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      A8DDT.RNO Version 1


      1.0 SUMMARY


      A8DDT is a TECO macro which allows use of basic File DDT functions
      on  A8  format  files,  particularly  the  DX10  microcode.   This
      provides a means to inspect and patch the A8 file prior to loading
      it  into  the  PDP-8.  The code operates only in octal type in/out
      mode and is intended for use by  experienced  systems  programmers
      and DEC service personnel.



      2.0 INITIALIZING A8DDT


      A8DDT IS A TECO macro which is started in the following manner:

        .R TECO
        *ERA8DDT.TEC$YAHXAMA$$

      at  this  point   control   will   be   transferred   to   A8DDT's
      initialization code which will prompt with

        FILE:

      Answer this prompt with the full file spec of the A8 file you wish
      to  use.  A8DDT will accept any file spec construct which would be
      accepted  by  TECO  including  SFD's.   The  file  spec  must   be
      terminated with a carriage return.  While accepting your file spec
      A8DDT allows use of <DEL>, ^U, and ^R for editing.  When your file
      spec has been accepted A8DDT will respond with

        A8DDT <cr><lf>

      to let you know it is ready to accept commands.



      3.0 A8DDT COMMANDS


      A8DDT is a subset of DDT which operates exclusively in octal  type
      in/out mode.  The following commands are accepted:

      n/        open location n (up to 4 digits) and display its
                contents
      ./        display the contents of the last opened location
      <cr>      close the location and store any numeric supplied (up to
                4 digits) as the new contents
      <lf>      perform <cr> processing and then open the next
                consecutive location
      <del>     delete the last numeric argument typed

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      ?         enter/exit debug (this is used to activate TECO's ? mode
                mode for tracing the macro's operation)
      ^Z        exit the program (A8DDT will close the edited file and
                produce a backup file)



      4.0 INTERNALS INFORMATION


      The following information is not necessary for use  of  A8DDT  but
      may  be  helpful  to  those  wishing a better understanding of the
      program and its operational peculiarities.



      4.1 A8 FILE FORMAT


      A8 files are produced by the  CONVRT  program  (diagnostic  DDQDG)
      from  BIN  files  output by PAL10.  The files are in the following
      format:

      8 wc,adr,12,12,12, . . . . .12,cks
      8 wc,adr,12,12,12, . . . . .12,cks
      8 0,adr,cks
      where:
      8         pdp-8 file identifier
                space,sifnifies asciized file
      wc        word count
      adr       load address
      12        up to 2 characters for each 12-bit data word
      cks       file line checksum
      <cr><lf> indicates end of line
      As many line of this type as required to load the PDP-8  file  are
      in the file followed by the EOF indicator:

      8         PDP-8 identifier
                space,signifies asciized file
      0         word count of 0 signifies transfer block
      adr       PDP-8 start address
      cks       file line checksum
      <cr><lf> indicates end of line

      The file line checksum is the 12-bit negated checksum.   The  word
      count,  the  load  address,  the data, and the line checksum added
      together produces a 12-bit zero sum.



      4.2 SPECIAL CONSIDERATIONS


      A8DDT does not support the A8 "octal  file  format"  option.   The
      decision  to  impose  this  restriction  was based on two factors.

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      First, no currently available or planned  software  produces  this
      format.   Second, this format was intended to allow patch overlays
      which  could  not  be  handled  by  A8DDT  without   significantly
      degrading response.


      A8DDT  was  designed  to  be  easily  modified  to  handle   other
      "asciized"   formats.   To  do  this  the  references  to  the  A8
      identifier "8" would have to  be  changed.   These  occur  at  tag
      EXAMINE-T+1  line where <cr><lf>8 is searched, at tag CRLF+2 lines
      where 8<space>, is searched, and at tag CRLF+3 lines where a blank
      entry  is created.  All occurances of ^O7777 would also have to be
      changed to mask the word length for the corresponding processor.



      4.3 Q-REGISTER USEAGE


      The following defines the standard Q-register use within A8DDT:

      A         Code. Holds the A8DDT macro itself.
      C         Data. Used to pass arguments to subroutines.
                Usually contains the contents of currently open
                location.
      D         Code. Subroutine to convert numeric in Q-register C to
                A8 format and store at the current pointer position.
      E         Code. Subroutine to type the contents of Q-register 1.
      F         Flag. Indicates a location is currently opened.
      G         Flag. Indicates a numeric argument was the last thing
                typed.
      I         Code. Subroutine to convert the asciized data at the
                current pointer position to binary and return result in
                Q-register 1.
      T         Data. Address of the currently open location.
      X,Y       Data. Hold start address and length of PDP-8 data
                defined within currently used line in A8 file.
      1,2       Data. Temporary use for assembling numeric arguments and
                passing data to/from subroutines.
      7         Data. Holds last character typed for parser.
      [end]