NASM strange behavior with al

Asked Mar 12 '14 at 16:42

Active Jul 02 '15 at 21:19

Viewed 339 times

I tried to run the following program:

C code :

int  main()
    {
     char  *s1 = "hello";

     printf("string : %s\n", strchr(s1, 'l'));
    }

assembly code:

global strchr

    section .text

strchr:
    push rbp
    mov rbp, rsp

strchr_loop:
    mov al, byte [rsi]       ; My bug come from here
    cmp byte [rdi], al       ; and from here
    je strchr_end
    cmp byte[rdi], 0
    jz strchr_nul
    inc rdi
    jmp strchr_loop

strchr_end:
    mov rax, rdi
    mov rsp, rbp
    pop rbp

    ret

strchr_nul:
    mov rax, 0
    mov rsp, rbp
    pop rbp

    ret

When I executed this, I got a Segmentation fault.

But when I replace the bug line with the value 'l' instead of use al, the program is working

cmp byte [rdi], 'l'       ; It's working

edited Mar 12 '14 at 17:05

asked Mar 12 '14 at 16:42

Jérôme Campeaux

S1 is in rdi and 'l' is in rsi. My code is working when I put manually **'l'** in the code – Jérôme Campeaux Mar 12 '14 at 16:54
3

Your second argument is a `char`, yet your assembly code appears to treat it as a `char*`. – Michael Mar 12 '14 at 16:56
Ok, so i use sil, the 8 bits register of rsi and now it's working Thank's – Jérôme Campeaux Mar 12 '14 at 17:00
2

Use `cmp byte [rdi], sil` (and delete the `mov al, [rsi]`). *I see you figured it out :) – Jester Mar 12 '14 at 17:00

NASM strange behavior with al

0 Answers0