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How to reverse a string, various methods for reversing strings in Assembly Language.

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dqtln

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Group: Members
Posts: 4
Member No.: 854
Joined: 13-December 04

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hi all

string db "dqtln123"
how to reverse it : "321nltqd"
thanks

shoorick

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Group: Admins
Posts: 2314
Member No.: 160
Joined: 22-June 04

at first tell us how we can find end of string:
1. place terminator at the and (zero, or say "$")
2. place string length at the start
3. suppose fixed length of 8 bytes

last case is simpliest:

CODE

mov eax,dword [string]
mov edx,dword [string + 4]
bswap eax
bswap edx
mov dword [string + 4],eax
mov dword [string],edx


dqtln
New Member Group: Members Posts: 4 Member No.: 854 Joined: 13-December 04	string db "12345678" reverse it : "78563412" view tack hex dump of victim , kaka i done ... thanks for reply

guyenWinasm

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Group: Members
Posts: 47
Member No.: 1386
Joined: 22-July 05

QUOTE (shoorick @ Feb 12 2007, 02:31 AM)

at first tell us how we can find end of string:
1. place terminator at the and (zero, or say "$")
2. place string length at the start
3. suppose fixed length of 8 bytes

last case is simpliest:

CODE

mov eax,dword [string]
mov edx,dword [string + 4]
bswap eax
bswap edx
mov dword [string + 4],eax
mov dword [string],edx

bswap doesn't work when lower then .486 specified.

idea is push and reset pointer to beginning of string again pop

CODE

string db '1234567890'
strlen dw $-string

IRP StackAction <push, pop> //repeat first time push second time pop
LOCAL loop1 // local variable in macro

lea di, string // di=ptr to string
mov cx, strlen // cx= string length

loop1:
IFIDNI &StackAction&, <push> //
mov al, byte ptr ds:[di] // al= di -th byte
ENDIF

&StackAction& ax //replace with macro parameter

IFIDNI &StackAction&, <pop> // if pop
mov byte ptr ds:[di], al // [di] = char in al
ENDIF

inc di // di= next char
dec cx // go toward 0
or cx, cx // is counter 0?
jnz loop1 // if not loop ever

endm

haven't tested. but u got the idea

shoorick

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Group: Admins
Posts: 2314
Member No.: 160
Joined: 22-June 04

QUOTE (guyenWinasm @ Feb 24 2007, 05:30 AM)

bswap doesn't work when lower then .486 specified.

what cpu required?

CODE

string db "dqtln123"

� �lxi d,string
� �lxi h,7
� �mvi b,4
� �dad d
loop:
� �ldax d
� �mov c,m
� �xchg
� �mov m,c
� �stax d
� �inx h
� �dcx d
� �xchg
� �dec b
� �jnz loop

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