2009-04-20 23:44:48

by Dragoslav Zaric

Subject: Address of .globl _start

Hello,

I have compiled test.s (bellow) with 'as' compiler:

-------------------------------------------------
.section .data

.globl var
var:
.long 10

.section .text

.globl _start
_start:
movl $_start, %ebx
jmp loop_exit

loop_exit:
movl $1, %eax #1 is the exit() syscall
int $0x80

-------------------------------------------------
and got output with echo $? (reads %ebx register)
116

After that I compiled test.s (bellow) with 'as' compiler:

-------------------------------------------------
.section .data

.globl var
var:
.long 10

.section .text

.globl _start
_start:
movl _start, %ebx
jmp loop_exit

loop_exit:
movl $1, %eax #1 is the exit() syscall
int $0x80

-------------------------------------------------
and got output with echo $? (reads %ebx register)
139

After that I compiled test.s (bellow) with 'as' compiler:

-------------------------------------------------
.section .data

.globl var
var:
.long 10

.section .text

.globl _start
_start:
movl var, %ebx
jmp loop_exit

loop_exit:
movl $1, %eax #1 is the exit() syscall
int $0x80

-------------------------------------------------
and got output with echo $? (reads %ebx register)
10

After that I compiled test.s (bellow) with 'as' compiler:

-------------------------------------------------
.section .data

.globl var
var:
.long 10

.section .text

.globl _start
_start:
movl $var, %ebx
jmp loop_exit

loop_exit:
movl $1, %eax #1 is the exit() syscall
int $0x80

-------------------------------------------------
and got output with echo $? (reads %ebx register)
132

So my question is, what these numbers mean ? From which
point in memory they are counted from ? From this I can conclude
that value for "movl _start, %ebx" is always 139 and it can be on
various addresses. Is this means that beginning of 'as' assembly program is
signed with value 139 ???

thanks,

Dragoslav Zaric,
[Programmer; M Sc Astrophysics]

2009-04-21 00:13:15

by Dragoslav Zaric

Subject: Re: Address of .globl _start

Ok Steve,

So what do you think, where should I post this one ?
I don't see what is wrong with this question ?
Maybe you can answer if it is off topic for the LKML.

thanks

Dragoslav Zaric
[Programmer; M Sc Astrophysics]