why does this give a segmentation fault? it should just exit

Question

Hi, I am trying to learn low-level development. By putting 0 in ebx and 1 in eax (for exit() syscall) and calling int 0x80, it should exit the program. I have a simple c program that runs fine, but when I paste this in, instead of exiting as expected, I get a segmantation fault. Why does this happen?

THANK YOU!

 __asm__ ("xor %ebx, %ebx;"
                 "mov %al, 1;"
                "int $80;"
);

edit: thanks for advice, still nothing but seg faults, though. here are modifications i've made:

  __asm__ ("xor %ebx, %ebx;"
           "xor %eax, %eax;"
                "mov $1, %eax;"
                "int $80;"
);

edit: after modifying this example from http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html

asm("movl $1,%%eax;         /* SYS_exit is 1 */
             xorl %%ebx,%%ebx;      /* Argument is in ebx, it is 0 */
             int  $0x80"            /* Enter kernel mode */
             );

This finally worked for me:

asm("   movl $1,%eax;
        xorl %ebx,%ebx;
        int  $0x80
"
        );

thanks for looking and offering advice.

Answer 1

This looks like AT&T assembler syntax, so operations are in the order "operation source, target" as opposed to "operation target, source" in the more common Intel syntax.

Knowing this, mov %al, 1; tries to write the content of the al register to the memory position 1. Change it to mov 1, %al; and it should work IMO. Note that I have never used AT&T syntax, so I'm not sure whether I interpreted it correctly.

Edit: And Jeff M is right, the syscall number must be stored in the eax register, so make sure it is cleared.

Answer 2

Are you sure the rest of eax is cleared? Try moving 1 into eax and not just al or at least clear it first.

__asm__ ("xor %ebx, %ebx;"
         "mov $1, %eax;"
         "int $0x80;"
);

edit: If AndiDog is right about AT&T syntax.

edit: It's been a while since I've used gas but 80₁₆ is $0x80. $80 is 80₁₀. This should fix the last of it.