Passing a long* from a C Module to an Assembly Module

• Microsoft Macro Assembler for MS-DOS, versions 5.1, 5.1a, 6.0, 6.0a, and 6.0b

SUMMARY

The following code demonstrates passing a LONG INT (4 bytes) via a far pointer (4 bytes) to an assembly routine that accesses the LONG integer.

Each integer is incremented in the assembly routine and its new value returned to the calling C program.

MORE INFORMATION

Sample Code

/* Compile options needed: /AL
*/

#include <stdio.h>
#include <process.h>
#include <conio.h>

void main()

   long *n1,*n2;               /* 4 byte pointers  */

   long int t1 = 9999999L;     /* 4 byte variables */
   long int t2 = 2256789L;

   n1 = &t1;
   n2 = &t2;

   /* initial values */
   printf( "The values are  %ld and  %ld\n ", *n1, *n2 );

   printf( "Incrementing values...\n" );
   IncLongs( n1, n2 );

   /* values returned by the assembly routine */
   printf( "The values are  %ld and  %ld\n ", *n1, *n2 );

; Assemble options needed: none

         .model large, c
         .data
         .code

         PUSH es                 ; save registers
         PUSH si
         LES si,arg1             ; load in es:si the seg:offset of n1
         INC word ptr es:[si]
         JNC doarg2
         INC word ptr es:[si+2]  ; if carry, increment high word of n1

         LES si,arg2             ; load in es:si the seg:offset of n2
         INC word ptr es:[si]
         JNC finis
         INC word ptr es:[si+2]  ; if carry, increment high word of n2

         POP si                  ; restore registers
         POP es
         MOV sp,bp
         RET

         END