2.2 KiB
Executable File
NOP - No Operation
Description
This instruction performs no operation. It is a one-byte or multi-byte NOP that takes up space in the instruction stream but does not impact machine context, except for the EIP register.
Instruction
| Opcode | Assembly | Op/En | Modern Mode | Legacy Mode | Description |
|---|---|---|---|---|---|
| 90 | NOP | NP | Valid | Valid | One byte no-operation instruction. |
| 0F 1F /0 | NOP r/m16 | M | Valid | Valid | Multi-byte no-operation instruction. |
| 0F 1F /0 | NOP r/m32 | M | Valid | Valid | Multi-byte no-operation instruction. |
Information
The multi-byte form of NOP is available on processors with model encoding:
- CPUID.01H.EAX (Bytes 11:8) = 0110B or 1111B
The multi-byte NOP instruction does not alter the content of a register and will not issue a memory operation. The instruction’s operation is the same in non-64-bit modes and 64-bit mode.
Operation
The one-byte NOP instruction is an alias mnemonic for the XCHG (E)AX, (E)AX instruction. The multi-byte NOP instruction performs no operation on supported processors and generates undefined opcode exception on processors that do not support the multi-byte NOP instruction.
The memory operand form of the instruction allows software to create a byte sequence of “no operation” as one instruction. For situations where multiple-byte NOPs are needed, the recommended operations (32-bit mode and 64-bit mode) are:
- 2 bytes
66 NOP 66 90H - 3 bytes
NOP DWORD ptr (EAX) 0F 1F 00H - 4 bytes
NOP DWORD ptr (EAX + 00H) 0F 1F 40 00H - 5 bytes
NOP DWORD ptr (EAX + EAX*1 + 00H) 0F 1F 44 00 00H - 6 bytes
66 NOP DWORD ptr (EAX + EAX*1 + 00H) 66 0F 1F 44 00 00H - 7 bytes
NOP DWORD ptr (EAX + 00000000H) 0F 1F 80 00 00 00 00H - 8 bytes
NOP DWORD ptr (EAX + EAX*1 + 00000000H) 0F 1F 84 00 00 00 00 00H - 9 bytes
66 NOP DWORD ptr (EAX + EAX*1 + 00000000H) 66 0F 1F 84 00 00 00 00 00H