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what is NOP?

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wiz561
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Joined: 12 Aug 2003
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PostPosted: Thu Sep 23, 2004 7:18 pm    Post subject: what is NOP? Reply with quote
Hi!

Stupid question, but as I read more and more about exploits, I see the phrase "NOP" used a lot of the time. Can anybody give me a hint on what this stands for and how to figure out what it means? Embarassed


Thank you in advance!
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capi
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PostPosted: Thu Sep 23, 2004 8:08 pm    Post subject: Reply with quote
No Operation.

Assembly mnemonic for the instruction with opcode 0x90 (in Intel and compatible CPUs).

Intel Arch. Software Dev's Manual wrote:
Description

Performs no operation. This instruction is a one-byte instruction that takes up space in the
instruction stream but does not affect the machine context, except the EIP register.
The NOP instruction is an alias mnemonic for the XCHG (E)AX, (E)AX instruction.

Flags Affected
None.

Exceptions (All Operating Modes)
None.

_________________
main(_){for(_=')';_;_+=~!&_["]["]){char l;write(!_!=_,(l=_["mI}., m0:0,$6/\3,\
$6/m/&\"10*\177c,$6\17cm\4c/&\"10\12"]^unix["CC me on *nix"],&l),_==_);}}

Israel G. Lugo
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Zarnick
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PostPosted: Thu Sep 23, 2004 9:20 pm    Post subject: Reply with quote
A good use of a NOP instruction(a buffer of NOPs) is when you are trying to guess the return code. In that way, if you exploits picks any nops, the buffer will be read, and the shellcode will be executed correctly.
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-==Peacemaker==- Just for fun.
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