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<h4 class="subsection">9.16.17 Notes</h4>

<p><a name="index-i386-_0040code_007bmul_007d_002c-_0040code_007bimul_007d-instructions-1273"></a><a name="index-g_t_0040code_007bmul_007d-instruction_002c-i386-1274"></a><a name="index-g_t_0040code_007bimul_007d-instruction_002c-i386-1275"></a><a name="index-g_t_0040code_007bmul_007d-instruction_002c-x86_002d64-1276"></a><a name="index-g_t_0040code_007bimul_007d-instruction_002c-x86_002d64-1277"></a>There is some trickery concerning the &lsquo;<samp><span class="samp">mul</span></samp>&rsquo; and &lsquo;<samp><span class="samp">imul</span></samp>&rsquo;
instructions that deserves mention.  The 16-, 32-, 64- and 128-bit expanding
multiplies (base opcode &lsquo;<samp><span class="samp">0xf6</span></samp>&rsquo;; extension 4 for &lsquo;<samp><span class="samp">mul</span></samp>&rsquo; and 5
for &lsquo;<samp><span class="samp">imul</span></samp>&rsquo;) can be output only in the one operand form.  Thus,
&lsquo;<samp><span class="samp">imul %ebx, %eax</span></samp>&rsquo; does <em>not</em> select the expanding multiply;
the expanding multiply would clobber the &lsquo;<samp><span class="samp">%edx</span></samp>&rsquo; register, and this
would confuse <code>gcc</code> output.  Use &lsquo;<samp><span class="samp">imul %ebx</span></samp>&rsquo; to get the
64-bit product in &lsquo;<samp><span class="samp">%edx:%eax</span></samp>&rsquo;.

   <p>We have added a two operand form of &lsquo;<samp><span class="samp">imul</span></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><span class="samp">%eax</span></samp>&rsquo; by 69, for
example, can be done with &lsquo;<samp><span class="samp">imul $69, %eax</span></samp>&rsquo; rather than &lsquo;<samp><span class="samp">imul
$69, %eax, %eax</span></samp>&rsquo;.

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