forked from NRZCode/ia32-64
72 lines
3.3 KiB
HTML
72 lines
3.3 KiB
HTML
<!DOCTYPE html>
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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>FDECSTP
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— Decrement Stack-Top Pointer</title></head><body><header><nav><ul><li><a href='index.html'>Index</a></li><li>December 2023</li></ul></nav></header><h1>FDECSTP
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— Decrement Stack-Top Pointer</h1>
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<table>
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<tr>
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<th>Opcode </th>
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<th></th>
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<th>Mode</th>
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<th>Leg Mode</th>
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<th>Description</th></tr>
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<tr>
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<td>D9 F6 </td>
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<td></td>
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<td></td>
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<td></td>
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<td>Decrement TOP field in FPU status word.</td></tr></table>
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<h2 id="description">Description<a class="anchor" href="#description">
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¶
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</a></h2>
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<p>Subtracts one from the TOP field of the FPU status word (decrements the top-of-stack pointer). If the TOP field contains a 0, it is set to 7. The effect of this instruction is to rotate the stack by one position. The contents of the FPU data registers and tag register are not affected.</p>
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<p>This instruction’s operation is the same in non-64-bit modes and 64-bit mode.</p>
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<h2 id="operation">Operation<a class="anchor" href="#operation">
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¶
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</a></h2>
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<pre>IF TOP = 0
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THEN TOP := 7;
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ELSE TOP := TOP – 1;
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FI;
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</pre>
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<h2 id="fpu-flags-affected">FPU Flags Affected<a class="anchor" href="#fpu-flags-affected">
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¶
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</a></h2>
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<p>The C1 flag is set to 0. The C0, C2, and C3 flags are undefined.</p>
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<h2 class="exceptions" id="floating-point-exceptions">Floating-Point Exceptions<a class="anchor" href="#floating-point-exceptions">
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¶
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</a></h2>
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<p>None.</p>
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<h2 class="exceptions" id="protected-mode-exceptions">Protected Mode Exceptions<a class="anchor" href="#protected-mode-exceptions">
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¶
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</a></h2>
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<table>
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<tr>
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<td>#NM</td>
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<td>CR0.EM[bit 2] or CR0.TS[bit 3] = 1.</td></tr>
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<tr>
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<td>#MF</td>
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<td>If there is a pending x87 FPU exception.</td></tr>
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<tr>
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<td>#UD</td>
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<td>If the LOCK prefix is used.</td></tr></table>
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<h2 class="exceptions" id="real-address-mode-exceptions">Real-Address Mode Exceptions<a class="anchor" href="#real-address-mode-exceptions">
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¶
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</a></h2>
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<p>Same exceptions as in protected mode.</p>
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<h2 class="exceptions" id="virtual-8086-mode-exceptions">Virtual-8086 Mode Exceptions<a class="anchor" href="#virtual-8086-mode-exceptions">
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¶
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</a></h2>
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<p>Same exceptions as in protected mode.</p>
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<h2 class="exceptions" id="compatibility-mode-exceptions">Compatibility Mode Exceptions<a class="anchor" href="#compatibility-mode-exceptions">
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¶
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</a></h2>
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<p>Same exceptions as in protected mode.</p>
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<h2 class="exceptions" id="64-bit-mode-exceptions">64-Bit Mode Exceptions<a class="anchor" href="#64-bit-mode-exceptions">
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¶
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</a></h2>
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<p>Same exceptions as in protected mode.</p><footer><p>
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This UNOFFICIAL, mechanically-separated, non-verified reference is provided for convenience, but it may be
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inc<span style="opacity: 0.2">omp</span>lete or b<sub>r</sub>oke<sub>n</sub> in various obvious or non-obvious
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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.
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</p></footer></body></html>
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