looping_code

DISCUSSION ssams 4 participants 8 messages Aug 26, 2002 – Sep 1, 2002

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ssams

author

2002-08-26 09:19 UTC #0 lore raw

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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Maciej Hrebien

2002-08-26 14:59 UTC #1 lore raw

Quoted 7 lines · 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....?

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

Quoted 9 lines

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

Why $10 if Your msg is two bytes long?

Quoted 9 lines

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

Signature · 1 line

Maciej Hrebien

Scott Lanning

2002-08-31 06:17 UTC #2 lore raw

In reply to ssams

Quoted 6 lines · ssams

.data
msg: .byte 65,10
.text
.globl mulai
mulai:

Quoted 6 lines

    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)

Quoted 5 lines

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

Robert Plantz

2002-08-31 15:05 UTC #3 lore raw

Quoted 9 lines · Scott Lanning

Quoted 8 lines · ssams
.data
msg: .byte 65,10
.text
.globl mulai
mulai:
    mov $10,%edx

Quoted 7 lines

Quoted 4 lines
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

Scott Lanning

2002-08-31 17:36 UTC #4 lore raw

Quoted 9 lines · Robert Plantz

Quoted 5 lines · Scott Lanning
Quoted 1 line · ssams
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.

Maciej Hrebien

2002-08-31 15:51 UTC #5 lore raw

In reply to Scott Lanning

Quoted 15 lines · Scott Lanning

Quoted 6 lines · ssams
.data
msg: .byte 65,10
.text
.globl mulai
mulai:
Quoted 6 lines
    mov $10,%edx
ulang:
mov $4,%eax
mov $1,%ebx
mov $msg,%ecx

Quoted 5 lines

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,

Signature · 1 line

Maciej Hrebien

Robert Plantz

2002-08-31 17:10 UTC #6 lore raw

Quoted 9 lines · Maciej Hrebien

Quoted 9 lines · ssams
Quoted 8 lines
.data
msg: .byte 65,10
.text
.globl mulai
mulai:
    mov $10,%edx

Quoted 9 lines

Quoted 4 lines
Quoted 4 lines
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

Maciej Hrebien

2002-09-01 09:21 UTC #7 lore raw

Quoted 17 lines · Robert Plantz

Quoted 5 lines
"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

Quoted 5 lines

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,

Signature · 1 line

Maciej Hrebien