INFO: Passing C Arrays 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 C arrays of type char, short, and long to a Microsoft Macro Assembler (MASM) procedure 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 model 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++ 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
   #include <stdio.h>

   #ifdef __cplusplus
   extern "C" {
   #endif

   void MasmSub (char *, short *, long *);

   #ifdef __cplusplus
   }
   #endif

   char chararray[4] = "abc";
   short shortarray[3]  = {1, 2, 3};
   long longarray[3] = {32768, 32769, 32770};

   main ()
   {
      printf ("%s\n", chararray);
      printf ("%d %d %d\n", shortarray[0], shortarray[1], shortarray[2]);
      printf ("%ld %ld %ld\n", longarray[0], longarray[1], longarray[2]);
      MasmSub (chararray, shortarray, longarray);
      printf ("%s\n", chararray);
      printf ("%d %d %d\n", shortarray[0], shortarray[1], shortarray[2]);
      printf ("%ld %ld %ld", longarray[0], longarray[1], longarray[2]);
   }

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
   .286
   .CODE

   MasmSub PROC uses si, \
      arraychar:PTR, \
      arrayshort:PTR, \
      arraylong:PTR

      mov si, arraychar  ; Load SI with the address of the char array.
      mov BYTE PTR [si], "x"       ; Since a char is 1 byte long, each
      mov BYTE PTR [si+1], "y"     ; successive element can be accessed
      mov BYTE PTR [si+2], "z"     ; by adding 1 more to si.
      mov si, arrayshort ; Load SI with the address of the short array.
      add WORD PTR [si], 7         ; Since a short is 2 bytes long, each
      add WORD PTR [si+2], 7       ; successive element can be accessed
      add WORD PTR [si+4], 7       ; by adding 2 more to si.
      mov si, arraylong  ; Load SI with the address of the long array.
      add WORD PTR [si], 1         ; Since a long is 4 bytes long, each
      adc WORD PTR [si+2], 0       ; successive element in the array
      add WORD PTR [si+4], 1       ; can be accessed by adding 4 more
      adc WORD PTR [si+6], 0       ; to si (or 4 more to si+2 to access
      add WORD PTR [si+8], 1       ; the high word of each element).
      adc WORD PTR [si+10], 0
      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
   .286
   .CODE

   MasmSub PROC uses es si, \
      arraychar:PTR, \
      arrayshort:PTR, \
      arraylong:PTR

      les si, arraychar  ; Load ES:SI with the address of the char array.
      mov BYTE PTR es:[si], "x"    ; Since a char is 1 byte long, each
      mov BYTE PTR es:[si+1], "y"  ; successive element can be accessed
      mov BYTE PTR es:[si+2], "z"  ; by adding 1 more to si.
      les si, arrayshort ; Load ES:SI with the address of the short array.
      add WORD PTR es:[si], 7      ; Since a short is 2 bytes long, each
      add WORD PTR es:[si+2], 7    ; successive element can be accessed
      add WORD PTR es:[si+4], 7    ; by adding 2 more to si.
      les si, arraylong  ; Load ES:SI with the address of the long array.
      add WORD PTR es:[si], 1      ; Since a long is 4 bytes long, each
      adc WORD PTR es:[si+2], 0    ; successive element in the array
      add WORD PTR es:[si+4], 1    ; can be accessed by adding 4 more
      adc WORD PTR es:[si+6], 0    ; to si (or 4 more to si+2 to access
      add WORD PTR es:[si+8], 1    ; the high word of each element).
      adc WORD PTR es:[si+10], 0
      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 flat, C
   .CODE

   MasmSub PROC uses esi, \
      arraychar:PTR, \
      arrayshort:PTR, \
      arraylong:PTR

      mov esi, arraychar ; Load ESI with the address of the char array.
      mov BYTE PTR [esi], "x"      ; Since a char is 1 byte long, each
      mov BYTE PTR [esi+1], "y"    ; successive element can be accessed
      mov BYTE PTR [esi+2], "z"    ; by adding 1 more to esi.
      mov esi, arrayshort; Load ESI with the address of the short array.
      add WORD PTR [esi], 7        ; Since a short is 2 bytes long, each
      add WORD PTR [esi+2], 7      ; successive element can be accessed
      add WORD PTR [esi+4], 7      ; by adding 2 more to esi.
      mov esi, arraylong ; Load ESI with the address of the long array.
      inc DWORD PTR [esi]          ; Since a long is 4 bytes long, each
      inc DWORD PTR [esi+4]        ; successive element can be accessed
      inc DWORD PTR [esi+8]        ; by adding 4 more to esi.
      ret
   MasmSub ENDP
   END

  abc
  1 2 3
  32768 32769 32770
  xyz
  8 9 10
  32769 32770 32771