11-2
                  PHYSICAL AND VIRTUAL ADDRESSING COMMANDS
                  PHYSICAL AND VIRTUAL ADDRESSING COMMANDS


   COMMAND                  EXPLANATION

   page<ESC>0U

        This command causes memory mapping to occur through the executive
                                                             ____
        process table (EPT) that is located at physical page page.

   offset<ESC><ESC>0U

        This command produces the same  effect  as  page<ESC>0U  (above),
                    ______
        except that offset is an offset (in words) into the SPT.

   page1<page2<ESC>0U

        This command is an exception to the general syntax, and is  legal
        only  under  KI-paging.   You can select both the user page table
        (UPT) and the executive page table (EPT) with this command, where
        _____                                    _____
        page1 is the page number of the UPT, and page2 is the page number
        of the EPT.  Follow page1 with a left angle bracket (<).

   page<ESC>1U

        This command causes memory mapping  to  occur  through  the  user
                                                              ____
        process  table (UPT) that is located at physical page page.  With
        this command, you can bypass the EPT.

   offset<ESC><ESC>1U

        This command produces the same  effect  as  page<ESC>1U  (above),
                    ______
        except that offset is an offset (in words) into the SPT.

   page<ESC>2U

        This command causes mapping to occur through the section  map  at
                        ____
        physical  page  page.  This command is legal only if KL-paging is
        in effect.

   offset<ESC><ESC>2U

        This command produces the same  effect  as  page<ESC>2U  (above),
                      ______
        except  that  offset  is an offset (in words) into the SPT.  This
        command is legal only if KL-paging is in effect.












                                    11-3
                  PHYSICAL AND VIRTUAL ADDRESSING COMMANDS
                  PHYSICAL AND VIRTUAL ADDRESSING COMMANDS


   COMMAND                  EXPLANATION

   n<ESC>3U

        This command determines  whether  DDT  interprets  references  to
        memory  locations 0-17 as references to hardware registers, or to
        DDT's  own  internal  "registers"  (which  normally  contain  the
        user-program ACs), by setting or resetting DDT's FAKEAC flag.

        If n=0, reset FAKEAC flag (use the hardware registers 0-17).
        If n is nonzero, set FAKEAC flag (use  DDT's  internal  registers
        0-17).

        If you enter a nonzero value for n, DDT stores the value -1.

   n<ESC>4U

        This  command  tells  DDT  to  copy  hardware  register  block  n
        (0<=n<=7)  to  its  own  internal  register block, set the FAKEAC
        flag, and use hardware register block n as its own registers.  If
        the  FAKEAC  flag  is  set  when you give this command, DDT first
        restores the contents of  its  internal  register  block  to  the
        hardware  register  block  from  which  they  were  copied.  This
        command is legal in executive mode EDDT and KDDT only.  Note that
        the  microcode uses register block 7, and any attempt to use this
        block produces an almost immediate system crash.

   addr<ESC>5U

                                                                 ____
        This command copies the 20 (octal) word block located at addr  to
        DDT's internal "registers" and sets the FAKEAC flag.

   addr<ESC>6U

        This command sets the special pages table (SPT) to addr.

   addr<ESC>7U

        This command sets the core status table address (CST) to addr.

   addr<ESC>8U

                                                              ____
        This command sets the address relocation  factor  to  addr.   DDT
        adds addr to all user addresses that you enter.

   addr<ESC>9U

                                                                     ____
        This command read-and-write-protects  all  addresses  above  addr
        (before adding relocation factor).





                                    11-4
                  PHYSICAL AND VIRTUAL ADDRESSING COMMANDS
                  PHYSICAL AND VIRTUAL ADDRESSING COMMANDS


   COMMAND                  EXPLANATION

   n<ESC>10U

        This command controls whether KI paging is enabled or cleared.

        If n is nonzero, KI paging is enabled.
        If n=0, KI paging is cleared.

        If you enter a nonzero value for n, DDT stores the value -1.

|       This command is illegal in executive mode EDDT.

   n<ESC>11U

        This command controls whether KL paging is enabled or cleared.

        If n is nonzero, KL paging is enabled.
        If n=0, KL paging is cleared.

        If you enter a nonzero value for n, DDT stores the value -1.

|       This command is illegal in executive mode EDDT.

|  22<ESC>U
|  23<ESC>U
|  
|       These commands specify the type of CPU on which  the  program  is
|       being  debugged.   22<ESC>U  refers  to a KL processor.  23<ESC>U
|       refers to a KS processor.
|  
|       For DDT, this  command  is  meaningless,  because  DDT  uses  the
|       current  CPU  type.   However,  these  commands may be useful for
|       FILDDT.

   You can interrogate DDT  to  determine  the  last  virtual  addressing
   command that was given for a specific function.  The command:

        <ESC>nU

   where 0<=n<=11, returns the address of a DDT  location  that  contains
                               __ ___ _______ ___ ____ ________ ___ ____
   the argument that was given if the command for that function was used,
   and returns the default value if that function was not used.   If  you
   entered  a nonzero argument to a command that requires zero or nonzero
   values (or if the default is nonzero), this location contains -1.  You
   can use DDT commands to examine this location.








                                    11-5
                  PHYSICAL AND VIRTUAL ADDRESSING COMMANDS
                  PHYSICAL AND VIRTUAL ADDRESSING COMMANDS


   The command:

        <ESC><ESC>nU

   where 0<=n<=2, returns the address of a  DDT  location  that  contains
   information  that  indicates  which function you used, and whether you
   set a page address or an offset.  You can use DDT commands to  examine
   this  location.   This command is illegal for all functions where n>2.
   If you did not enter any commands affecting functions  0-2  since  the
   last <ESC>U command, the right half of this DDT location word contains
   zero.  Otherwise, the right half contains n+1, where n is  the  number
   of  the  command  function  you used.  If you set a page address (with
   arg<ESC>nU), bit 1 of this word is reset.  If you set an offset  (with
   arg<ESC><ESC>nU), bit 1 of the word is set.








































                                    11-6












                                 CHAPTER 12
                                 CHAPTER 12

                            EXTENDED ADDRESSING
                            EXTENDED ADDRESSING



   12.1  LOADING DDT INTO AN EXTENDED SECTION
   12.1  LOADING DDT INTO AN EXTENDED SECTION

   If your program is loaded in a nonzero section, merge  DDT  with  your
   program with the TOPS-20 command:

        @DDT

   DDT is loaded into the highest-numbered  free  (nonexistent)  section.
   If  your program has a TOPS-10-style entry vector in section zero, the
   EXEC merges the UDDT stub into the section  that  contains  the  entry
   vector, and places that section's JOBDAT symbol table pointers (.JBSYM
   and .JBUSY) into the DDT locations pointed to by UDDT locations 770001
   and  770002.   UDDT  then  loads  XDDT  into the highest-numbered free
   section.  You can load DDT into a specific  section  with  the  EXEC's
   /USE-SECTION switch, as:

        @DDT/USE-SECTION:n

   where n is the (octal) section number  of  a  section  that  does  not
   already exist.

   You can load DDT into an existing section with  the  /USE-SECTION  and
   /OVERLAY  switches.   You  must be careful, however, that your program
                                                        _______  ________
   does not use any pages  that  DDT  uses.   See  the  TOPS-20  COMMANDS
   _________ ______
   REFERENCE MANUAL for more information about the use of these switches.














                                    12-1
                            EXTENDED ADDRESSING
                            EXTENDED ADDRESSING


   12.2  EXAMINING AND CHANGING MEMORY
   12.2  EXAMINING AND CHANGING MEMORY

   The commands /, [ (left square bracket), ] (right square bracket),  !,
   \  and  <TAB> (see Section 4.4.3) open a memory location at an address
   calculated from an expression typed in or defaulted  in  the  command.
   The syntax of the command is

        {expr}{<ESC>{<ESC>}}c

   where c is the command, and expr is any  legal  DDT  expression  (expr
   defaults to $Q, the current quantity).

   In nonzero sections, you can cause DDT to utilize all indirection  and
   indexing  indicated  by  EFIWs  (extended  format  indirect  words) to
   calculate 30-bit global addresses by using the format

        {expr}<ESC><ESC>c

   This format also recognizes and utilizes instruction  format  indirect
   words  (IFIW).   These  commands are thoroughly described in Chapter 4
   (Displaying and Modifying Memory).



   12.3  BREAKPOINTS
   12.3  BREAKPOINTS

   If DDT is running in a nonzero section, breakpoints can be set in  any
   section.



   12.3.1  The Breakpoint Block
   12.3.1  The Breakpoint Block

   To set breakpoints in a section external to the  one  containing  DDT,
   DDT  requires  an area of contiguous storage in the section containing
   the breakpoint.  This area is known as the  "breakpoint  block".   The
   extra  storage is required for saving global addresses for transfer of
   control between your program and DDT, and also for  the  execution  of
   single-stepped  instructions  that reference memory locations that are
   not in their section.

   Each section within your program space that contains a breakpoint must
   have  one breakpoint block.  Breakpoint blocks are located at the same
   relative local address within each section (the  default  is  777700),
   and are 100 (octal) words in size.

   Each  breakpoint  block  is  always  contiguous  within  one  section.
   Breakpoint  blocks  never  extend  across section boundaries and never
   "wrap around" the end of a section to the beginning of the section.





                                    12-2
                            EXTENDED ADDRESSING
                            EXTENDED ADDRESSING


   DDT creates a  breakpoint  block  in  each  section  as  required,  if
   inter-section breakpoints are enabled (see below).

   You (or your program) can reference memory within a breakpoint  block,
   but any information stored there can be overwritten by DDT.



   12.3.2  Enabling and Disabling Inter-section Breakpoints
   12.3.2  Enabling and Disabling Inter-section Breakpoints

   The section-relative (18-bit) address of the  breakpoint  block(s)  is
   stored  in  an internal DDT location.  The command <ESC>4M returns the
   address of that DDT location.   The  symbol  $4M  refers  to  the  DDT
   location   at   the   address   returned  by  <ESC>4M.   Inter-section
   breakpoints are enabled as long as $4M contains  the  address  of  the
   breakpoint  block.   At startup, DDT enables inter-section breakpoints
   by default.

   To change the address of the breakpoint block, enter:

        n<ESC>4M

   where n is the address of the breakpoint block, and can be  any  legal
   DDT  expression  (20<=n<=777700).   DDT uses only the right half of n,
   and changes only the right half of the DDT location at $4M.

   By default, the section-relative breakpoint block address  is  777700,
   placing  the  breakpoint  block at the top of the section.  To display
   the address of the breakpoint block, enter:

        <ESC>4M/

   Inter-section breakpoints are disabled when $4M contains zero.

                                    NOTE

                  In MDDT, $4M  defaults  to  MDDBLK.   In
                  KDDT,  $4M  defaults  to  EDDBLK.   Each
                  symbol denotes the start of  a  100-word
                  (octal)  block contained in page zero of
                  the monitor.  Page zero of  the  monitor
                  is   mapped   into  every  section  that
                  contains monitor code.


      _______ _____________ ___________
   To disable inter-section breakpoints, enter:

        0<ESC>4M






                                    12-3
                            EXTENDED ADDRESSING
                            EXTENDED ADDRESSING


   While  inter-section  breakpoints  are  disabled,  you  cannot  set  a
   breakpoint  in  a section external to DDT, and any breakpoints already
   set in such a section are lost when you begin program  execution  with
   <ESC>G,   or   continue  program  execution  with  <ESC>P.   For  each
   breakpoint lost, DDT displays:

        % CAN'T INSERT $nB - IN NON-DDT SECTION

   where n is the breakpoint number.

   While inter-section breakpoints are disabled, DDT cannot  execute  the
   <ESC>X command when:

         o  you try to execute the instr<ESC>X command, and  the  default
            section is not the section that contains DDT.

         o  you try  to  single-step  a  dangerous  instruction  and  the
            user-program PC is not in the section that contains DDT.

|  In these cases, when you try to use <ESC>X,  DDT  rings  the  terminal
|  bell or buzzer and sets its error message text to:
|  
|       Intersection reference and no $4M global breakpoint/execute block



   12.4  DISPLAYING SYMBOLS IN NONZERO SECTIONS
   12.4  DISPLAYING SYMBOLS IN NONZERO SECTIONS

   DDT normally uses right-halfword values when searching  symbol  tables
   for symbols to display.  However, code linked in a nonzero section has
   symbols defined with the section number  in  the  left-halfword.   DDT
   uses  a  30-bit  value  when  searching  for a symbol in the following
   circumstances:

         o  when displaying the address of a location

         o  when displaying the contents of a location as an address

         o  when displaying the Y field of an instruction

   When displaying an address, DDT searches for a symbol defined with the
   30-bit  value  of  the  address.   If  such a symbol is not found, DDT
   displays the address in halfword format.

   When displaying the Y field of an  instruction,  DDT  searches  for  a
   symbol defined with a 30-bit value consisting of:

         o  the section number of the address of the word being displayed

         o  the section-relative address contained in the Y field of  the
            instruction



                                    12-4
                            EXTENDED ADDRESSING
                            EXTENDED ADDRESSING


   If DDT does not find a symbol defined with that 30-bit value, it looks
   for a symbol defined with the 18-bit value contained in the Y field of
   the instruction.

   Assume a program with the following conditions:

        Symbol LABL1 is defined as 0,,300
        Symbol LABL2 is defined as 3,,300
        Location 1,,300 contains 3,,300
        Location 1,,301 contains 2,,300
        Location 3,,400 contains 200040,,300
               (MOVE contents of location 300 to AC 1)

   When displaying the contents of location 1,,300, DDT displays:

        1,,LABL1/   LABL2

   When displaying the contents of location 1,,301, DDT displays:

        1,,LABL1+1/   2,,LABL1

   When displaying the contents of location 3,,400, DDT displays:

        LABL2+100/   MOVE 1,LABL2



   12.5  DEFAULT SECTION NUMBERS
   12.5  DEFAULT SECTION NUMBERS

   To reduce the need to type in  the  section  number  as  part  of  the
   address when you specify a location, DDT uses a default section number
   when you do not specify one.  DDT has two section defaulting options:

         o  permanent default section

         o  floating default section

   The command <ESC>6M returns the address of an  internal  DDT  location
   that  contains  section default information.  The symbol $6M refers to
   the DDT location at the address returned by the command <ESC>6M.

   When DDT is in section zero, the  default  section  number  is  always
   zero, regardless of the contents of $6M.


                                    NOTE

                  When you  use  KDDT  in  user-mode,  $6M
                  defaults  to  0,,0.  In all other cases,
                  $6M defaults to 1,,0.




                                    12-5
                            EXTENDED ADDRESSING
                            EXTENDED ADDRESSING


   12.5.1  Permanent Default Section
   12.5.1  Permanent Default Section

|  If the value contained in $6M is non-negative (bit zero is reset), the
   permanent  default  section  option  is in effect.  DDT then takes the
   left half of $6M as the section number of any address that you type in
   without a section number.

   Set the permanent default section by entering:

        n,,0<ESC>6M

   where n is the section number, and can be any legal DDT expression.



   12.5.2  Floating Default Section
   12.5.2  Floating Default Section

   If the value contained in $6M is  negative  (bit  zero  is  set),  the
   floating  default  section  option  is in effect.  This is the default
   option (at start-up, DDT initializes $6M  to  -1).   DDT  selects  the
   floating default section as follows:

         o  If you enter DDT from its normal start address, DDT sets  the
            default section to:

             >  the section that contains the program  entry  vector  (if
                there is one)  OR

             >  section zero.

         o  If you enter DDT from a  breakpoint,  DDT  sets  the  default
            section to the section that contains the breakpoint.

         o  If you open a local address between 20 and 777777,  DDT  sets
            the  default  section  to  the section that contains the open
            address.

         o  If you type in an address  that  contains  a  section  number
            (including  a  symbol that is defined with a section number),
            DDT sets the default section to the one in  the  address  you
            entered.

   If you exit DDT with <CTRL/C> or <CTRL/Z>, and then reenter  DDT,  the
   current  location  does  not change.  If you give a command that takes
   the current location as its default address  argument,  DDT  sets  the
   floating default section to the section of the current location.

   In the following example, the DDT screen display is on the  left,  and
   explanatory  comments  are  on  the  right.   The  entry  vector is in
   section 1.  Symbol START is not defined with a section  number.   User
   input is in lowercase.



                                    12-6
                            EXTENDED ADDRESSING
                            EXTENDED ADDRESSING


          SCREEN DISPLAY       USER INPUT        EXPLANATION
          SCREEN DISPLAY       USER INPUT        EXPLANATION

                               3,,place/   Examine location 3,,PLACE.

        LABL1                              DDT displays the contents.

                               <LF>        Type in <LF> to examine
                                           the next location.

        3,,PLACE+1/   LABL1+2              DDT displays the next
                                           location. The floating
                                           default section = 3.

                               <CTRL/C>    Exit with <CTRL/C>.
                                           The current location is
                                           3,,PLACE+1.

        @                                  TOPS-20 prompts you.

                               @ddt        Reenter DDT.

        DDT                                DDT is loaded and ready
                                           for your command. The 
                                           floating default section is
                                           1, because the entry vector
                                           is in section 1.

                               <LF>        Type in <LF> to examine
                                           the next location.

        3,,START+2/   LABL1+4              DDT displays the address
                                           and contents of the next
                                           location. DDT doesn't use
                                           the floating default section,
                                           because your <LF> command
                                           defaults addr to the current 
                                           location, and uses its 
                                           section number (3).

                               start/      Examine location START.
                                           DDT uses the floating default 
                                           section number because symbol
                                           START is defined with no
                                           section number.

        JFCL 0                             DDT displays the contents.

                               <LF>        Type in <LF> to examine
                                           the next location.

        1,,START+1/   MOVE 1,LABL1         DDT displays the address
                                           and contents of the location.


                                    12-7
                            EXTENDED ADDRESSING
                            EXTENDED ADDRESSING


   12.6  EXECUTING SINGLE INSTRUCTIONS
   12.6  EXECUTING SINGLE INSTRUCTIONS

   Instructions that are executed by means of the command

        instr<ESC>X

   where instr is the instruction for DDT to execute, are executed within
   the  current  default  section.   If  that section is not the one that
   contains DDT, DDT uses the breakpoint block in that section to execute
   instr.   If  the  floating default section option is in effect and you
   are unsure of the current default section, use the  addr/  command  to
   open a location in the section in which you wish DDT to execute instr.
   This sets the default section to the section specified by addr.

   If DDT is to execute the instruction in a section other than  the  one
   that contains DDT, inter-section breakpoints must be enabled.

   If you try to execute instr outside DDT's section  while  intersection
   breakpoints  are  disabled,  DDT  sounds  the terminal buzzer or bell,
   displays "?" and sets its error string to:

|       Intersection reference and no $4M global breakpoint/execute block



   12.7  ENTERING PATCHES IN EXTENDED SECTIONS
   12.7  ENTERING PATCHES IN EXTENDED SECTIONS

   You cannot enter a patch if a patching area  does  not  exist  in  the
   section  that  contains the word to be replaced.  To ensure that there
   is a patching area for each section that contains  user-program  code,
   do one of the following:

         o  reserve the same part of each section for patches, and define
            the  patch symbol as 0,,addr, where addr is the local address
            of the patching area

         o  use only one patching area and map it into all  the  sections
            that  contain  user-program code.  Define the patch symbol as
            0,,addr, where addr is the  local  address  of  the  patching
            area.

         o  define a different symbol for each section's  patching  area,
            and use the symbol appropriate to the section being patched

   If the left half of expr is zero, DDT  defaults  the  section  to  the
   section  that contains the open location.  If the left half of expr is
   a value that is not the section that contains the open  location,  DDT
   displays:

        ?CAN'T PATCH ACROSS SECTIONS




                                    12-8












                                 APPENDIX A
                                 APPENDIX A

                               ERROR MESSAGES
                               ERROR MESSAGES



   DDT and FILDDT display error messages to indicate the results of  your
   commands.   DDT sometimes (and FILDDT usually) displays these messages
   on the screen, and at other times displays only a question mark.  When
   only  a question mark is displayed, a location internal to DDT usually
   points to a text string that is the error  message.   To  display  the
   error message, enter the command:

        <ESC>?
o  
   Following is a list of DDT messages together with explanations of what
   the messages indicate.


   ? ABOVE PROTECTION REGISTER LIMIT

        The address of the location you tried to  display  or  modify  is
        above the protection register limit, which is set by n<ESC>9U.


   ? ACTUAL REFERENCE FAILED

        A memory reference failed unexpectedly (the page  exists  and  is
        readable, but the reference failed anyway).


   ? ADDRESS GREATER THAN 777777

        An address to be mapped through a section  table  has  a  nonzero
        section  number.   This can occur only if you specified a section
        table with the n<ESC>{<ESC>}2U command.









                                    A-1
                               ERROR MESSAGES
                               ERROR MESSAGES


|  ? ADDRESS BEYOND END OF PHYSICAL MEM
|  
|       You attempted to examine a physical memory  location  beyond  the
|       end  of physical memory.  This error occurs only if you have used
|       the <ESC><ESC>U command to enable physical addressing.


   ? Bad format for .EXE file

        You specified a file that appears to have an .EXE directory,  but
        the directory is badly formatted or DDT cannot read it because of
        some other reason.


   ? BAD $4M VALUE

        You used the n<ESC>4M command where 777700<n<20.


   ? BAD POINTER ENCOUNTERED

        DDT does not recognize the type code  contained  in  a  page  map
        pointer.   This  can  occur only if you are trying to do your own
        virtual address mapping, and used the expr<ESC>{<ESC>}nU command,
        where 0<=n<=2.


   ? CAN'T BE WRITE ENABLED

        Even though you have automatic write-enable  turned  on,  DDT  is
        unable to write-enable a page that exists and is write-protected.


   ? CAN'T CREATE PAGE

        DDT attempted to create a page and failed, or else cannot attempt
        to create the page (see the <ESC>1W command).


   ? CAN'T DEPOSIT INTO SYMBOL TABLE BECAUSE .....

        You tried to define or kill a  symbol,  but  DDT  was  unable  to
        modify  the  symbol  table.  Look up the second part of the error
        message in this appendix.


   ? CAN'T DEPOSIT INTO SYMBOL TABLE BECAUSE DEPOSIT FAILED

        You tried to define or kill a  symbol,  but  DDT  was  unable  to
        modify the symbol table, and cannot identify the specific reason.




                                    A-2
                               ERROR MESSAGES
                               ERROR MESSAGES


   % CAN'T INSERT $nB BECAUSE .....

        DDT is not able to access the location where  you  inserted  your
        breakpoint.  Look up the second part of the error message in this
        appendix.  This situation occurs  before  DDT  tries  to  execute
        <ESC>G, <ESC>P, <ESC>X, or <ESC><ESC>X.


   % CAN'T INSERT $nB BECAUSE BREAKPOINT IS IN DIFFERENT SECTION

        DDT is not able to access the location where  you  inserted  your
        breakpoint  because  inter-section  breakpoints  are  not enabled
        (<ESC>4M contains zero).  This error occurs before DDT  tries  to
        execute  <ESC>G,  <ESC>P,  <ESC>X,  or  <ESC><ESC>X.   To  enable
        inter-section breakpoints, deposit the breakpoint  block  address
        in the location addressed by the command <ESC>4M.


   % CAN'T INSERT $nB BECAUSE MEM REF FAILED

        DDT is not able to access the location where  you  inserted  your
        breakpoint.  DDT is not able to identify the reason.  This occurs
        before  DDT  tries  to  execute  <ESC>G,   <ESC>P,   <ESC>X,   or
        <ESC><ESC>X.   A typical occurrence is when you have a breakpoint
        set in the swappable monitor (set in KDDT in executive mode), but
        the swappable monitor is not locked in memory.


   ? CAN'T PATCH ACROSS SECTIONS

        You tried to insert a patch in a section other than the one  that
        contains the patching area.


   ? CAN'T PEEK PHYSICAL

        You attempted to PEEK at the monitor  but  have  specified  other
        than normal virtual addressing (FILDDT only).


   % CAN'T REMOVE $nB BECAUSE .....

        DDT is not able to access the location where  you  inserted  your
        breakpoint.  Look up the second part of the error message in this
        appendix.  This error occurs when your program enters DDT from  a
        breakpoint.








                                    A-3
                               ERROR MESSAGES
                               ERROR MESSAGES


   % CAN'T REMOVE $nB BECAUSE BREAKPOINT IS IN DIFFERENT SECTION

        DDT is not able to access the location where  you  inserted  your
        breakpoint.   because  inter-section  breakpoints are not enabled
        (<ESC>4M contains zero).  This error  occurs  when  your  program
        enters   DDT   from   a   breakpoint.   To  enable  inter-section
        breakpoints, deposit the breakpoint block address in the location
        addressed by the command <ESC>4M.


   % CAN'T REMOVE $nB BECAUSE MEM REF FAILED

        DDT is not able to access the location where  you  inserted  your
        breakpoint.   DDT is not able to identify the reason.  This error
        occurs when your program enters DDT from a breakpoint.  A typical
        occurrence  is  when  you  have a breakpoint set in the swappable
        monitor (set in  KDDT  in  executive  mode),  but  the  swappable
        monitor is not locked in memory.


   % CAN'T SET BREAKPOINT, $4M NOT SET

        You attempted to set a breakpoint in a section other than the one
        containing DDT while inter-section breakpoints were not enabled.


|  ? FAILURE ON SWITCHING ADDRESS SPACE
|  
|       EDDT (Executive mode EDDT only) encountered an error while trying
|       to  access  the  virtual  address space where monitor symbols are
|       kept.


   ? Garbage at end-of-command

        FILDDT encountered extra text at a place  in  the  command  where
        there should have been only <RET>.
|  
|  
|  Intersection reference and no $4M global breakpoint/execute block
|  
|       Inter-section  breakpoints  are  not  enabled,  and  one  of  the
|       following is true:
|  
|        o  you tried to execute the command instr<ESC>X but the  default
|           section is not the section that contains DDT.
|  
|        o  you tried to single-step  a  dangerous  instruction  but  the
|           user-program PC is not in the section that contains DDT.





                                    A-4
                               ERROR MESSAGES
                               ERROR MESSAGES


   ? I/O error

        Some kind of I/O error occurred when FILDDT attempted to read  or
        write to the unit specified in a DRIVE or STRUCTURE command.


   ? Illegal channel number

        You entered a DRIVE command that  contained  an  illegal  channel
        number.


   ? Illegal controller number

        You entered a DRIVE command that contained an illegal  controller
        number.


   ? Illegal unit number

        You entered a  DRIVE  command  that  contained  an  illegal  unit
        number.


   ? Incorrect symbol table pointer

        FILDDT is unable to read the symbol table specified by the symbol
        table pointer in the file.


   ? Input device must be a disk

        The device you specified is not a disk.


   ? Insufficient memory to read EXE file directory

        FILDDT does not have enough free memory to read in the  directory
        section of the .EXE file that you specified.















                                    A-5
                               ERROR MESSAGES
                               ERROR MESSAGES


   ? Insufficient memory to read PDV list

        FILDDT does not have enough free memory to read in  the  list  of
        PDVs in the .EXE file that you specified.


                                      NOTE

                       FILDDT sometimes runs out of memory
                       when  you  use the <CTRL/E> command
                       to  save  files   without   exiting
                       FILDDT.   If this is the case, exit
                       with the <CTRL/Z> command, and then
                       restart  FILDDT  with  the  TOPS-20
                       FILDDT command.




   ? INVALID DDT INTERNAL ADDRESS

        You addressed an internal location that is not defined.  This  is
        most likely to occur after you use a command that returns a value
        (such as <ESC>M) to examine a DDT location and then use  <LF>  or
        <BKSP> to look at nearby memory.


   ? Invalid file specification, message

        where message is a TOPS-20 error string.  FILDDT could not  parse
        a filespec given to a LOAD or GET command.


   ? Invalid guide phrase input

        where input is a guide (or noise) phrase that you typed  in,  and
        does not match FILDDT's guide phrase.


   M

        You entered a symbol that is defined in  more  than  one  module.
        You  can  select  the  correct symbol by opening the symbol table
                                                       ____________
        associated with that module, using the command module<ESC>:.


   ? MDDT BREAKPOINT BLOCK ALREADY IN USE

        Only one fork may have breakpoints set in MDDT at one time.   You
        attempted  to  set  a  breakpoint  in MDDT while another fork had
        already set an MDDT breakpoint.



                                    A-6
                               ERROR MESSAGES
                               ERROR MESSAGES


   ? Missing or extra units in structure

        The number of units with the name supplied in a STRUCTURE command
        does  not  agree  with the number of units in the first structure
        with that name returned by MSTR%.


   ? No keyword input

        where input is a word that you  typed  in.   You  entered  ENABLE
        without the DATA, PATCHING, or THAWED qualifier.


   ? NO READ ACCESS

        You tried to display a word in a page to which you  do  not  have
        read access.


   ? No such command as input

        where input is a word that you typed in.  You entered  a  command
        that FILDDT does not recognize.


   ? No such file structure

        COMND% and DEVST% think you supplied a disk name in  a  STRUCTURE
        command, but no unit with that name was returned by MSTR%.


   ? Not enough memory for file pages

        FILDDT does not  have  enough  free  memory  for  its  file  page
        buffers.

                                      NOTE

                FILDDT sometimes runs out of memory when you  use
                the   <CTRL/E>  command  to  save  files  without
                exiting FILDDT.  If this is the case,  exit  with
                the  <CTRL/Z>  command,  and  then restart FILDDT
                with the TOPS-20 FILDDT command.



   ? Not enough memory for symbols

        FILDDT does not have enough free memory to  read  in  the  symbol
        table from the specified .EXE file.  See the note above.




                                    A-7
                               ERROR MESSAGES
                               ERROR MESSAGES


   ? NOT IN CORE

        You tried to map through a page map pointer (in a  UPT,  SPT,  or
        section  table)  that addresses a page that is swapped out.  This
        can occur only if you are trying to do your own  virtual  address
        mapping, and used the expr<ESC>{<ESC>}nU command, where 0<=n<=2.


   % Not in .EXE format -- Data file assumed.

        A GET command  without  a  /DATA  switch  or  a  previous  ENABLE
        DATA-FILE  command  specified  a  file  which is not in .EXE file
        format.  FILDDT assumes it is a data file.


   ? NOT WRITABLE

        You tried to modify a word in a write-protected page.  To  enable
        writing on protected pages, use the <ESC>0W command.
|  
|  
|  ? Null filename illegal
|  
|       You did not enter a file specification to a command that requires
|       one.


   ? PAGE DOES NOT EXIST

        You tried to display a word in a nonexistent page.

   ? Patching is not enabled

        You attempted to modify (with FILDDT) a  file,  a  disk,  or  the
        monitor, but did not use the /PATCH switch or the ENABLE PATCHING
        command.


   % Patching the running monitor is illegal

        You entered an ENABLE PATCHING  command  and  then  gave  a  PEEK
        command.


   ? PEEK FAILED

        You tried to PEEK at the  monitor,  but  do  not  have  WHEEL  or
        OPERATOR privileges enabled.






                                    A-8
                               ERROR MESSAGES
                               ERROR MESSAGES


   % Symbols cannot be extracted from a data file

                                 ___ _____________
        You  used  the  command  GET filnam/SYMBOL.   Either   the   file
        specified  by  filnam is not an .EXE file, or you previously used
                    ______ _________
        the command ENABLE DATA-FILE.


   ? Symbols cannot be extracted from a data file

                              ____ ______
        You used the command  LOAD filnam,  and  the  file  specified  by
        filnam is not an .EXE file.


   U

        You entered a symbol that DDT cannot locate in any symbol  table.
        Cure  this  by  entering  the  correct symbol, or by defining the
                                ______________
        symbol with the command {expr<}symbol:.


   ? UNEXPECTED MOVEM FAILURE

        DDT could not deposit to  memory  even  though  the  page  exists
        exists and is write-enabled.

                                      NOTE

                                           ____
                In the following messages, unit  is  one  of  the
                following:

                 o  "Unit" (if you used the DRIVE command)

                 o  "Unit u on controller k on channel c" (if you
                    used the STRUCTURE command, where u, k, and c
                    are the arguments you entered)



     ____
   % Unit has a bad BAT block

        The unit that you specified in a DRIVE or STRUCTURE command has a
        bad BAT block.


     ____
   % Unit has a bad HOME block

        The unit that you specified in a DRIVE or STRUCTURE command has a
        bad HOME block.






                                    A-9
                               ERROR MESSAGES
                               ERROR MESSAGES


     ____
   ? Unit is off line

        The unit that you specified in a DRIVE or  STRUCTURE  command  is
        off line.


     ____
   % Unit is write locked

        You  used  an  ENABLE  PATCHING  command  and  then  specified  a
        write-locked unit in a DRIVE or STRUCTURE command.


   % Update of file's symbol table failed

        FILDDT was unable to write the modified symbol table back to  the
        file  after  you  gave  a <CTRL/Z> or <CTRL/E> command.  This may
        also occur when you use the n<ESC>5M command.





































                                    A-10













                                  GLOSSARY
                                  GLOSSARY



   bit

          Bit is a contraction of "binary digit".  A bit is the  smallest
          unit  of information in a binary system of notation.  It is the
          choice between two possible states, usually designated as  zero
          and  one.   Bits  of  data  are often used as flags to indicate
          on/off or yes/no conditions.

   breakpoint

          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.

   current display mode

          The current display mode is the mode in which DDT displays  the
          next  word (unless there is an intervening command that changes
          the current display mode).  Also known as the  current  typeout
          mode.

   current quantity

          The current quantity is the most recent of:

           o  the last 36-bit quantity that DDT displayed

           o  the 36-bit evaluation  of  the  last  expression  that  you
              entered as an argument to a command that deposits to memory

          This value is often used as the default argument for  the  next
          command.  Also known as the last value typed.







                                  Gloss-1
                                  GLOSSARY
                                  GLOSSARY


   current location

          The current location is a memory word that has been  referenced
          by an earlier DDT command.  The address of the current location
          is the default address for most DDT commands.

   current location stack entry

          The location that will become the current location as a  result
          of the next <ESC><RET> command.

   current radix

          The current radix is the radix in which  DDT  displays  numeric
          values.

   current typeout mode

          See current display mode.

   debugging

          Debugging is the process of finding  and  removing  programming
          errors from programs.

   EDDT

          EDDT is the DDT variant that is used  to  debug  executive-mode
          programs.

   FILDDT

          FILDDT is the DDT variant that is used to  examine  and  modify
          disk files and disk structures.  FILDDT is also used to examine
          (but not modify) the running monitor.

   jiffy

          A jiffy is a unit  of  time  defined  as  one  AC  (alternating
          current)  cycle.  If your line power has a frequency of 60 Hz.,
          a jiffy is one sixtieth of a second  (about  16  milliseconds).
          If  your  line  power has a frequency of 50 Hz., a jiffy is one
          fiftieth of a second (20 milliseconds).

   KDDT

          KDDT is the DDT variant used to debug the monitor.  You can set
          breakpoints,  single-step  instructions,  and perform any other
          DDT function.





                                  Gloss-2
                                  GLOSSARY
                                  GLOSSARY


   last value typed

          See current quantity.

   location

          A location is a numbered or named place in  storage  or  memory
          where  a  unit  of  data or an instruction can be stored.  This
                                     ____     ______ ____
          manual also uses the terms word and memory word.

   location counter

          The location counter is a memory word that contains the address
          of the current location.

   location sequence stack

          The location sequence stack is a stack in which DDT stores  the
          addresses  of  locations  used  earlier.  DDT uses the stack to
          access these locations  again  without  having  you  explicitly
          enter  the  address  of  each of the locations.  DDT references
          these addresses in a last-in, first-out manner.

   MDDT

          MDDT is the DDT mode used to  examine  and  patch  the  running
          monitor.

   open location

          The open location is a memory word that  you  can  modify  with
          your next DDT command.

   prevailing display mode

          The prevailing display mode is a user-defined  default  display
          mode.   DDT displays memory words in the prevailing mode unless
          you specify a temporary display  mode.   You  can  restore  the
          prevailing   mode  with  the  <RET>  command.   See  Chapter  4
          (Displaying and Modifying Memory) for a list of other  commands
          that restore the prevailing display mode.

   reset

          Reset refers to the zero condition of a bit  or  flag.   A  bit
                                                __ _____
          that  is  zero  is said to be reset.  To reset is the verb that
          refers to the act of turning the bit off, "clearing"  the  bit,
          or making it zero.






                                  Gloss-3
                                  GLOSSARY
                                  GLOSSARY


   set

          Set refers to the nonzero condition of a bit or  flag.   A  bit
                                                  __ ___
          that  is  nonzero  is  said to be set.  To set is the verb that
          refers to the act of turning the bit on, or making it nonzero.

   single-stepping

          Single-stepping   is   the   process   of   executing   program
          instructions  one  at a time using DDT, to verify the result of
          each instruction.

   target

          Target refers to the contents of the virtual address space that
          FILDDT  is  accessing.  The virtual address space may contain a
          disk structure, a disk file, or the running monitor.

   temporary display mode

          The temporary  display  mode  is  a  short-term,  user-selected
          display  mode  which  overrides  the  prevailing  display mode.
          Temporary display mode remains in effect until you enter  <RET>
          or <TAB>.  Also known as the temporary typeout mode.

   temporary typeout mode

          See temporary display mode.


























                                  Gloss-4
                                   INDEX



   $, 2-2                                  ], 4-9, 4-12, 4-13, 4-15,
   $$., 5-2, 5-3                               12-2
   $., 5-2, 5-3                            ^, 4-9
                                           Backslash, 4-9, 4-12, 4-13,
   ASCIZ strings, 4-19                         4-16, 12-2
   Automatic page-creation, 4-21           <BKSP>, 2-3, 4-9
   Automatic proceed                       <CTRL/U>, 2-3
     terminating, 5-8                      <CTRL/Z>, 2-3
   Automatic proceed flag, 5-7             deleting, 2-3
   Automatic write-enable, 4-20            Equal sign, 4-6
                                           <ESC>, 2-3, 4-12
   BACKSPACE key, 2-3                      <ESC>", 3-6
   <BKSP>, 2-3                             <ESC>"5, 3-7
   $0BPT, 5-10                             <ESC>"c<ESC>, 3-7
   Breakpoint block, 12-2                  <ESC>., 5-2
   Breakpoints, 2-5, 5-1                   <ESC>0<, 8-6
     conditional, 5-9                      <ESC>0T, 4-5
     DDT action at, 5-3                    <ESC>1:, 7-2
     display additional location at,       <ESC>1M, 4-23, 4-25
         5-4                               <ESC>1S, 4-5
     display address of, 5-6               <ESC>1W, 4-21
     executing command strings at,         <ESC>2M, 4-3
         5-5                               <ESC>3M, 4-3
     executing instructions at, 5-11       <ESC>4M, 12-3
     executing subroutines at, 5-13        <ESC>5M, 7-6
     inter-section, 12-3                   <ESC>5T, 4-5
     proceeding from, 5-3, 5-6             <ESC>6M, 12-5
     removing, 5-6                         <ESC>6T, 4-5
     setting, 5-3                          <ESC>7T, 4-5
     single-stepping at, 5-11              <ESC>:, 7-1, 7-2
     unsolicited, 5-10                     <ESC><, 8-1
   Byte pointers, 4-6                      <ESC><BKSP>, 4-11, 4-12
                                           <ESC><ESC>., 5-2
                                           <ESC><ESC>1:, 7-7
   Commands                                <ESC><ESC>1W, 4-21
     DDT                                   <ESC><ESC>1X, 5-15
       , 4-6                               <ESC><ESC>:, 7-6
       !, 4-9, 4-12, 4-13, 4-16,           <ESC><ESC><, 8-3
           4-17, 4-18, 12-2                <ESC><ESC>K, 7-3
       ", 3-5                              <ESC><ESC>nU, 11-2
       ""<""">, 3-6                        <ESC><ESC>P, 5-8
       ., 4-7                              <ESC><ESC>Q, 4-8
       /, 4-9, 4-12, 4-13, 4-14,           <ESC><ESC>U, 11-2
           12-2                            <ESC><ESC>W, 4-20
       ;, 4-6                              <ESC><ESC>X, 5-14, 5-16
       =, 4-6                              <ESC><LF>, 4-11
       ?, 4-19, 4-23, 5-14, 6-3, 7-5       <ESC><RET>, 4-11
       [, 4-9, 4-12, 4-13, 4-14,           <ESC>>, 8-2
           12-2                            <ESC>?, 2-3
       \, 4-9, 4-12, 4-13, 4-16,           <ESC>B, 5-6, 11-2
           12-2                            <ESC>C, 4-5


                                  Index-1
       <ESC>D, 7-3                     DDT variants, 1-2
       <ESC>E, 6-3                     Default section
       <ESC>F, 4-5                       floating, 12-6
       <ESC>G, 5-1, 5-6, 5-16, 11-1      permanent, 12-6
       <ESC>H, 4-5                     Display mode
       <ESC>I, 5-16                      C, 4-5
       <ESC>K, 7-3                       current, 4-3
       <ESC>M, 6-4                       F, 4-5
       <ESC>N, 6-2                       H, 4-5
       0<ESC>nB, 5-6                     O, 4-5
       <ESC>nB, 3-4, 5-3, 5-4, 5-6       prevailing, 4-2
       <ESC>nI, 3-4                      1S, 4-5
       <ESC>nM, 3-4                      S, 4-5
       <ESC>nT, 4-5                      symbolic, 4-1
       <ESC>nU, 3-4, 11-2                0T, 4-5
       <ESC>O, 4-5                       5T, 4-5
       <ESC>P, 5-6, 5-7, 5-16, 11-1      6T, 4-5
       <ESC>Q, 4-8                       temporary, 4-2
       <ESC>S, 4-5
       <ESC>U, 5-17, 11-2              EFIW, 4-13, 4-18
       <ESC>V, 4-23                    <ESC>, 2-2, 2-3
       <ESC>W (search), 6-1            ESCAPE key, 2-2, 2-3
       <ESC>W (write-protect), 4-20    Expression operators, 3-7
       <ESC>X, 5-11, 5-13, 5-16,       Expressions, 3-2
           11-1, 11-2                  Extended format indirect word,
       <ESC>Z, 4-19                        4-13
       Exclamation point, 4-9, 4-12,
           4-13, 4-16, 4-17, 4-18,     IFIW, 4-13, 4-18
           12-2                        Initializing memory, 4-19
       Left square bracket, 4-9,       Input
           4-12, 4-13, 4-14, 12-2        ASCII character, 3-6
       <LF>, 2-3, 4-9                    ASCII string, 3-5
       Period, 4-7                       decimal integer, 3-3
       <RET>, 2-3, 4-9, 4-10             floating point, 3-3
       Reverse slash, 4-9, 4-12,         halfwords, 3-10
           4-13, 4-16, 12-2              instructions, 3-10
       Right square bracket, 4-9,        long text string, 3-4
           4-12, 4-13, 4-15, 12-2        octal integer, 3-2
       Semicolon, 4-6                    RADIX50 word, 3-7
       Slash, 4-9, 4-12, 4-13, 4-14,     SIXBIT character, 3-7
           12-2                          SIXBIT string, 3-6
       <TAB>, 2-3, 4-9, 4-12, 4-13,      text, 3-4
           4-17, 12-2                    value returned by a command,
       Underscore, 4-6                       3-3
     <ESC>L (page access), 4-22        Input to DDT, 3-2
     FILDDT                            Instruction format indirect word,
       <CTRL/Z>, 9-9                       4-13
   CONTROL key, 2-3
   CPU type for FILDDT, 11-5           Last quantity typed, 4-8
   Current display mode, 4-3           <LF>, 2-3
   Current location, 2-4, 4-7          LINE FEED key, 2-3
   Current location stack entry, 4-8   Location counter, 2-4, 4-7
   Current quantity, 2-4, 4-8          Location sequence stack, 2-4, 4-8
    
   Dangerous instructions, 5-15        $4M, 12-3


                                  Index-2
   $5M, 7-6                              for address, 6-3
   $6M, 12-5                             for matching value, 6-1
   Mask                                  for non-matching value, 6-2
     output byte size, 4-3               terminate, 6-3
     search, 6-4                       Search mask, 6-4
     TTY control, 4-23                 Single-stepping, 5-11
   Maximum symbolic offset, 4-4        Symbol table
   Memory protection, 4-20               closing, 7-2
   Memory watch, 4-23                    finding name of, 7-2
                                         opening, 7-1
   $nB, 5-2                            Symbolic debugging, 1-1
                                       Symbols
   Open location, 2-4, 4-7               creating undefined, 7-4
   Operators                             defining new, 7-2
     in expressions, 3-7                 deleting, 7-3
   Output byte size mask, 4-3            listing specific, 7-5
                                         listing undefined, 7-5
   Page accessibility, 4-22              locating, 7-4
   Patch                                 multiply-defined, 7-1
     abort, 8-6                          reactivating typeout of, 7-3
     before instruction, 8-1             redefining old, 7-2
     following instruction, 8-3          suppressing typeout of, 7-3
     terminate, 8-2, 8-3               TAB key, 2-3
   Prevailing display mode, 4-2        <TAB>, 2-3
   Proceed count, 5-7                  Temporary display mode, 4-2
                                       TTY control mask, 4-23
   $$Q, 4-8
   $Q, 4-8                             Unsolicited breakpoint, 5-10
                                       User-program context, 5-16
   <RET>, 2-3
   RETURN key, 2-2, 2-3                Watching memory, 4-23
    
   Search                              Zeroing memory, 4-19
























                                  Index-3
