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DDT v43(627) - Warnings and Hints


This version of DDT contains all development work  currently
planned.   Following  is  a  list  of  hints,  aspects,  and
hitherto undocumented features, and a  brief  discussion  of
useful information in DDT's Program Data Vector.

HINTS/ASPECTS:

     1.  DDT v43 is not officially supported on TOPS-20  5.1
         or  4.1 systems.  However, user DDTs may be used on
         a 5.1 system  which  has  installed  the  following
         edit:

         Autopatch tape #8 includes  edit  3122  to  TOPS-20
         which  modifies the PDVOP% JSYS.  This edit changes
         the monitor's interpretation of the .PVNAM word  of
         the  Program  Data  Vector,  to agree with LINK and
         DDT.  In order to use DDT  v43  successfully  on  a
         TOPS-20 5.1 system, you must either install tape #8
         or install the following patch:

         $get system:monitr
         $start 140
         DDT
         ponam+6/   TLNN T1,777777   tlne t1,400000
         ^Z
         $save system:monitr
          SYSTEM:MONITR.EXE.2 Saved
         $

     2.  Monitor symbols from MONSYM and MACSYM can be  made
         available from XDDT by typing the command
                        $$:/DDT%/
         which causes DDT to select its own symbol table for
         use.   This  is  equivalent to running SDDT in v42.
         Note that this symbol table may be selected at  any
         time,  even  when  debugging  another program;  the
         program's symbol table may  be  restored  with  the
         command 0$5M.

     3.  Users of VT200-series terminals will note that  the
         grave  character (`) is equivalent to the escape in
         DDT commands.  This change does not affect  use  of
         the grave in character strings.

                                                      Page 2


DDT'S PROGRAM DATA VECTOR

XDDT has a Program Data Vector  associated  with  it,  named
"DDT%".   This  PDV has several useful pieces of information
in it:

      o  A symbol table vector, pointing  to  symbol  tables
         loaded from MONSYM and MACSYM.

      o  An  "exported   information"   vector,   containing
         pointers to useful DDT internal locations.

As noted above, DDT's symbol table may be selected with  the
command  $$:/DDT%/.   This  causes symbols defined in MONSYM
and MACSYM to be available for use.

The "exported information" vector  is  pointed  to  by  word
.PVEXP  in  the  PDV.   This  is  a  counted vector with the
following format:

DDTEXP: EXP     EXP.L           ;LENGTH OF BLOCK
        IFIW    DDT             ;DDT'S START ADDRESS
        IFIW    $0BPT           ;UNSOLICITED BREAKPOINT ADDRESS
        IFIW    $5M             ;SYMTAB LOCATION INFO
        IFIW    SECUDD          ;SECTION OF UDDT STUB

        EXP.L==.-DDTEXP

This vector may be used, for example, to enter  DDT  at  the
unsolicited  breakpoint.  Assuming that your program and DDT
are both in non-zero sections,  and  that  T1  contains  the
address  of  DDT's PDV, the following code sequence could be
used:

        XMOVEI  T2,@.PVEXP(T1)  ;GET ADDRESS OF EXPORT VECTOR
        JSR     @2(T2)          ;ENTER AT UNSOLICITED BREAKPOINT

DDT's exported information vector is  reserved  to  Digital.
Customers  wishing  to  make  use  of  DDT's  PDV  for other
purposes should use word .PVCST of the PDV.