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Corruptor
Advanced Cheater
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Joined: 10 Aug 2011
Posts: 82
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 Posted: Wed May 08, 2013 1:01 pm Post subject: Confusing Assembly instructions |
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Hi there, im back with assembler im to stupid to understand for.
| Code: | worldedit.exe+1158A9 - 33 C0 - xor eax,eax
worldedit.exe+1158AB - 39 82 18020000 - cmp [edx+00000218],eax
worldedit.exe+1158B1 - 5E - pop esi
worldedit.exe+1158B2 - 0F95 C0 - setnc al
worldedit.exe+1158B5 - C3 - ret
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[edx+218] is a flag that either is 0 or 1 (a bool if you will). Because of the xor eax,eax, eax is always 0, so the cmp seemingly compares the flag to 0. The function is supposed to return 0 when the flag is 0 and 1 if the flag is 1. Why not just return the flag itself? boolean-paranoia i guess.
However, i wonder how this works. A cmp is a subtraction, so it subtracts eax from the flag and sets the flags according to this subtraction. The Carry flag is the flag that is set when an overflow occurs (unlike the overflow flag, which is set when the msb is changed... douh)
But subtracting a zero from anything will never change the value and could thus never result in the carry flag being set, so setnc should always set al to 1. However, somehow the carry flag IS set when the cmp kicks in. Why does this happen?
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TsTg
Master Cheater
 Reputation: 5
Joined: 12 Dec 2012
Posts: 340
Location: Somewhere....
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| Posted: Fri May 10, 2013 11:21 am Post subject: |
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because the CMP command will compare between the values in [edx+00000218] and in EAX(which is always zero), if they are equal: the ZERO or (Z) flag is set to 1, if not equal, the Z flag is set to 0, this flag is what a following conditional jump decides what to do (jump or don't), this doesn't have a relation with the value in AL as actually it's a part of eax's value (ex: eax = 00342015, then AL = 15), the SETNC will set it's value to 1, but the Z flag is not touched.
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Corruptor
Advanced Cheater
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Joined: 10 Aug 2011
Posts: 82
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| Posted: Sat May 11, 2013 4:08 pm Post subject: |
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| setnc is short for "set if not carry", so it's supposed to check the carry flag, not the zero flag. The zero flag is sort of insignificant for this case, isnt it?
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TsTg
Master Cheater
Reputation: 5
Joined: 12 Dec 2012
Posts: 340
Location: Somewhere....
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| Posted: Sun May 12, 2013 6:40 am Post subject: |
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yes, the SETNC will change the value of AL to 1 if the Carry flag = 0 (or condition is true), or will set the AL to zero if the carry flag = 1 (condition false), the CMP instruction is the one that will change the value of the carry flag according to the compare result(equal or not), beside changing the zero flag.
So, in your code, the whole thing depends on the comparison instruction.
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