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<h4 class="subsection">9.15.17 Notes</h4>
<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>’
instructions that deserves mention. The 16-, 32-, 64- and 128-bit expanding
multiplies (base opcode ‘<samp><span class="samp">0xf6</span></samp>’; extension 4 for ‘<samp><span class="samp">mul</span></samp>’ and 5
for ‘<samp><span class="samp">imul</span></samp>’) can be output only in the one operand form. Thus,
‘<samp><span class="samp">imul %ebx, %eax</span></samp>’ does <em>not</em> select the expanding multiply;
the expanding multiply would clobber the ‘<samp><span class="samp">%edx</span></samp>’ register, and this
would confuse <code>gcc</code> output. Use ‘<samp><span class="samp">imul %ebx</span></samp>’ to get the
64-bit product in ‘<samp><span class="samp">%edx:%eax</span></samp>’.
<p>We have added a two operand form of ‘<samp><span class="samp">imul</span></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><span class="samp">%eax</span></samp>’ by 69, for
example, can be done with ‘<samp><span class="samp">imul $69, %eax</span></samp>’ rather than ‘<samp><span class="samp">imul
$69, %eax, %eax</span></samp>’.
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