1. Translate C Language Program into x86 Assembly Language
Chung-Yuan Christian University
Information & Computer Eng. Dept.
Teddy Hsiung

2. From “Hello, world” Example
#include <stdio.h>
int main(void) {
printf("Hello, world\n");
return 0; }
Why include file “stdio.h” ?
Every include file might includes…
Macro & constant definitions.
User defined data type definitions.
External data reference declarations.
External function call prototype declarations.

3. Equivalent “Hello, world” Program
#include <stdio.h>
int main(void) {
printf("Hello, world\n");
return 0; }
int printf (const char *__format, ...);
int main(void) {
printf("Hello, world\n");
return 0; }

4. Translate C to Assembly Program
#include <stdio.h>
int x = 168;
int y;
int main(void) {
int z = 166;
y = 188;
printf("x=%d,y=%d,z=%d\n",x,y,z);
return 0; }
DATA Segment
BSS Segment
STACK Segment
CODE Segment

5. C to Assembly: Code Segment
#include <stdio.h>
int x = 168;
int y;
int main(void) {
int z = 166;
y = 188;
printf("x=%d,y=%d,z=%d\n",x,y,z);
return 0; }
Code Segment contains CPU executable machine code.
Basically, assembly inst. is 1-to-1 directly mapping to machine code.
_TEXT segment byte public 'CODE' ;
; int main(void)
assume cs:_TEXT
_main proc near
push bp
mov bp,sp
push si
; {
; int z = 166;
mov si,166
; y = 188;
mov word ptr DGROUP:_y,188
; printf("x=%d,y=%d,z=%d\n",x,y,z);
push si
push word ptr DGROUP:_y
push word ptr DGROUP:_x
mov ax,offset
push ax
call near ptr _printf
add sp,8 ;
; return 0;
xor ax,ax
; }
pop si
pop bp
ret
_main endp
_TEXT ends

6. C to Assembly: Code Segment
_TEXT segment byte public 'CODE'
assume cs:_TEXT
_main proc near
push bp
B EC mov bp,sp
push si
0004 BE 00A6 mov si,166
0007 C R 00BC mov word ptr DGROUP:_y,188
000D push si
000E FF R push word ptr DGROUP:_y
0012 FF R push word ptr DGROUP:_x
0016 B R mov ax,offset
push ax
001A E E call near ptr _printf
001D 83 C add sp,8
C0 xor ax,ax
E pop si
D pop bp
0024 C3 ret
_main endp
_TEXT ends

7. C to Assembly: Data Segment
_DATA segment word public 'DATA'
label byte
label word
_DATA ends
_x label word
db 168
db 0
label byte
db 'x=%d,y=%d,z=%d'
db 10
#include <stdio.h>
int x = 168;
int y;
int main(void) {
int z = 166;
y = 188;
printf("x=%d,y=%d,z=%d\n",x,y,z);
return 0; }
Data segment contains the initialized data in the program.
The initialized data include the initialized variable and constant string.

8. C to Assembly: BSS Segment
#include <stdio.h>
int x = 168;
int y;
int main(void) {
int z = 166;
y = 188;
printf("x=%d,y=%d,z=%d\n",x,y,z);
return 0; }
_BSS segment word public 'BSS'
label byte
label word
_BSS ends
_y label word
db 2 dup (?)
?debug C E9
BSS: BLOCK STARTED BY SYMBOL
BSS Segment contains the uninitialized data in the program.
The whole BSS Segment usually filled with zero at the program startup.

9. C to Assembly: Stack Segment
#include <stdio.h>
int x = 168;
int y;
int main(void) {
int z = 166;
y = 188;
printf("x=%d,y=%d,z=%d\n",x,y,z);
return 0; }
;In file ~\EXAMPLES\STARTUP\C0.ASM
_STACK SEGMENT
db dup(?)
ENDS
Stack Segment contains:
The return address in the function call.
The parameter passed in the function call.
Non-static local variable within the function.
Defined in the C startup code “C0.ASM”.