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69 lines
3.8 KiB
HTML
69 lines
3.8 KiB
HTML
4 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) 1991-2015 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 1.3
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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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</head>
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<body>
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<div class="node">
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<a name="i386-Notes"></a>
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<a name="i386_002dNotes"></a>
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<p>
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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>
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</div>
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<h4 class="subsection">9.15.17 Notes</h4>
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<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>’
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instructions that deserves mention. The 16-, 32-, 64- and 128-bit expanding
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multiplies (base opcode ‘<samp><span class="samp">0xf6</span></samp>’; extension 4 for ‘<samp><span class="samp">mul</span></samp>’ and 5
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for ‘<samp><span class="samp">imul</span></samp>’) can be output only in the one operand form. Thus,
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‘<samp><span class="samp">imul %ebx, %eax</span></samp>’ does <em>not</em> select the expanding multiply;
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the expanding multiply would clobber the ‘<samp><span class="samp">%edx</span></samp>’ register, and this
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would confuse <code>gcc</code> output. Use ‘<samp><span class="samp">imul %ebx</span></samp>’ to get the
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64-bit product in ‘<samp><span class="samp">%edx:%eax</span></samp>’.
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<p>We have added a two operand form of ‘<samp><span class="samp">imul</span></samp>’ 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 ‘<samp><span class="samp">%eax</span></samp>’ by 69, for
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example, can be done with ‘<samp><span class="samp">imul $69, %eax</span></samp>’ rather than ‘<samp><span class="samp">imul
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$69, %eax, %eax</span></samp>’.
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<!-- Copyright (C) 2000-2015 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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