Microsoft KB Archive/104617

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Article ID: 104617

Article Last Modified on 6/1/2005



APPLIES TO

  • Microsoft Visual C++ 1.0 Professional Edition
  • Microsoft Visual C++ 1.5 Professional Edition
  • Microsoft Visual C++ 2.0 Professional Edition
  • Microsoft Visual C++ 4.0 Standard Edition
  • Microsoft Visual C++ 5.0 Enterprise Edition
  • Microsoft Visual C++ 6.0 Enterprise Edition
  • Microsoft Visual C++ 5.0 Professional Edition
  • Microsoft Visual C++ 6.0 Professional Edition
  • Microsoft Visual C++ 6.0 Standard Edition



This article was previously published under Q104617


SUMMARY

The sample code below demonstrates how to pass a short from a program that is written in Microsoft C to a procedure that is written with the Microsoft Macro Assembler (MASM). The MASM function also returns a short 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)
                

For 32-bit code, such as a Windows NT program, use the following conventions for returning data to a C program:

char                   AL
short                  AX
long, int, *           EAX
                

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

short MasmSub(short);

#ifdef __cplusplus
}
#endif

main ()
{
   short var = 1;
   printf ("%d\n", var);
   printf ("%d", MasmSub(var));
}
                

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, \ 
   sVar:WORD

   mov ax, sVar     ; Load the short into AX.
   add ax, 32766    ; Because the function returns a short (a 2-byte
   ret              ; value), C will get the return value from AX.
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, \ 
   sVar:WORD

   mov ax, sVar     ; Load the short into AX.
   add ax, 32766    ; Because the function returns a short (a 2-byte
   ret              ; value), C will get the return value from AX.
MasmSub ENDP
END
                


The following is the output of the program:

  1
  32767
                


Additional query words: mixed language

Keywords: kbhowtomaster kblangc kbcode KB104617