Intel 64 and IA-32 Architectures Software Developers Manual Volume 2B, Instruction Set Reference, N-Z

4-4 Vol. 2B
INSTRUCTION SET REFERENCE, N-Z
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.
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:
Opcode
Instructio
n
64-Bit
Mode
Compat/
Leg Mode Description
90 NOP Valid Valid One byte no-operation instruction.
0F 1F /0 NOP
r/m16
Valid Valid Multi-byte no-operation instruction.
0F 1F /0 NOP
r/m32
Valid Valid Multi-byte no-operation instruction.
Table 4-1. Recommended Multi-Byte Sequence of NOP Instruction
Length Assembly Byte Sequence
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