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<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>DEC
— Decrement by 1</title></head><body><header><nav><ul><li><a href='index.html'>Index</a></li><li>December 2023</li></ul></nav></header><h1>DEC
— Decrement by 1</h1>
<table>
<tr>
<th>Opcode</th>
<th>Instruction</th>
<th>Op/En</th>
<th>64-Bit Mode</th>
<th>Compat/Leg Mode</th>
<th>Description</th></tr>
<tr>
<td>FE /1</td>
<td>DEC <em>r/m8</em></td>
<td>M</td>
<td>Valid</td>
<td>Valid</td>
<td>Decrement <em>r/m8</em> by 1.</td></tr>
<tr>
<td>REX + FE /1</td>
<td>DEC <em>r/m8</em><sup>*</sup></td>
<td>M</td>
<td>Valid</td>
<td>N.E.</td>
<td>Decrement <em>r/m8</em> by 1.</td></tr>
<tr>
<td>FF /1</td>
<td>DEC <em>r/m16</em></td>
<td>M</td>
<td>Valid</td>
<td>Valid</td>
<td>Decrement <em>r/m16</em> by 1.</td></tr>
<tr>
<td>FF /1</td>
<td>DEC <em>r/m32</em></td>
<td>M</td>
<td>Valid</td>
<td>Valid</td>
<td>Decrement <em>r/m32</em> by 1.</td></tr>
<tr>
<td>REX.W + FF /1</td>
<td>DEC <em>r/m64</em></td>
<td>M</td>
<td>Valid</td>
<td>N.E.</td>
<td>Decrement <em>r/m64</em> by 1.</td></tr>
<tr>
<td>48+rw</td>
<td>DEC <em>r16</em></td>
<td>O</td>
<td>N.E.</td>
<td>Valid</td>
<td>Decrement <em>r16</em> by 1.</td></tr>
<tr>
<td>48+rd</td>
<td>DEC <em>r32</em></td>
<td>O</td>
<td>N.E.</td>
<td>Valid</td>
<td>Decrement <em>r32</em> by 1.</td></tr></table>
<blockquote>
<p>* In64-bitmode,r/m8cannotbeencodedtoaccessthefollowingbyteregistersifaREXprefixisused:AH,BH,CH,DH.</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>
<tr>
<td>O</td>
<td>opcode + rd (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>Subtracts 1 from the destination operand, while preserving the state of the CF flag. The destination operand can be a register or a memory location. This instruction allows a loop counter to be updated without disturbing the CF flag. (To perform a decrement operation that updates the CF flag, use a SUB instruction with an immediate operand of 1.)</p>
<p>This instruction can be used with a LOCK prefix to allow the instruction to be executed atomically.</p>
<p>In 64-bit mode, DEC r16 and DEC r32 are not encodable (because opcodes 48H through 4FH are REX prefixes). Otherwise, the instructions 64-bit mode default operation size is 32 bits. Use of the REX.R prefix permits access to additional registers (R8-R15). Use of the REX.W prefix promotes operation to 64 bits.</p>
<p>See the summary chart at the beginning of this section for encoding data and limits.</p>
<h2 id="operation">Operation<a class="anchor" href="#operation">
</a></h2>
<pre>DEST := DEST 1;
</pre>
<h2 id="flags-affected">Flags Affected<a class="anchor" href="#flags-affected">
</a></h2>
<p>The CF flag is not affected. The OF, SF, ZF, AF, and PF flags are set according to the result.</p>
<h2 class="exceptions" id="protected-mode-exceptions">Protected Mode Exceptions<a class="anchor" href="#protected-mode-exceptions">
</a></h2>
<table>
<tr>
<td rowspan="3">#GP(0)</td>
<td>If the destination operand 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>
<tr>
<td>#UD</td>
<td>If the LOCK prefix is used but the destination is not a memory operand.</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>#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>
<tr>
<td>#UD</td>
<td>If the LOCK prefix is used but the destination is not a memory operand.</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>#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>
<tr>
<td>#UD</td>
<td>If the LOCK prefix is used but the destination is not a memory operand.</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>#GP(0)</td>
<td>If the memory address is in a non-canonical form.</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>
<tr>
<td>#UD</td>
<td>If the LOCK prefix is used but the destination is not a memory operand.</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 Developers Manual</a> for anything serious.
</p></footer></body></html>