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<title>FDECSTP: Decrement Stack-Top Pointer (x86 Instruction Set Reference)</title>
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<div class="main_container"><h1>x86 Instruction Set Reference</h1>
<title>FDECSTP: Decrement Stack-Top Pointer (x86 Instruction Set Reference)</title>
<h1>FDECSTP</h1>
<h2>Decrement Stack-Top Pointer</h2>
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<th>Opcode</th>
<th>Mnemonic</th>
<th>Description</th>
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<td class="grid"><code>D9 F6</code></td>
<td class="grid"><code>FDECSTP</code></td>
<td class="grid">Decrement TOP field in FPU status word.</td>
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<th>Description</th>
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<p>Subtracts one from the TOP field of the FPU status word (decrements the top-of-stack pointer).</p>
<p>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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<th>Operation</th>
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<pre><span class="keyword">if</span><span class="operator">(</span>Top <span class="operator">==</span> <span class="number">0</span><span class="operator">)</span> Top <span class="operator">=</span> <span class="number">7</span><span class="operator">;</span>
<span class="keyword">else</span> Top <span class="operator">=</span> Top <span class="operator">-</span> <span class="number">1</span><span class="operator">;</span>
</pre>
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<th>FPU flags affected</th>
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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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None.
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<th>Protected Mode Exceptions</th>
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<td class="instruction_set_reference_box">
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<tr><td><code>#NM</code></td><td>EM or TS in CR0 is set.</td></tr>
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<th>Real-Address Mode Exceptions</th>
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<tr><td><code>#NM</code></td><td>EM or TS in CR0 is set.</td></tr>
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<th>Virtual-8086 Mode Exceptions</th>
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<td class="instruction_set_reference_box">
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<table class="operations_table">
<tr><td><code>#NM</code></td><td>EM or TS in CR0 is set.</td></tr>
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</td>
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