NOP—No Operation

Opcode

Instruction

Op/

64-Bit

Compat/

Description

En Mode Leg Mode
NP 90 NOP ZO Valid Valid One byte no-operation instruction.
NP 0F 1F /0 NOP r/m16 M Valid Valid Multi-byte no-operation instruction.
NP 0F 1F /0 NOP r/m32 M Valid Valid Multi-byte no-operation instruction.

Instruction Operand Encoding

Op/En Operand 1 Operand 2 Operand 3 Operand 4
ZO NA NA NA NA
M ModRM:r/m (r) NA NA NA

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.

The multi-byte form of NOP is available on processors with model encoding:

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:

Table 4-12. Recommended Multi-Byte Sequence of NOP Instruction

Length

Assembly

Byte Sequence

2 bytes

3 bytes

4 bytes

5 bytes

6 bytes

7 bytes

8 bytes

9 bytes

66 NOP

NOP DWORD ptr [EAX]

NOP DWORD ptr [EAX + 00H]

NOP DWORD ptr [EAX + EAX*1 + 00H]

66 NOP DWORD ptr [EAX + EAX*1 + 00H]

NOP DWORD ptr [EAX + 00000000H]

NOP DWORD ptr [EAX + EAX*1 + 00000000H]

66 NOP DWORD ptr [EAX + EAX*1 + 00000000H]

66 90H

0F 1F 00H

0F 1F 40 00H

0F 1F 44 00 00H

66 0F 1F 44 00 00H

0F 1F 80 00 00 00 00H

0F 1F 84 00 00 00 00 00H

66 0F 1F 84 00 00 00 00 00H

Flags Affected

None

Exceptions (All Operating Modes)

#UD

If the LOCK prefix is used.