Registers_-

 A register may be used to pass variables between subroutines
 as  shown  in the following example.  The address of a block
 of data (BLOCK) is loaded  into  register  9  prior  to  the
 subroutine   call.    By   putting  $RSAVL=8  prior  to  the
 Subroutine statement, register  9  would  not  be  saved  on
 entering the subroutine, and its contents can be accessed by
 the subroutine.


         SUBROUTINE TEST_IT
 DM                      ; Data memory
 BLOCK:  DS      6       ; Block of data
 PM                      ; Program memory
         LA      9,BLOCK ; Load R9 with address of block of data
         CALL    ZERO_BLOCK  
         RETURN
         END
 $RSAV0=0
 $RSAVL=8               ; save registers 0-8
         SUBROUTINE ZERO_BLOCK
 PM                           ; Program memory 
         LA      12,U_ZERO(,) ; load address of zero block
         NOP                  ; avoid index register contention
         NOP
         LM      0,5,0(12)    ; load some zeroes into registers 0-5
         STM     0,5,0(9)     ; store contents of registers 0-5 in BLOCK
         RETURN
         END




 Globals_-

 The BNL787 fixed locations (defined as global  symbols)  are
 accessible  to  all subroutines.  However, users may wish to
 pass symbols that are not  BNL787  fixed  locations  between
 subroutines.   Since  the BNL787 fixed locations do not take
 up the whole of the first 4K bytes of DM, a  file  has  been
 created  to  reserve  space  for any symbols that a user may
 need.  This file MUST be  named  USER_RES.MAR.   An  example
 file is found in the directory 787EXAMPLES:.  This file must
 be present in the same directory as the files  being  linked
 with  the  DLINK  command,  but  may  be only a dummy if not
 needed.  It is up to  the  user  to  ensure  that  the  user
 reserved  locations  do  not  exceed  the  first  4K of data
 memory.

 The USER_RES.MAR may also contain marker blocks (see  above)
 if  several  subroutines  need access to the data stored.  A


 convention has been established in the  BNL787  USER_RES.MAR
 that  the  symbols  should be preceded by "U_" .  This makes
 the  identifications  of  USER_RES  symbols  in  subroutines
 obvious.   Thus  a programmer will be aware that all symbols
 beginning with "$" (see  BNL787_DEFINED_LOCATIONS)  or  "U_"
 should  be followed by "(,)" to denote that no base register
 is to be used.

 A feature of the BNL standard cross-assembler is that  these
 "absolute" symbols cannot be used as variables of a DO loop.
 Thus the statement

      DO 5,U_START(,),U_STOP(,)

 will be  rejected  by  the  cross-assembler.   This  can  be
 corrected  by  loading the values of U_START and U_STOP into
 registers or local variables.



 Argument_list_-

 A new feature introduced  in  BNLSYS  version  4.00  is  the
 possibility  of  passing variables between subroutines using
 an argument list.  This is really a special case of  passing
 variables  via registers, as described above.  In this case,
 a  list  of  arguments  follows  the  Call  statement.   The
 arguments  are automatically stored in consecutive locations
 in DM, and register 1 is used to point to the address of the
 first variable (see BNLSSP:|SSP_PROGRAMMER.MEM).

 Note that if an argument list is used with a Call statement,
 the  contents of register 1 is overwritten.  If no arguments
 are used, register 1 is not changed, and can be used to pass
 information  to the subroutine - thus BNLSYS version 4.00 is
 downwards compatible.

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