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98 lines
4.3 KiB
HTML
98 lines
4.3 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- This file documents the GNU Assembler "as".
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Copyright (C) 1991-2019 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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-->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>i386-Notes (Using as)</title>
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<meta name="description" content="i386-Notes (Using as)">
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<meta name="keywords" content="i386-Notes (Using as)">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="AS-Index.html#AS-Index" rel="index" title="AS Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="i386_002dDependent.html#i386_002dDependent" rel="up" title="i386-Dependent">
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<link href="IA_002d64_002dDependent.html#IA_002d64_002dDependent" rel="next" title="IA-64-Dependent">
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<link href="i386_002dBugs.html#i386_002dBugs" rel="prev" title="i386-Bugs">
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</style>
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</head>
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<body lang="en">
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<a name="i386_002dNotes"></a>
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<div class="header">
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<p>
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Previous: <a href="i386_002dBugs.html#i386_002dBugs" accesskey="p" rel="prev">i386-Bugs</a>, Up: <a href="i386_002dDependent.html#i386_002dDependent" accesskey="u" rel="up">i386-Dependent</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Notes-2"></a>
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<h4 class="subsection">9.15.17 Notes</h4>
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<a name="index-i386-mul_002c-imul-instructions"></a>
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<a name="index-mul-instruction_002c-i386"></a>
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<a name="index-imul-instruction_002c-i386"></a>
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<a name="index-mul-instruction_002c-x86_002d64"></a>
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<a name="index-imul-instruction_002c-x86_002d64"></a>
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<p>There is some trickery concerning the ‘<samp>mul</samp>’ and ‘<samp>imul</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>0xf6</samp>’; extension 4 for ‘<samp>mul</samp>’ and 5
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for ‘<samp>imul</samp>’) can be output only in the one operand form. Thus,
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‘<samp>imul %ebx, %eax</samp>’ does <em>not</em> select the expanding multiply;
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the expanding multiply would clobber the ‘<samp>%edx</samp>’ register, and this
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would confuse <code>gcc</code> output. Use ‘<samp>imul %ebx</samp>’ to get the
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64-bit product in ‘<samp>%edx:%eax</samp>’.
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</p>
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<p>We have added a two operand form of ‘<samp>imul</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>%eax</samp>’ by 69, for
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example, can be done with ‘<samp>imul $69, %eax</samp>’ rather than ‘<samp>imul
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$69, %eax, %eax</samp>’.
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</p>
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</body>
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</html>
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