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

dqtln
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hi all :)
string db "dqtln123"
how to reverse it : "321nltqd"
thanks
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shoorick
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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
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dqtln
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string db "12345678"
reverse it : "78563412"
view tack hex dump of victim , kaka
i done ... thanks for reply
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guyenWinasm
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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
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shoorick
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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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