HOWTO: Pass Common C Types to MASM by Reference

• Microsoft C for MS-DOS, versions 5.1, 5.1a, 6.0, 6.0a, 6.0ax
• Microsoft C for OS/2, versions 5.1, 6.0, 6.0a
• Microsoft C/C++ for MS-DOS, version 7.0
• Microsoft Visual C++ for Windows, versions 1.0, 1.5
• Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 4.0, 5.0

SUMMARY

The sample code below demonstrates how to pass variables of type char, short, and long from a Microsoft C program to a Microsoft Macro Assembler (MASM) program by reference.

MORE INFORMATION

The samples below include one C file and three different assembly files. The three assembly files demonstrate how to pass variables by reference in small and large model for MS-DOS and in flat model for Windows NT. Link only the appropriate assembly module to the C module.

Note that MASM 6.1 or later and the C/C++ version 8.0 32-bit compiler that ships with Visual C++, 32-bit Edition, are required to build the flat model Windows NT version.

Sample Code

   // Filename: CMAIN.C
   // Compile options needed: /c (and /AL for large model)

   #include <stdio.h>
   extern void  MasmSub (char *, short *, long *);

   char charvar = 'a';
   short shortvar = 1;
   long longvar = 32768L;

   void main (void)
   {
      printf ("%c %d %ld\n", charvar, shortvar, longvar);
      MasmSub (&charvar, &shortvar, &longvar);
      printf ("%c %d %ld", charvar, shortvar, longvar);
   }

Sample Code for MS-DOS Small Model Version

   ; Filename: MASMSUB.ASM
   ; Assemble options needed for MASM: /MX
   ; Assemble options needed for ML: /c /Cx

   .MODEL small, C      ;.MODEL ... C tells the assembler that
   .286                ; parameters are pushed from right to left.
   .CODE

   MasmSub PROC uses si, \
      pchar:PTR, \
      pshort:PTR, \
      plong:PTR

      mov si, pchar      ;Load SI with the address of the char variable.
      mov BYTE PTR [si], "z"
      mov si, pshort      ;Load SI with the address of the short variable.
      add WORD PTR [si], 9
      mov si, plong      ;Load SI with the address of the long variable.
      add WORD PTR [si], 1  ;Increment the low word of the long variable
                            ;by 1.
      adc WORD PTR [si+2],0 ;Increment the high word.
      ret
   MasmSub ENDP
   END

Sample Code for MS-DOS Large Model Version

   ; Filename: MASMSUB.ASM
   ; Assemble options needed for MASM: /MX
   ; Assemble options needed for ML: /c /Cx

   .MODEL large, C      ;.MODEL ... C tells the assembler that
   .286                ; parameters are pushed from right to left.
   .CODE

   MasmSub PROC uses es si, \
      pchar:PTR, \
      pshort:PTR, \
      plong:PTR

      les si, pchar      ;Load ES:SI with the address of the char variable.
      mov BYTE PTR es:[si], "z"
      les si, pshort      ;Load ES:SI with the address of the short
                          ;variable.
      add WORD PTR es:[si], 9
      les si, plong      ;Load ES:SI with the address of the long variable.
      add WORD PTR [si], 1  ;Increment the low word of the long variable
                            ;by 1.
      adc WORD PTR [si+2],0 ;Increment the high word.
      ret
   MasmSub ENDP
   END

Sample Code for Windows NT Flat Model Version

   ; Filename: MASMSUB.ASM
   ; Assemble options needed for ML: /c /Cx /coff

   .386                ;.MODEL ... C tells the assembler that
   .MODEL flat, C      ; parameters are pushed from right to left.
   .CODE

   MasmSub PROC uses esi, \
      pchar:PTR, \
      pshort:PTR, \
      plong:PTR

      mov esi, pchar      ;Load ESI with the address of the char variable.
      mov BYTE PTR [esi], "z"
      mov esi, pshort      ;Load ESI with the address of the short
                           ;variable.
      add WORD PTR [esi], 9
      mov esi, plong      ;Load ESI with the address of the long variable.
      inc DWORD PTR [esi]      ;Increment the long variable by 1.
      ret
   MasmSub ENDP
   END

  a 1 32768
  z 10 32769