# 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`