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Subject: Re: Rybka code

Author: Chrilly Donninger

Date: 09:18:50 12/12/05

Go up one level in this thread

>>.text:0040D156                 call    sub_40D1B0
>>.text:0040D15B                 mov     esi, eax
>>.text:0040D15D                 lea     eax, [esp+278h+var_238]
>>.text:0040D161                 push    eax
>>.text:0040D162                 push    edi
>>.text:0040D163                 neg     esi
>>.text:0040D165                 call    sub_40A780
>>.text:0040D16A                 add     esp, 18h
>>.text:0040D16D                 cmp     esi, [esp+268h+var_25C]
>>.text:0040D171                 jle     short loc_40D183
>>.text:0040D173                 cmp     esi, ebx
>>.text:0040D175                 mov     [esp+268h+var_25C], esi
>>.text:0040D179                 jle     short loc_40D183
>>.text:0040D17B                 cmp     esi, [ebp+arg_4]
>>.text:0040D17E                 mov     [ebp+arg_0], esi
>>.text:0040D181                 jge     short loc_40D13A
>How can you tell?  And how on earth did you find it?

There is a nice tool, called IDA-Pro. (IDA=Interactive DissAssembler).
IDA produces the code above. And then one needs some (assembler) experience to
read this.
E.g. [ebp+arg_0] means the first argument to the function. And if one follows
somewhat the logic, one sees that this is alpha, [ebp+arg_4] is argument 1 or
In Pseudo-Code the code above reads as:
if(val>bestscore) { bestscore=val; }
if(val>alpha) { alpha=val; }
if(val>beta) { return val; } // Actually goto return-sequence.

All programms are open-source. Some opensource is only more difficult to read
than other.

Dissassembling is no efficient way to improve one owns code. It does not help,
if one knows, that programm X has a nice trick. The trick must fit in one owns
programm/architecture. Usually it does not. E.g the well documented Fruit
History-Pruning did not work at all in Hydra. Dissassembling is just an old
hackers fun.


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