Why %ebx value is getting corrupted when I try to use it as an exit code?

Asked Apr 28 '15 at 19:06

Active Apr 28 '15 at 19:40

Viewed 67 times

My code to find the largest value:

# %edi - Holds the current index
# %ebx - Largest element so far
# %eax - Current element

.section .data

    data_items:
        .int 3,67,34,222,45,75,65,55,45,35,100,300,0
.section .text

.global _start
_start:
    movl $0, %edi
    movl data_items (,%edi,4), %eax
    movl %eax, %ebx

    start_loop:
            cmpl $0, %eax
            je loop_exit
            incl %edi
            movl data_items(,%edi,4), %eax
            cmpl %ebx, %eax
            jle start_loop
            movl %eax, %ebx
            jmp start_loop
    loop_exit:
            movl $1,%eax
            int $0x80

The output of this code: echo $? is getting printed as 44. But when I debug it using gdbtui I am getting the value 300 in %ebx. Then why is it corrupting the value of %ebx while calling the program exit interrupt?

edited Apr 28 '15 at 19:40

Raymond Chen

44,448
11
96
135

asked Apr 28 '15 at 19:06

Sreeraj Chundayil

5,548
3
29
68

5

300-256=44, exit codes are somewhat limited. – ninjalj Apr 28 '15 at 19:12
So how do I print that value. I just started learning assembly. – Sreeraj Chundayil Apr 28 '15 at 19:13
You should probably link against libc and call printf, unless you want to write your own routine to print numbers. – ninjalj Apr 28 '15 at 19:16
I want to do entirely in assembly, with no external dependencies. – Sreeraj Chundayil Apr 28 '15 at 19:19
allocate a buffer, convert the number to decimal string in buffer, call `write()`. – EOF Apr 28 '15 at 19:26
Thnks. With my current knowledge it is difficult to implement this. But I understood the method, soon I will implement this :) – Sreeraj Chundayil Apr 28 '15 at 19:27
Regarding the limitations of exit code/exit status, see the answers to http://unix.stackexchange.com/questions/99112/default-exit-code-when-process-is-terminated – ninjalj Apr 28 '15 at 19:57

Why %ebx value is getting corrupted when I try to use it as an exit code?

0 Answers0