Author Topic: Scan of string  (Read 7078 times)

Offline akus85

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Scan of string
« on: May 03, 2010, 04:19:59 PM »
Hi,i'm a noob in nasm and have a problem to scan a string.
For a string "hello" i should obtain:
68
65
6C
6C
6F

Where i wrong ?  ???
Code: [Select]
%include "asm_io.inc"

segment .data
        ;; ostring dd '1','2','3','4','5'                                                                                                     
        ostring db "hello",0

segment .bss

segment .text
        global _asm_main
_asm_main:
        enter 0,0
        pusha

        xor ebx,ebx
cycle:
        cmp ebx,5
        jae ex
        mov eax,[ostring+ebx*1]
        call print_int
        call print_nl
inc ebx
jmp cycle


ex:
        mov eax,0
        leave
        popa
        ret


Offline Bryant Keller

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Re: Scan of string
« Reply #1 on: May 03, 2010, 04:51:56 PM »
I don't have a copy of asm_io.inc, but if it works the way I think it does, then this should get you going.
Code: [Select]
%include "asm_io.inc"

segment .data
ostring db "hello"
ostring_size equ ($-ostring)

segment .bss

segment .text

global _asm_main
_asm_main:
enter 0,0
pusha

xor ebx,ebx
xor eax,eax
cycle:
push ebx
mov al,[ostring+ebx*1]
call print_int
call print_nl
pop ebx
inc ebx
cmp ebx, ostring_size
jl cycle

ex:
mov eax,0
leave
popa
ret

I'm assuming that ebx is being corrupted by either print_int or print_nl. When I rewote this to make use of libc it worked fine. So I'm pretty certain that's your problem.

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Offline akus85

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Re: Scan of string
« Reply #2 on: May 03, 2010, 05:18:38 PM »
my error was here :
mov eax,[ostring+ebx*1] ---> mov al,[ostring+ebx*1]

Thaks  ;)

For information asm_io.inc is :

Code: [Select]
        extern  read_int, print_int, print_string
        extern  read_char, print_char, print_nl
        extern  sub_dump_regs, sub_dump_mem, sub_dump_math, sub_dump_stack

%macro  dump_regs 1
        push      dword %1
        call      sub_dump_regs
%endmacro


;
; usage: dump_mem label, start-address, # paragraphs
%macro  dump_mem 3
        push     dword %1
        push     dword %2
        push     dword %3
        call     sub_dump_mem
%endmacro

%macro  dump_math 1
        push     dword %1
        call     sub_dump_math
%endmacro

%macro  dump_stack 3
        push     dword %3
        push     dword %2
        push     dword %1
        call     sub_dump_stack
%endmacro



Offline Frank Kotler

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Re: Scan of string
« Reply #3 on: May 04, 2010, 03:53:56 AM »
The relevant code is actually in asm_io.asm (which is assembled to asm_io.o and linked with your code)...

Code: [Select]
print_int:
enter 0,0
pusha
pushf

push eax
push dword int_format
call _printf
pop ecx
pop ecx

popf
popa
leave
ret

As you can see, it even preserves flags (print_nl likewise). You should be able to remove the "push ebx", "pop ebx" (both or neither, please!) and still have it work. What Bryant did that actually fixed it was:

Code: [Select]
xor eax,eax
cycle:
push ebx
mov al,[ostring+ebx*1]

instead of:

Code: [Select]
mov eax,[ostring+ebx*1]

With eax there, you get 4 bytes of your string in eax, and print_int prints all 4 of 'em as a single number. By zeroing eax first, and then loading just al from your string, print_int prints all of eax as a single number - but it's the number we want. :)

Or is it? Your example appears to show the bytes of the string represented in hex. That's not what print_int does (it prints a decimal representation). It would be trivial (for an experienced programmer) to add a "print_hex" function - a duplicate of "print_int", but with "push dword hex_format"... added above as hex_format db "%X", 0... add print_hex to the list of globals here, and to the list of externs in asm_io.inc... reassemble asm_io.asm... and you should be able to use print_hex just like print_int. I don't imagine you're supposed to do that as homework, though. :)

If it's really supposed to be hex, you may be expected to convert to a hex representation and use print_char to display it. If that's the case, you have more work to do. :(

Best,
Frank


Offline Bryant Keller

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Re: Scan of string
« Reply #4 on: May 07, 2010, 12:31:10 AM »
What Bryant did that actually fixed it was:

Code: [Select]
xor eax,eax
cycle:
push ebx
mov al,[ostring+ebx*1]

instead of:

Code: [Select]
mov eax,[ostring+ebx*1]

Yea, I have a habit of subliminally correcting things like that when I'm "fixing" code to work on my setup. I had to change this code from print_int/print_nl to use printf() so I instinctively modified the mov al, [ostring+ebx*1]. :D

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