Q102434: Accessing Global Symbols Defined in C from MASM
Article: Q102434
Product(s): Microsoft Macro Assembler
Version(s): 6.0,6.0a,6.0b,6.1,6.11,6.1a
Operating System(s):
Keyword(s):
Last Modified: 06-MAY-2001
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The information in this article applies to:
- Microsoft Macro Assembler (MASM), versions 6.0, 6.0a, 6.0b, 6.1, 6.1a, 6.11
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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;
struct var_tag
{
int var3;
int var4;
} * struct_ptr, struct_var = {5, 7}, v_ray[2] = {{9, 11}, {13, 15}};
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 ?
info ends
.MODEL large, C ; Specify a language for the externals
.8086
.FARDATA
EXTERN var1:word
EXTERN ptr2:dword ; pointer to var2
EXTERN struct_var:dword
EXTERN struct_ptr:dword ; pointer to array of structures
.CODE
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
;;;;;;;;;;;;;;;;;;;;;; Use pointer to variable ;;;;;;;;;;;;;;;;;;;;
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
;;;;;;;;;;;;;;;;;;;;; Use structure ;;;;;;;;;;;;;;;;;;;;;;;;;;;
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
;;;;;;;;;;;;;;;;;; Use pointer to array of structures ;;;;;;;;;
; ===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
func1 ENDP
END
Additional query words: 6.00 6.00a 6.00b 6.10 6.10a s_c
======================================================================
Keywords :
Technology : kbMASMsearch kbAudDeveloper kbMASM600 kbMASM610 kbMASM611 kbMASM610a kbMASM600a kbMASM600b
Version : :6.0,6.0a,6.0b,6.1,6.11,6.1a
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