CSC231 Buggy Code 1
--D. Thiebaut 09:40, 9 November 2012 (EST)
The code below is buggy... can you explain why?
section .data
hexChars: db "0123456789ABCDEF"
section .text
global _start
_start:
mov al,0xf5
and al,0x0f ;al <-- 05
mov ebx, hexChars ;ebx <-- address hexChars
add bl, al ;ebx <-- hexChars+05
mov al,[ebx] ;al <-- '5'
call printChar
;;; exit
mov ebx, 0
mov eax, 1
int 0x80
;;;----------------------------------------------------
;;; printChar: prints char in al to screen
;;;----------------------------------------------------
section .data
pc_temp: db 0
section .text
printChar: pushad
mov [pc_temp],al ;store char in string
mov eax, 4 ;print string
mov ebx, 1
mov ecx, pc_temp
mov edx, 1
int 0x80
popad
ret
Solution
You may have guessed that the culprit was the add instruction highlighted in the code below. The reason is that what we want is the 32-bit address of the character in the hexChars array, but we perform an 8-bit addition, between bl and al, hoping that ebx is the correct address. Unfortunately there might be a carry from the addition of al to bl that will not affect bh and the higher part of ebx...
section .data
hexChars: db "0123456789ABCDEF"
section .text
global _start
_start:
mov al,0xf5
and al,0x0f ;al <-- 05
mov ebx, hexChars ;ebx <-- address hexChars
add bl, al ;ebx <-- hexChars+05
mov al,[ebx] ;al <-- '5'
call printChar
;;; exit
mov ebx, 0
mov eax, 1
int 0x80
;;;----------------------------------------------------
;;; printChar: prints char in al to screen
;;;----------------------------------------------------
section .data
pc_temp: db 0
section .text
printChar: pushad
mov [pc_temp],al ;store char in string
mov eax, 4 ;print string
mov ebx, 1
mov ecx, pc_temp
mov edx, 1
int 0x80
popad
ret
