The NOP (no-op) instruction is an instruction that does nothing. In x86, a 1byte

Question

The NOP (no-op) instruction is an instruction that does nothing. In x86, a 1byte opcode 0x90 is designated as NOP instruction. In addition to this single-byte NOP instruction, x86 defines/recommends multi-byte NOP instructions as follows:

    2 bytes: 0x66 0x90                              /*0x66; NOP */  

3 bytes: 0x0f 0x1f 0x00                      /* NOP DWORD ptr [EAX] */  

4 bytes: 0x0f 0x1f 0x40 0x00             /* NOP DWORD ptr [EAX + 00h]*/  

5 bytes: 0x0f 0x1f 0x44 0x00 0x00   /* NOP DWORD ptr [EAX + EAX*1 + 00h]*/

  6 bytes: 0x66 0x0f 0x1f 0x44 0x00 0x00 /*0x66; NOP DWORD ptr [EAX+EAX*1 + 00h]*/ and so on.

Explain why multi-byte NOP instructions are needed by pointing out the advantage of using the above multi-byte NOP instruction instead of just multiple single-byte NOPs.

Explanation / Answer

The NOP instructions do not change the state of the registers, memory , etc. The are mainly used in the programs for timing purposes , memory alignment . The use of multi byte NOP instructions is useful because they require lesser amount of time to be spend on them unlike using many number of NOP instructions . They are commonly used to align the code from 8 to 16 bytes of boundaries. They are used for caching and for faster read operation of memory . Instead of using multiple single byte NOPS the use of multi byte NOP instruction improves the count cycle and increases the speed and reduces the strain on decoder.

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.