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<h4 class="subsection">9.13.15 Notes</h4>
<p><a name="index-i386-_0040code_007bmul_007d_002c-_0040code_007bimul_007d-instructions-961"></a><a name="index-_0040code_007bmul_007d-instruction_002c-i386-962"></a><a name="index-_0040code_007bimul_007d-instruction_002c-i386-963"></a><a name="index-_0040code_007bmul_007d-instruction_002c-x86_002d64-964"></a><a name="index-_0040code_007bimul_007d-instruction_002c-x86_002d64-965"></a>There is some trickery concerning the <span class="samp">mul</span> and <span class="samp">imul</span>
instructions that deserves mention. The 16-, 32-, 64- and 128-bit expanding
multiplies (base opcode <span class="samp">0xf6</span>; extension 4 for <span class="samp">mul</span> and 5
for <span class="samp">imul</span>) can be output only in the one operand form. Thus,
<span class="samp">imul %ebx, %eax</span> does <em>not</em> select the expanding multiply;
the expanding multiply would clobber the <span class="samp">%edx</span> register, and this
would confuse <code>gcc</code> output. Use <span class="samp">imul %ebx</span> to get the
64-bit product in <span class="samp">%edx:%eax</span>.
<p>We have added a two operand form of <span class="samp">imul</span> when the first operand
is an immediate mode expression and the second operand is a register.
This is just a shorthand, so that, multiplying <span class="samp">%eax</span> by 69, for
example, can be done with <span class="samp">imul $69, %eax</span> rather than <span class="samp">imul
$69, %eax, %eax</span>.
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