Unable to use a variable in instruction

Unable to use a variable in instruction

[A D]

I'm having strange problem accessing a variable in a gnu assembly language program.
When i write a code:

.section .data
   x = 4
mm:
   .int 1, 2

.section .text
.globl _start
_start:
   nop
   movl x, %ecx   #can't do this 
   movl $1, %eax
   movl $0, %ebx
   int $0x80

I can't move the contents of variable x to %ecx register. This program compiles 
and links without giving any error message. But when i run the program I get:

Segmentation fault(core dumped)


Debugging with gdb using core:


Core was generated by `./test'.
Program terminated with signal 11, Segmentation fault.
#0  _start () at test.s:14
10         movl x, %ecx   #can't do this


Another odd thing about it is when i modify the program:

.section .data
   x = 4
mm:
   .int 1, 2

.section .text
.globl _start
_start:
   nop
   movl $0, %ecx
   movl mm(, %ecx, x), %eax
   movl $1, %eax
   movl $0, %ebx
   int $0x80

the code runs without any problem. Even %ecx register contains the correct value.
Is it possible to explain the reason why I can't move directly the value of x in the first
place? Isn't x a variable? If not how come it can store value(in this case 4) and 
be used as a size of an array's element? Thanks.


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Re: Unable to use a variable in instruction

[Frank Kotler]

A D wrote:
> I'm having strange problem accessing a variable in a gnu assembly language program.
> When i write a code:
> 
> .section .data
>    x = 4
> mm:
>    .int 1, 2
> 
> .section .text
> .globl _start
> _start:
>    nop
>    movl x, %ecx   #can't do this 

You're trying to access memory at address 4, not move the immediate 
value 4. Try "movl $x, %ecx".

Best,
Frank

Re: Unable to use a variable in instruction

[Robert Plantz]

On Sun, 2007-10-14 at 19:37 -0400, A D wrote:
> I'm having strange problem accessing a variable in a gnu assembly language program.
> When i write a code:
> 
> .section .data
>    x = 4

This is equivalent to writing
        .equ  x,4

> mm:
>    .int 1, 2
> 
> .section .text
> .globl _start
> _start:
>    nop

So this instruction:
>    movl x, %ecx   #can't do this 

is trying to move the 4 bytes stored at memory location 4 into the ecx
register. As the OS told you, you cannot do that.

The way to allocate an initialized 32-bit int in the .data section is:

x:     .long  4

RE: Unable to use a variable in instruction

[A D]

>A D wrote:
> I'm having strange problem accessing a variable in a gnu assembly language program.
> When i write a code:
> 
> .section .data
>    x = 4
> mm:
>    .int 1, 2
> 
> .section .text
> .globl _start
> _start:
>    nop
>    movl x, %ecx   #can't do this 
>
>
>
>


Kotler wrote: 
>>You're trying to access memory at address 4, not move the immediate 
>>value 4. Try "movl $x, %ecx".
>>
>>
>>


Plantz wrote:
>>So this instruction:
>>    movl x, %ecx   #can't do this 
>> 
>>is trying to move the 4 bytes stored at memory location 4 into the ecx
>>register. As the OS told you, you cannot do that.
>> 
>>The way to allocate an initialized 32-bit int in the .data section is:
>> 
>>x:     .long  4
 

This problem is resolved now. Thanks for clarifying this.

--DC

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