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67 lines
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HTML
67 lines
3.5 KiB
HTML
15 years ago
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<title>i386-Notes - Using as</title>
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<meta name="description" content="Using as">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="i386_002dDependent.html#i386_002dDependent" title="i386-Dependent">
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<link rel="prev" href="i386_002dBugs.html#i386_002dBugs" title="i386-Bugs">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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This file documents the GNU Assembler "as".
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Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001, 2002,
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2006, 2007 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.1
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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Back-Cover Texts. A copy of the license is included in the
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section entitled ``GNU Free Documentation License''.
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man end-->
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--></style>
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<body>
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<div class="node">
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<p>
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<a name="i386_002dNotes"></a>Previous: <a rel="previous" accesskey="p" href="i386_002dBugs.html#i386_002dBugs">i386-Bugs</a>,
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Up: <a rel="up" accesskey="u" href="i386_002dDependent.html#i386_002dDependent">i386-Dependent</a>
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<hr><br>
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</div>
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<h4 class="subsection">9.13.15 Notes</h4>
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<p><a name="index-i386-_0040code_007bmul_007d_002c-_0040code_007bimul_007d-instructions-961"></a><a name="index-_0040code_007bmul_007d-instruction_002c-i386-962"></a><a name="index-_0040code_007bimul_007d-instruction_002c-i386-963"></a><a name="index-_0040code_007bmul_007d-instruction_002c-x86_002d64-964"></a><a name="index-_0040code_007bimul_007d-instruction_002c-x86_002d64-965"></a>There is some trickery concerning the <span class="samp">mul</span> and <span class="samp">imul</span>
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instructions that deserves mention. The 16-, 32-, 64- and 128-bit expanding
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multiplies (base opcode <span class="samp">0xf6</span>; extension 4 for <span class="samp">mul</span> and 5
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for <span class="samp">imul</span>) can be output only in the one operand form. Thus,
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<span class="samp">imul %ebx, %eax</span> does <em>not</em> select the expanding multiply;
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the expanding multiply would clobber the <span class="samp">%edx</span> register, and this
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would confuse <code>gcc</code> output. Use <span class="samp">imul %ebx</span> to get the
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64-bit product in <span class="samp">%edx:%eax</span>.
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<p>We have added a two operand form of <span class="samp">imul</span> when the first operand
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is an immediate mode expression and the second operand is a register.
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This is just a shorthand, so that, multiplying <span class="samp">%eax</span> by 69, for
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example, can be done with <span class="samp">imul $69, %eax</span> rather than <span class="samp">imul
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$69, %eax, %eax</span>.
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<!-- Copyright 2000, 2003 Free Software Foundation, Inc. -->
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<!-- This is part of the GAS manual. -->
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<!-- For copying conditions, see the file as.texinfo. -->
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</body></html>
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