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Old 06-16-2010, 02:15 AM   #1
tushs
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Registered: Apr 2010
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how to traverse byte array in assembly using nasm as assembler?


hello, I am trying to learn assembly using nasm as assembler. I did not found example related to string operation. I had following example code,

global asm_strlen
section .data
section .bss
section .text
asm_strlen:
push ebp ; remember base pointer
mov ebp, esp ; init current base pointer
mov eax,0
mov ecx, [ebp + 8] ; get pointer to string
check:
movsb edx,[ecx] ; first byte
cmp edx,0
je out
inc ecx
inc eax
jmp check
out:
pop ebp ; pop return address in ebp
ret

my problem is at highlighted line, I am not getting how to copy a byte into 4 byte reg. rather what is syntax of mov instruction to mov byte to WORD DWORD etc.
if possible please give some example, and alternative solution if known.
 
Old 06-17-2010, 02:32 AM   #2
resetreset
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Location: India
Distribution: Dynebolic, Ubuntu 10.10
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from: http://en.wikibooks.org/wiki/X86_Assembly/Data_Transfer

"he movsb instruction copies one byte from the location specified in esi to the location specified in edi."

so you, i think, can't use it with edx and ecx, that's your problem.
 
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Old 06-21-2010, 03:45 AM   #3
tushs
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Registered: Apr 2010
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Thanks for reply, I got solution as bellow,

movzx edx,byte [ecx] ; move single byte

I can use byte, WORD, DWORD keywords to move 1 2 4 bytes.
after this I got stuck that how to move byte to a byte pointer, I mean, if I have two byte pointers how can I copy them, as u explained I can do that with esi and edi, can u give some code example how to do this ?
 
Old 06-25-2010, 01:18 AM   #4
tushs
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Registered: Apr 2010
Posts: 17

Original Poster
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I got solution as,
[CODE]

mov dh, byte [edi] ;copy byte to reg
mov [esi], dh ; copy byte to mem

[CODE]
 
  


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