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Previous: <a href="i386_002dBugs.html#i386_002dBugs" accesskey="p" rel="prev">i386-Bugs</a>, Up: <a href="i386_002dDependent.html#i386_002dDependent" accesskey="u" rel="up">i386-Dependent</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Notes-2"></a>
<h4 class="subsection">9.15.17 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 ‘<samp>mul</samp>’ and ‘<samp>imul</samp>’
instructions that deserves mention. The 16-, 32-, 64- and 128-bit expanding
multiplies (base opcode ‘<samp>0xf6</samp>’; extension 4 for ‘<samp>mul</samp>’ and 5
for ‘<samp>imul</samp>’) can be output only in the one operand form. Thus,
‘<samp>imul %ebx, %eax</samp>’ does <em>not</em> select the expanding multiply;
the expanding multiply would clobber the ‘<samp>%edx</samp>’ register, and this
would confuse <code>gcc</code> output. Use ‘<samp>imul %ebx</samp>’ to get the
64-bit product in ‘<samp>%edx:%eax</samp>’.
</p>
<p>We have added a two operand form of ‘<samp>imul</samp>’ when the first operand
is an immediate mode expression and the second operand is a register.
This is just a shorthand, so that, multiplying ‘<samp>%eax</samp>’ by 69, for
example, can be done with ‘<samp>imul $69, %eax</samp>’ rather than ‘<samp>imul
$69, %eax, %eax</samp>’.
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