Article ID: 104645
Article Last Modified on 7/5/2005
APPLIES TO
- Microsoft C Professional Development System 6.0a
- Microsoft C Professional Development System 6.0a
- 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 Q104645
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)
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
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
The following is the output of the program:
98304 + 147456 = 245760
Additional query words: mixed language
Keywords: kbinfo kblangc KB104645
