What is a NOP instruction in x86 and x64 assembly?

by | May 31, 2022 | Reverse Engineering

NOP (No Operation) is an assembly language instruction that does nothing. This instruction does not change the state of any registers or status flags except the EIP register in x86 assembly or RIP register in x64 assembly. It does not access any memory also.

A NOP instruction takes a specific number of clock cycles and it does nothing after the execution. The NOP instruction executes and changes the instruction pointer so that the next assembly instruction can be executed.

A NOP instruction can have different purposes. For example, this instruction can force memory alignment of the next assembly instruction. And, sometimes this instruction is used as a placeholder. For example, a developer may use a number of NOP instructions as a placeholder so that later those NOP instructions can be replaced by other assembly instructions.

Please note that a NOP instruction can be a single-byte instruction or a multi-byte instruction. A multi-byte NOP instruction can take 2 to 9 bytes and it generates a byte sequence of no operation as a single instruction. A multi-byte NOP instruction is often used for the purpose of memory alignment.

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Amrita Mitra

Author

Ms. Amrita Mitra is an author, who has authored the books “Cryptography And Public Key Infrastructure“, “Web Application Vulnerabilities And Prevention“, “A Guide To Cyber Security” and “Phishing: Detection, Analysis And Prevention“. She is also the founder of Asigosec Technologies, the company that owns The Security Buddy.

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