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<a name="Notes-2"></a>
<h4 class="subsection">9.16.18 Notes</h4>

<a name="index-i386-mul_002c-imul-instructions"></a>
<a name="index-mul-instruction_002c-i386"></a>
<a name="index-imul-instruction_002c-i386"></a>
<a name="index-mul-instruction_002c-x86_002d64"></a>
<a name="index-imul-instruction_002c-x86_002d64"></a>
<p>There is some trickery concerning the &lsquo;<samp>mul</samp>&rsquo; and &lsquo;<samp>imul</samp>&rsquo;
instructions that deserves mention.  The 16-, 32-, 64- and 128-bit expanding
multiplies (base opcode &lsquo;<samp>0xf6</samp>&rsquo;; extension 4 for &lsquo;<samp>mul</samp>&rsquo; and 5
for &lsquo;<samp>imul</samp>&rsquo;) can be output only in the one operand form.  Thus,
&lsquo;<samp>imul %ebx, %eax</samp>&rsquo; does <em>not</em> select the expanding multiply;
the expanding multiply would clobber the &lsquo;<samp>%edx</samp>&rsquo; register, and this
would confuse <code>gcc</code> output.  Use &lsquo;<samp>imul %ebx</samp>&rsquo; to get the
64-bit product in &lsquo;<samp>%edx:%eax</samp>&rsquo;.
</p>
<p>We have added a two operand form of &lsquo;<samp>imul</samp>&rsquo; when the first operand
is an immediate mode expression and the second operand is a register.
This is just a shorthand, so that, multiplying &lsquo;<samp>%eax</samp>&rsquo; by 69, for
example, can be done with &lsquo;<samp>imul $69, %eax</samp>&rsquo; rather than &lsquo;<samp>imul
$69, %eax, %eax</samp>&rsquo;.
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