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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 ‘<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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