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<a name="i386-Notes"></a>
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<a name="i386_002dNotes"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="i386_002dBugs.html#i386_002dBugs">i386-Bugs</a>,
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Up: <a rel="up" accesskey="u" href="i386_002dDependent.html#i386_002dDependent">i386-Dependent</a>
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<h4 class="subsection">9.15.17 Notes</h4>
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<p><a name="index-i386-_0040code_007bmul_007d_002c-_0040code_007bimul_007d-instructions-1159"></a><a name="index-g_t_0040code_007bmul_007d-instruction_002c-i386-1160"></a><a name="index-g_t_0040code_007bimul_007d-instruction_002c-i386-1161"></a><a name="index-g_t_0040code_007bmul_007d-instruction_002c-x86_002d64-1162"></a><a name="index-g_t_0040code_007bimul_007d-instruction_002c-x86_002d64-1163"></a>There is some trickery concerning the ‘<samp><span class="samp">mul</span></samp>’ and ‘<samp><span class="samp">imul</span></samp>’
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instructions that deserves mention. The 16-, 32-, 64- and 128-bit expanding
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multiplies (base opcode ‘<samp><span class="samp">0xf6</span></samp>’; extension 4 for ‘<samp><span class="samp">mul</span></samp>’ and 5
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for ‘<samp><span class="samp">imul</span></samp>’) can be output only in the one operand form. Thus,
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‘<samp><span class="samp">imul %ebx, %eax</span></samp>’ does <em>not</em> select the expanding multiply;
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the expanding multiply would clobber the ‘<samp><span class="samp">%edx</span></samp>’ register, and this
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would confuse <code>gcc</code> output. Use ‘<samp><span class="samp">imul %ebx</span></samp>’ to get the
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64-bit product in ‘<samp><span class="samp">%edx:%eax</span></samp>’.
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<p>We have added a two operand form of ‘<samp><span class="samp">imul</span></samp>’ when the first operand
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is an immediate mode expression and the second operand is a register.
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This is just a shorthand, so that, multiplying ‘<samp><span class="samp">%eax</span></samp>’ by 69, for
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example, can be done with ‘<samp><span class="samp">imul $69, %eax</span></samp>’ rather than ‘<samp><span class="samp">imul
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$69, %eax, %eax</span></samp>’.
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