INFO: Passing a long from C to MASM by Value & Returning a long

• 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 a long from a program written in Microsoft C to a procedure written with the Microsoft Macro Assembler (MASM). The MASM function also returns a long to the C program.

Registers are used to return values of simple data types. For 16-bit code, such as an MS-DOS program, use the following conventions for returning data to a C program:

   char                   AL
   short, int, near *     AX
   long, far *            DX:  High order portion (segment)
                          AX:  Low order portion (offset)

   char                   AL
   short                  AX
   long, int, *           EAX

MORE INFORMATION

The samples below include one C file and two different assembly files. The two assembly files demonstrate how to pass a variable in small 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

   long MasmSub (long, long);

   #ifdef __cplusplus
   }
   #endif

   main ()
      {
      long var1 = 98304, var2 = 147456;
      printf ("%ld + %ld = %ld", var1, var2, MasmSub (var1, var2));
      }

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, \
      lVar1:DWORD, \
      lVar2:DWORD

      mov ax, WORD PTR lVar1       ; Load the first long into DX:AX.
      mov dx, WORD PTR lVar1+2
      add ax, WORD PTR lVar2       ; Add the second long to DX:AX
      adc dx, WORD PTR lVar2+2
      ret              ; Because the function returns a long (a 4-byte
   MasmSub ENDP        ; value), C will get the return value from DX:AX.
   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, \
      lVar1:DWORD, \
      lVar2:DWORD

      mov eax, lVar1   ; Load the first long into EAX.
      add eax, lVar2   ; Add the second long to EAX.
      ret              ; Because the function returns a long (a 4-byte
   MasmSub ENDP        ; value), C will get the return value from EAX.
   END

   98304 + 147456 = 245760