Accessing Global Symbols Defined in C from MASM

• Microsoft Macro Assembler for MS-DOS, versions 6.0, 6.0a, 6.0b, 6.1, 6.1a, and 6.11
• Microsoft Macro Assembler for OS/2, versions 6.0, 6.0a, and 6.0b

SUMMARY

The code example below demonstrates accessing the following variable types from an assembly language program when the data is defined in a C module:

   a global variable
   a global variable through a far pointer
   a global structure
   a global pointer to a global array of structures.

MORE INFORMATION

None of the global variables are specified as arguments in the call to the assembly language procedure.

The .FARDATA directive instructs the assembler to place initialized variables into the FAR_DATA segment. The .FARDATA? directive instructs the assembler to place unitialized variables into the FAR_BSS data segment. For more information on creating data segments, please refer to pages 44 and 45 of the Microsoft Macro Assembler "Programmer's Guide" for versions 6.0x and pages 39 and 40 of the Microsoft Macro Assembler "Programmer's Guide" for versions 6.1x.

Sample Code

/*
 * Compiler options needed: /AL
 */

#include <stdio.h>

int func1(void);       /* assembly language procedure */
int var1 = 1;
int var2 = 3;
int *ptr2;

   int var3;
   int var4;

void main(void)

   ptr2 = &var2;                /* pointer to variable             */
   struct_ptr = v_ray;          /* pointer to array of structures  */

   printf("After the call to func1(), all values should be doubled.\n");

   printf("\nBefore...\n\n");
   printf("VAR1 is %d\n", var1);
   printf("VAR2 is %d\n", var2);
   printf("struct_var.var3 = %2d,\tstruct_var.var4 = %2d\n",
      struct_var.var3, struct_var.var4);
   printf("v_ray[0].var3   = %2d,\tv_ray[0].var4   = %2d\n",
      struct_ptr[0].var3, struct_ptr[0].var4);
   printf("v_ray[1].var3   = %2d,\tv_ray[1].var4   = %2d\n",
      struct_ptr[1].var3, struct_ptr[1].var4);

   printf("\nCalling func1()...\n");

   func1();                     /* call to assembly procedure */

   printf("\nAfter...\n\n");
   printf("VAR1 is %d\n", var1);
   printf("VAR2 is %d\n", var2);
   printf("struct_var.var3 = %2d,\tstruct_var.var4 = %2d\n",
      struct_var.var3, struct_var.var4);
   printf("v_ray[0].var3   = %2d,\tv_ray[0].var4   = %2d\n",
      struct_ptr[0].var3, struct_ptr[0].var4);
   printf("v_ray[1].var3   = %2d,\tv_ray[1].var4   = %2d\n",
      struct_ptr[1].var3, struct_ptr[1].var4);

;===================================================================== ; Assembly module with procedure "func1" called from C module above. ; Assembler options needed: none

info struct                 ; template for structure
   variable_3 DW ?          ;  member offsets
   variable_4 DW ?

.MODEL large, C             ; Specify a language for the externals

.FARDATA EXTERN var1:word

EXTERN ptr2:dword           ; pointer to var2

EXTERN struct_ptr:dword     ; pointer to array of structures

func1 PROC uses ax dx di es

;;;;;;;;;;;;;;;;;;;;;;; Use Variable ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

   mov dx, SEG var1         ; get the segment of var1
   mov es, dx
   mov di, OFFSET var1      ; get the offset of var1

   mov dx, es:[di]          ; dereference address to modify value
   shl dx, 1
   mov es:[di], dx

   mov dx, SEG ptr2        ; get the segment of pointer
   mov es, dx
   mov di, OFFSET ptr2     ; get the offset of pointer

   mov dx, es:[di+2]       ; dereference ptr to get
                           ; the segment of var2
   mov ax, es:[di]         ; dereference ptr to get the offset of var2
   mov es, dx
   mov di, ax

   mov ax, es:[di]         ; dereference address to modify value
   shl ax, 1
   mov es:[di], ax

   mov dx, SEG struct_var      ; get structure segment
   mov es, dx
   mov di, OFFSET struct_var   ; get structure offset

   mov ax, es:[di]             ; dereference address to modify var3
   shl ax, 1
   mov es:[di], ax

   mov ax, (info ptr es:[di]).variable_4
                               ; offset determined by info template

   ; Alternative instruction to line of code above
   ; mov ax, es:[di+2]         ; dereference address to modify var4

   shl ax, 1
   mov (info ptr es:[di]).variable_4, ax

   ; Alternate instruction to line above
   ; mov es:[di+2], ax

   ; ===modifies structure[0]===
   mov dx, SEG struct_ptr      ; get pointer segment
   mov es, dx
   mov di, OFFSET struct_ptr   ; get pointer offset

   mov dx, es:[di+2]           ; dereference pointer to get
                               ; segment of v_ray
   mov ax, es:[di]             ; dereference pointer to get
                               ; offset of v_ray
   mov es, dx
   mov di, ax

   mov ax, es:[di]             ; dereference address to modify var3
   shl ax, 1
   mov es:[di], ax

   mov ax, (info ptr es:[di]).variable_4
                               ; offset determined by info template
   shl ax, 1
   mov (info ptr es:[di]).variable_4, ax

   ;=== modifies structure[1] ===
   mov ax, es:[di+4]           ; advance to next element of array
   shl ax, 1                   ; and modify var3
   mov es:[di+4], ax

   mov ax, (info ptr es:[di+4]).variable_4
                               ; offset determined by info template
   shl ax, 1
   mov (info ptr es:[di+4]).variable_4, ax

   ret

END