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Vol. 2A 3-153
INSTRUCTION SET REFERENCE, A-M
CMPXCHG8B/CMPXCHG16B—Compare and Exchange Bytes
CMPXCHG8B/CMPXCHG16B—Compare and Exchange Bytes
Description
Compares the 64-bit value in EDX:EAX (or 128-bit value in RDX:RAX if operand size
is 128 bits) with the operand (destination operand). If the values are equal, the
64-bit value in ECX:EBX (or 128-bit value in RCX:RBX) is stored in the destination
operand. Otherwise, the value in the destination operand is loaded into EDX:EAX (or
RDX:RAX). The destination operand is an 8-byte memory location (or 16-byte
memory location if operand size is 128 bits). For the EDX:EAX and ECX:EBX register
pairs, EDX and ECX contain the high-order 32 bits and EAX and EBX contain the low-
order 32 bits of a 64-bit value. For the RDX:RAX and RCX:RBX register pairs, RDX
and RCX contain the high-order 64 bits and RAX and RBX contain the low-order
64bits of a 128-bit value.
This instruction can be used with a LOCK prefix to allow the instruction to be
executed atomically. To simplify the interface to the processor’s bus, the destination
operand receives a write cycle without regard to the result of the comparison. The
destination operand is written back if the comparison fails; otherwise, the source
operand is written into the destination. (The processor never produces a locked read
without also producing a locked write.)
In 64-bit mode, default operation size is 64 bits. Use of the REX.W prefix promotes
operation to 128 bits. Note that CMPXCHG16B requires that the destination
(memory) operand be 16-byte aligned. See the summary chart at the beginning of
this section for encoding data and limits. For information on the CPUID flag that indi-
cates CMPXCHG16B, see page 3-173
.
IA-32 Architecture Compatibility
This instruction encoding is not supported on Intel processors earlier than the
Pentium processors.
Opcode* Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
0F C7 /1 m64 CMPXCHG8B m64 Valid Valid* Compare EDX:EAX with
m64. If equal, set ZF and
load ECX:EBX into m64.
Else, clear ZF and load m64
into EDX:EAX.
REX.W + 0F C7 /1
m128
CMPXCHG16B
m128
Valid N.E. Compare RDX:RAX with
m128. If equal, set ZF and
load RCX:RBX into m128.
Else, clear ZF and load
m128 into RDX:RAX.
NOTES:
* See IA-32 Architecture Compatibility section below.