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| <a name="Notes-3"></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> |
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| <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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