Q29113: Example of Passing C Strings to Assembly Language

Article: Q29113
Product(s): See article
Version(s): 5.00 5.10 6.00 6.00a | 5.10 6.00 6.00a
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | h_masm | mspl13_c
Last Modified: 5-FEB-1991

The following code is an example program that passes a string to an
assembly language routine. The routine will access the string, change
one character, and return.

Sample Code
-----------

/*  C program - compile with defaults (small model) */

#include <string.h>
#include <stdio.h>

void main(void);             /* Prototype for main */
void extern changit(char *); /* Assembly routine declaration*/
char s[10];                  /* String to be changed */

void main()
{
        strcpy(s,"xxxxxxxxx"); /* Initialize string to x's */
        printf("s = %s\n",s);
        changit(s);            /* Call assembly routine */
        printf("s = %s\n",s);  /* See if string changed */
}

=============================================================

;Assembly language routine -- assemble with /Mx
;

.MODEL SMALL
.CODE
        PUBLIC  _changit
_changit PROC
        push    bp                  ;Entry sequence
        mov     bp,sp

        mov     bx,[bp+4]           ;Put array address in bx
        mov     [bx+8],byte ptr 'Y' ;Change the 9th character
                                    ;0 is the first, 1 is second...

        pop     bp                  ; Exit sequence
        ret

_changit ENDP
        END

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