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<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:svg="http://www.w3.org/2000/svg" xmlns:x86="http://www.felixcloutier.com/x86"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><link rel="stylesheet" type="text/css" href="style.css"></link><title>CMPXCHG8B/CMPXCHG16B
— Compare and Exchange Bytes</title></head><body><header><nav><ul><li><a href='index.html'>Index</a></li><li>December 2023</li></ul></nav></header><h1>CMPXCHG8B/CMPXCHG16B
— Compare and Exchange Bytes</h1>
<table>
<tr>
<th>Opcode/Instruction</th>
<th>Op/En</th>
<th>64-Bit Mode</th>
<th>Compat/Leg Mode</th>
<th>Description</th></tr>
<tr>
<td>0F C7 /1 CMPXCHG8B m64</td>
<td>M</td>
<td>Valid</td>
<td>Valid*</td>
<td>Compare EDX:EAX with m64. If equal, set ZF and load ECX:EBX into m64. Else, clear ZF and load m64 into EDX:EAX.</td></tr>
<tr>
<td>REX.W + 0F C7 /1 CMPXCHG16B m128</td>
<td>M</td>
<td>Valid</td>
<td>N.E.</td>
<td>Compare RDX:RAX with m128. If equal, set ZF and load RCX:RBX into m128. Else, clear ZF and load m128 into RDX:RAX.</td></tr></table>
<blockquote>
<p>*See IA-32 Architecture Compatibility section below.</p></blockquote>
<h2 id="instruction-operand-encoding">Instruction Operand Encoding<a class="anchor" href="#instruction-operand-encoding">
</a></h2>
<table>
<tr>
<th>Op/En</th>
<th>Operand 1</th>
<th>Operand 2</th>
<th>Operand 3</th>
<th>Operand 4</th></tr>
<tr>
<td>M</td>
<td>ModRM:r/m (r, w)</td>
<td>N/A</td>
<td>N/A</td>
<td>N/A</td></tr></table>
<h2 id="description">Description<a class="anchor" href="#description">
</a></h2>
<p>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.</p>
<p>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.)</p>
<p>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 indicates CMPX-CHG16B, see page 3-243.</p>
<h2 id="ia-32-architecture-compatibility">IA-32 Architecture Compatibility<a class="anchor" href="#ia-32-architecture-compatibility">
</a></h2>
<p>This instruction encoding is not supported on Intel processors earlier than the Pentium processors.</p>
<h2 id="operation">Operation<a class="anchor" href="#operation">
</a></h2>
<pre>IF (64-Bit Mode and OperandSize = 64)
THEN
TEMP128 := DEST
IF (RDX:RAX = TEMP128)
THEN
ZF := 1;
DEST := RCX:RBX;
ELSE
ZF := 0;
RDX:RAX := TEMP128;
DEST := TEMP128;
FI;
FI
ELSE
TEMP64 := DEST;
IF (EDX:EAX = TEMP64)
THEN
ZF := 1;
DEST := ECX:EBX;
ELSE
ZF := 0;
EDX:EAX := TEMP64;
DEST := TEMP64;
FI;
FI;
FI;
</pre>
<h2 id="flags-affected">Flags Affected<a class="anchor" href="#flags-affected">
</a></h2>
<p>The ZF flag is set if the destination operand and EDX:EAX are equal; otherwise it is cleared. The CF, PF, AF, SF, and OF flags are unaffected.</p>
<h2 class="exceptions" id="protected-mode-exceptions">Protected Mode Exceptions<a class="anchor" href="#protected-mode-exceptions">
</a></h2>
<table>
<tr>
<td>#UD</td>
<td>If the destination is not a memory operand.</td></tr>
<tr>
<td rowspan="3">#GP(0)</td>
<td>If the destination is located in a non-writable segment.</td></tr>
<tr>
<td>If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.</td></tr>
<tr>
<td>If the DS, ES, FS, or GS register contains a NULL segment selector.</td></tr>
<tr>
<td>#SS(0)</td>
<td>If a memory operand effective address is outside the SS segment limit.</td></tr>
<tr>
<td>#PF(fault-code)</td>
<td>If a page fault occurs.</td></tr>
<tr>
<td>#AC(0)</td>
<td>If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.</td></tr></table>
<h2 class="exceptions" id="real-address-mode-exceptions">Real-Address Mode Exceptions<a class="anchor" href="#real-address-mode-exceptions">
</a></h2>
<table>
<tr>
<td>#UD</td>
<td>If the destination operand is not a memory location.</td></tr>
<tr>
<td>#GP</td>
<td>If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.</td></tr>
<tr>
<td>#SS</td>
<td>If a memory operand effective address is outside the SS segment limit.</td></tr></table>
<h2 class="exceptions" id="virtual-8086-mode-exceptions">Virtual-8086 Mode Exceptions<a class="anchor" href="#virtual-8086-mode-exceptions">
</a></h2>
<table>
<tr>
<td>#UD</td>
<td>If the destination operand is not a memory location.</td></tr>
<tr>
<td>#GP(0)</td>
<td>If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.</td></tr>
<tr>
<td>#SS(0)</td>
<td>If a memory operand effective address is outside the SS segment limit.</td></tr>
<tr>
<td>#PF(fault-code)</td>
<td>If a page fault occurs.</td></tr>
<tr>
<td>#AC(0)</td>
<td>If alignment checking is enabled and an unaligned memory reference is made.</td></tr></table>
<h2 class="exceptions" id="compatibility-mode-exceptions">Compatibility Mode Exceptions<a class="anchor" href="#compatibility-mode-exceptions">
</a></h2>
<p>Same exceptions as in protected mode.</p>
<h2 class="exceptions" id="64-bit-mode-exceptions">64-Bit Mode Exceptions<a class="anchor" href="#64-bit-mode-exceptions">
</a></h2>
<table>
<tr>
<td>#SS(0)</td>
<td>If a memory address referencing the SS segment is in a non-canonical form.</td></tr>
<tr>
<td rowspan="3">#GP(0)</td>
<td>If the memory address is in a non-canonical form.</td></tr>
<tr>
<td>If memory operand for CMPXCHG16B is not aligned on a 16-byte boundary.</td></tr>
<tr>
<td>If CPUID.01H:ECX.CMPXCHG16B[bit 13] = 0.</td></tr>
<tr>
<td>#UD</td>
<td>If the destination operand is not a memory location.</td></tr>
<tr>
<td>#PF(fault-code)</td>
<td>If a page fault occurs.</td></tr>
<tr>
<td>#AC(0)</td>
<td>If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.</td></tr></table><footer><p>
This UNOFFICIAL, mechanically-separated, non-verified reference is provided for convenience, but it may be
inc<span style="opacity: 0.2">omp</span>lete or b<sub>r</sub>oke<sub>n</sub> in various obvious or non-obvious
ways. Refer to <a href="https://software.intel.com/en-us/download/intel-64-and-ia-32-architectures-sdm-combined-volumes-1-2a-2b-2c-2d-3a-3b-3c-3d-and-4">Intel® 64 and IA-32 Architectures Software Developer’s Manual</a> for anything serious.
</p></footer></body></html>
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