DOS: TASM and C

c_asm.md

How to combine Assembly and C in DOS

This gist is showing an example of C program calling routine written in assembly.

Some theory

• C is case sensitive so it is necessary to use /ml switch in TASM
• C compiler uses leading underscore (_) character for function names
• parameters are passed to function in stack and integer return value is passed back in AX register
• stack cleaning is done by caller

Example:

extern int min(int p1, int p2)

SP + 4	p2
SP + 2	p1
SP:	offset of return value

in case of FAR calling:

SP + 6	p2
SP + 4	p1
SP + 2	segment of return value
SP:	offset of return value

Example using TASM and TC

We will implement two functions (max and swap) in assembler. Max function will accept two integers (a and b) and returns maximal value. Swap function accepts two pointers to two integers and swap their values.

;LIB.ASM

.MODEL SMALL
.CODE

_max PROC                ;return value is in AX
        PUBLIC _max
        PUSH BP
        MOV BP,SP
        MOV AX,[BP+4]    ;AX=a
        CMP AX,[BP+6]    ;a=b?
        JGE end          ;jump if a>=b
        MOV AX,[BP+6]    ;else AX=b
end:
        POP BP
        RET
_max ENDP


_swap PROC
        PUBLIC _swap
        PUSH BP
        MOV BP,SP
        MOV SI,[BP+4]    ;SI=adresa a
        MOV DI,[BP+6]    ;DI=adresa b
        MOV AX,[SI]      ;AX=a
        MOV BX,[DI]      ;BX=b
        MOV [SI],BX      ;a=b
        MOV [DI],AX      ;b=a
        POP BP
        RET
_swap ENDP

ENDS
END

C program is showing how to use functions described above.

/*
*       main.c
*/

#include <stdio.h>

extern int max(int a, int b);
extern void swap(int *a, int *b);

void main() {
        int a, b;

        printf("\nInsert two integers\n");
        printf("A=");
        scanf("%d", &a);
        printf("B=");
        scanf("%d", &b);

        printf("maximum=%d\n", max(a,b));

        swap(&a, &b);
        printf("a=%d, b=%d\n", a, b);
}

Finally we will compile them and link all together.

tasm lib.asm /ml
tcc -c main.c
tlink c:\tc\lib\c0s main lib,app,,c:\tc\lib\cs