BNL787_defined_locations

 It soon became clear that a number of defined  locations  in
 data  memory  would be highly desirable, so that all SSPs in
 the system would have  various  parameters  at  known  fixed
 locations  in  memory.   Locations  DM200 to DM3FC have been
 reserved for these parameters.  Thus the user code starts at
 DM400 in the BNL787 system.  The three files described below
 contain these fixed locations.


       o  The file 787SSP:|BNL787.INC gives  a  list  of  the
          addresses  in  DM  of  the defined locations in the
          form of a list of parameters.  This FORTRAN file is
          to be included in FORTRAN packages that need access
          to the defined locations.  Note that the values  of
          the parameters in BNL787.INC are the byte addresses
          in DM.

       o  787SSP:|BNL787_DEF.MAR defines the  BNL787  defined
          locations as symbols in a similar way to BNL787.INC
          in the FORTRAN case.

       o  The file  787SSP:|BNL787_RES.MAR  contains  exactly
          the  same  symbols as BNL787_DEF.MAR, but this file
          actually reserves the space in the SSP data memory.


 Note  that  all  the   names   of   defined   locations   in
 BNL787_DEF.MAR begin with the symbol "$", in common with the
 BNLSYS "system" symbols.

 If there are a lot of symbols to be resolved at  link  time,
 the   code   produced   in  the  listing  will  bear  little
 resemblance to that actually loaded  into  the  SSP,  making
 debugging  tricky.  The command "DCROSS" cross-assembles the
 source code, using symbols from  BNL787_DEF.MAR  to  resolve
 globals,  thus  making  the  code in the list file easier to
 read.

 Version 4.00 of the BNL cross-assembler includes support for
 resolving global symbols at link time, therefore the command
 "CROSS" may be used instead, if preferred.  By default,  the
 command "CROSS" does not produce a listing.

 Also located in these files are four symbols defined  as  MS
 code symbols (see MS_CODE_CONVENTIONS).

 The  BNL787  fixed  locations   are   described   fully   in
 787DOC:|787_FIXED_LOCATIONS.MEM.




 Marker_blocks

 Marker (or common) blocks can be used to pass blocks of data
 either to or from the host program.

 It is possible to  set  up  marker  blocks  of  data  in  an
 SSP-resident program that can easily be accessed by the host
 program.  To do this, the control statement MARKER is  used.
 The BNL787 convention is to set the first word of the marker


 block to contain the length of the following data  in  words
 (i.e.  the exclusive length).

 An example of a marker block in an SSP resident  program  is
 given below:

 NLEN:   DC      5       ; length of marker block following
 NGO:    DS      5       ; reserve 5 words of storage

 MARKER  NLEN,<Testblock for access by host> 

 This would be equivalent to a COMMON block in FORTRAN (say)
         COMMON TESTBLOCK/NLEN,NGO(5)/ 

 The locations of MARKER blocks are accessible to the host by
 a   call   to   the   appropriate  FORTRAN  subroutine  (see
 BNLSSP:|SSP_PROGRAMMER.MEM).

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