Using assembly routines with C on DOS

I've been playing with DOS real mode assembly for a while and now I want to utilize some routines in a C program. I'm using Turbo C 2.01 and TASM 3.0. I'm however unable to modif开发者_运维知识库y a variable passed by address, see the _setval routine below. I don't need/want inline assembly. A simple example:

foo.c

#include <stdio.h>
extern void setval(int *x, int *y);
extern int sum(int x, int y);
int main()
{
    int result, a, b;
    result = a = b = 0;
    setval(&a, &b);
    result = a + b;
    printf("a+b=%i, a=%i, b=%i\n", result, a, b);
    result = 0;
    a = 42;
    b = 19;
    result = sum(a, b);
    printf("a+b=%i, a=%i, b=%i\n", result, a, b);
    return 0;
}

foortn.asm

public _setval
public _sum
.model small
.stack
.data
.code
_setval proc near
push bp
mov bp, sp
mov word ptr [bp+4], 42
mov word ptr [bp+6], 19
pop bp
ret
endp
_sum proc near
push bp
mov bp, sp
mov ax, word ptr [bp+4]
add ax, word ptr [bp+6]
pop bp
ret
endp
end

I compile it like this:

tcc -c -ms foo.c
tasm /ml foortn.asm
tcc foo.obj foortn.obj

The result is:

a+b=0, a=0, b=0
a+b=61, a=42, b=19

I'm obviously missing something, but what?

Hans, Mark and Bill, thank you very much for your prompt and helpful responses.

Your current code is overwriting the passed pointer. You need to retrieve the pointer and write through it. Something like this:

mov ax, word ptr [bp+4]
mov word ptr [ax], 42

Write this code in C first and look at the assembly code that it generates to get this right.

Try replacing:

mov word ptr [bp+4], 42
mov word ptr [bp+6], 19

with

mov bx, word ptr [bp+4]
mov [bx], 42
mov bx, word ptr [bp+6]
mov [bx], 19

This:

mov word ptr [bp+4], 42
mov word ptr [bp+6], 19

is writing to the stack, not the addresses on the stack. You'll need to read the addresses on the stack, then write to them instead:

mov bx,[bp+4]  ; get the address of (a)
mov [bx],42    ; Write to that address
mov bx,[bp+6]  ; (b)
mov [bx],19    ; write

I don't know assembler ... but C passes everything by value.

In sum [bp+4] is 42 (or 19); in addsetval [bp+4] is 0xDEADBEEF 0xDECAFBAD (or whatever)