NOP

No Operation

Opcodes

Hex Mnemonic Encoding Long Mode Legacy Mode Description
0F 1F /0 NOP r/m32 B Valid Valid Multi-byte no-operation instruction.
0F 1F /0 NOP r/m16 B Valid Valid Multi-byte no-operation instruction.
90 NOP A Valid Valid One byte no-operation instruction.

Instruction Operand Encoding

Op/En Operand 0 Operand 1 Operand 2 Operand 3
A NA NA NA NA
B NA NA NA ModRM:r/m (r)

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:

= 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.

Pseudo Code

(* No operation *)

Flags Affected

None.

Exceptions

64-Bit Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.

Compatibility Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.

Virtual-8086 Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.

Real-Address Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.

Protected Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.