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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>SGDT
— Store Global Descriptor Table Register</title></head><body><header><nav><ul><li><a href='index.html'>Index</a></li><li>December 2023</li></ul></nav></header><h1>SGDT
— Store Global Descriptor Table Register</h1>
<p><strong>Opcode<sup>1</sup></strong></p>
<table>
<tr>
<th></th>
<th>Instruction</th>
<th>Op/En</th>
<th>64-Bit Mode</th>
<th>Compat/Leg Mode</th>
<th>Description</th></tr>
<tr>
<td>0F 01 /0</td>
<td></td>
<td></td>
<td>Valid</td>
<td>Valid</td>
<td>Store GDTR to <em>m.</em></td></tr></table>
<p>1. See the IA-32 Architecture Compatibility section below.</p>
<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 (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>Stores the content of the global descriptor table register (GDTR) in the destination operand. The destination operand specifies a memory location.</p>
<p>In legacy or compatibility mode, the destination operand is a 6-byte memory location. If the operand-size attribute is 16 or 32 bits, the 16-bit limit field of the register is stored in the low 2 bytes of the memory location and the 32-bit base address is stored in the high 4 bytes.</p>
<p>In 64-bit mode, the operand size is fixed at 8+2 bytes. The instruction stores an 8-byte base and a 2-byte limit.</p>
<p>SGDT is useful only by operating-system software. However, it can be used in application programs without causing an exception to be generated if CR4.UMIP = 0. See “LGDT/LIDT—Load Global/Interrupt Descriptor Table Register” in Chapter 3, Intel<sup>®</sup> 64 and IA-32 Architectures Software Developer’s Manual, Volume 2A, for information on loading the GDTR and IDTR.</p>
<h2 id="ia-32-architecture-compatibility">IA-32 Architecture Compatibility<a class="anchor" href="#ia-32-architecture-compatibility">
</a></h2>
<p>The 16-bit form of the SGDT is compatible with the Intel 286 processor if the upper 8 bits are not referenced. The Intel 286 processor fills these bits with 1s; processor generations later than the Intel 286 processor fill these bits with 0s.</p>
<h2 id="operation">Operation<a class="anchor" href="#operation">
</a></h2>
<pre>IF instruction is SGDT
IF OperandSize =16 or OperandSize = 32 (* Legacy or Compatibility Mode *)
THEN
DEST[0:15] := GDTR(Limit);
DEST[16:47] := GDTR(Base); (* Full 32-bit base address stored *)
FI;
ELSE (* 64-bit Mode *)
DEST[0:15] := GDTR(Limit);
DEST[16:79] := GDTR(Base); (* Full 64-bit base address stored *)
FI;
FI;
</pre>
<h2 id="flags-affected">Flags Affected<a class="anchor" href="#flags-affected">
</a></h2>
<p>None.</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 LOCK prefix is used.</td></tr>
<tr>
<td rowspan="4">#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 is used to access memory and it contains a NULL segment selector.</td></tr>
<tr>
<td>If CR4.UMIP = 1 and CPL &gt; 0.</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 CPL = 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 LOCK prefix is used.</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 LOCK prefix is used.</td></tr>
<tr>
<td rowspan="2">#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>If CR4.UMIP = 1.</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>#UD</td>
<td>If the LOCK prefix is used.</td></tr>
<tr>
<td rowspan="2">#GP(0)</td>
<td>If the memory address is in a non-canonical form.</td></tr>
<tr>
<td>If CR4.UMIP = 1 and CPL &gt; 0.</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 CPL = 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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