looping_code

looping_code

[ssams]

i really new in linux assembly. i want to know how i loop
some code..?
i feel it is different with tasm, i have try tasm way for
looping but
it was not run cleanly in gnu assembly. are there any body
want to help me....?
by this code:

.data
msg: .byte 65,10

.text
   .globl mulai
mulai:
    mov $10,%edx
 
ulang:
  mov $4,%eax
  mov $1,%ebx
  mov $msg,%ecx
  int $0x80
  loop ulang

  mov $1,%eax
  int $0x80
   
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Re: looping_code

[Maciej Hrebien]

ssams wrote:
> 
> i really new in linux assembly. i want to know how i loop
> some code..?
> i feel it is different with tasm, i have try tasm way for
> looping but
> it was not run cleanly in gnu assembly. are there any body
> want to help me....?

I think it's NTG but lets see...

> by this code:
> 
> .data
> msg: .byte 65,10
> 
> .text
>    .globl mulai
> mulai:
>     mov $10,%edx

Why $10 if Your msg is two bytes long?

> ulang:
>   mov $4,%eax
>   mov $1,%ebx
>   mov $msg,%ecx
>   int $0x80
>   loop ulang
> 
>   mov $1,%eax
>   int $0x80

-- 
Maciej Hrebien

Re: looping_code

[Scott Lanning]

On Mon, 26 Aug 2002, ssams wrote:
> .data
> msg: .byte 65,10
>
> .text
>    .globl mulai
> mulai:
>     mov $10,%edx
>
> ulang:
>   mov $4,%eax
>   mov $1,%ebx
>   mov $msg,%ecx

This moves the value at 'msg'
rather than a pointer to 'msg'.
Try this:

    leal msg, %ecx

Also, how many times do you think it will loop? :)
(hint: which register is the loop counter)

>   int $0x80
>   loop ulang
>
>   mov $1,%eax
>   int $0x80

Re: looping_code

[Robert Plantz]

On Friday, August 30, 2002, at 11:17  PM, Scott Lanning wrote:

> On Mon, 26 Aug 2002, ssams wrote:
>> .data
>> msg: .byte 65,10
>>
>> .text
>>    .globl mulai
>> mulai:
>>     mov $10,%edx
>>
>> ulang:
>>   mov $4,%eax
>>   mov $1,%ebx
>>   mov $msg,%ecx
>
> This moves the value at 'msg'
> rather than a pointer to 'msg'.

Sorry, but the gas assembler *will* treat the address
as an immediate. Proof:

When I did this ---
mov $msg,%ecx
leal msg,%edx

and looked in the debugger, I got ---
(gdb) i r
eax            0x4      4
ecx            0x80494c8        134517960
edx            0x80494c8        134517960
ebx            0x1      1

Furthermore, if you look at the listing --
   12 000f B9000000      mov $msg,%ecx
   12      00
   13 0014 8D150000      leal msg,%edx
   13      0000
you see that the leal instruction costs one more byte.

I have only done this in a very simple teaching environment
using linux. I may be missing something here in a dos or
windows environment. On the other hand, one probably would
use gas only in linux.

-- bob

-------------------------------------------------------
Robert G. Plantz, PhD			|
Professor of Computer Science	| cs.sonoma.edu/~bob
Sonoma State University			| plantz@sonoma.edu
1801 E. Cotati Ave.				| ph: 707.664.2806
Rohnert Park, CA 94928			| fax: 707.664.3012

Re: looping_code

[Scott Lanning]

On Sat, 31 Aug 2002, Robert Plantz wrote:
> On Friday, August 30, 2002, at 11:17  PM, Scott Lanning wrote:
> > On Mon, 26 Aug 2002, ssams wrote:
> >>   mov $msg,%ecx
> >
> > This moves the value at 'msg'
> > rather than a pointer to 'msg'.
>
> Sorry, but the gas assembler *will* treat the address
> as an immediate. Proof:

Right, sorry. ssams, ignore that part.
Fix the %ecx loop count.

Re: looping_code

[Maciej Hrebien]

Scott Lanning wrote:
> 
> On Mon, 26 Aug 2002, ssams wrote:
> > .data
> > msg: .byte 65,10
> >
> > .text
> >    .globl mulai
> > mulai:
> >     mov $10,%edx
> >
> > ulang:
> >   mov $4,%eax
> >   mov $1,%ebx
> >   mov $msg,%ecx
> 
> This moves the value at 'msg'
> rather than a pointer to 'msg'.
> Try this:
> 
>     leal msg, %ecx

"mov $msg,%ecx" and "lea msg,%ecx" does the same thing in this example -
moves ptr to 'msg' to the ecx register. The only difference is that the
first one is coded one byte shorter.

Regards,

-- 
Maciej Hrebien

Re: looping_code

[Robert Plantz]

On Saturday, August 31, 2002, at 08:51  AM, Maciej Hrebien wrote:

>> On Mon, 26 Aug 2002, ssams wrote:
>>> .data
>>> msg: .byte 65,10
>>>
>>> .text
>>>    .globl mulai
>>> mulai:
>>>     mov $10,%edx
>>>
>>> ulang:
>>>   mov $4,%eax
>>>   mov $1,%ebx
>>>   mov $msg,%ecx
>
> "mov $msg,%ecx" and "lea msg,%ecx" does the same thing in this example -
> moves ptr to 'msg' to the ecx register. The only difference is that the
> first one is coded one byte shorter.
>

which can be seen if you do:
mov $msg,%ecx
leal msg,%edx

and look in the debugger ---
(gdb) i r
eax            0x4      4
ecx            0x80494c8        134517960
edx            0x80494c8        134517960
ebx            0x1      1

Furthermore, if you look at the listing --
   12 000f B9000000      mov $msg,%ecx
   12      00
   13 0014 8D150000      leal msg,%edx
   13      0000


--- Bob Plantz

Re: looping_code

[Maciej Hrebien]

Robert Plantz wrote:
> 
> > "mov $msg,%ecx" and "lea msg,%ecx" does the same thing in this example -
> > moves ptr to 'msg' to the ecx register. The only difference is that the
> > first one is coded one byte shorter.
> >
> 
> which can be seen if you do:
> mov $msg,%ecx
> leal msg,%edx
> 
> and look in the debugger ---
> (gdb) i r
> eax            0x4      4
> ecx            0x80494c8        134517960
> edx            0x80494c8        134517960
> ebx            0x1      1
> 
> Furthermore, if you look at the listing --
>    12 000f B9000000      mov $msg,%ecx
>    12      00
>    13 0014 8D150000      leal msg,%edx
>    13      0000

Yes, that's right. Sorry for the mess - (don't know why) i received Your
"proof reply" just after i send my. In final we got two messages saying
the same :)

Regards,

-- 
Maciej Hrebien