104 lines
4.7 KiB
HTML
104 lines
4.7 KiB
HTML
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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) 1991-2016 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 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Using as: i386-Jumps</title>
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<meta name="description" content="Using as: i386-Jumps">
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<meta name="keywords" content="Using as: i386-Jumps">
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<meta name="resource-type" content="document">
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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="i386_002dFloat.html#i386_002dFloat" rel="next" title="i386-Float">
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<link href="i386_002dMemory.html#i386_002dMemory" rel="prev" title="i386-Memory">
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-->
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</style>
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="i386_002dJumps"></a>
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<div class="header">
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<p>
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Next: <a href="i386_002dFloat.html#i386_002dFloat" accesskey="n" rel="next">i386-Float</a>, Previous: <a href="i386_002dMemory.html#i386_002dMemory" accesskey="p" rel="prev">i386-Memory</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="Handling-of-Jump-Instructions"></a>
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<h4 class="subsection">9.15.8 Handling of Jump Instructions</h4>
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<a name="index-jump-optimization_002c-i386"></a>
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<a name="index-i386-jump-optimization"></a>
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<a name="index-jump-optimization_002c-x86_002d64"></a>
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<a name="index-x86_002d64-jump-optimization"></a>
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<p>Jump instructions are always optimized to use the smallest possible
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displacements. This is accomplished by using byte (8-bit) displacement
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jumps whenever the target is sufficiently close. If a byte displacement
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is insufficient a long displacement is used. We do not support
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word (16-bit) displacement jumps in 32-bit mode (i.e. prefixing the jump
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instruction with the ‘<samp>data16</samp>’ instruction prefix), since the 80386
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insists upon masking ‘<samp>%eip</samp>’ to 16 bits after the word displacement
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is added. (See also see <a href="i386_002dArch.html#i386_002dArch">i386-Arch</a>)
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</p>
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<p>Note that the ‘<samp>jcxz</samp>’, ‘<samp>jecxz</samp>’, ‘<samp>loop</samp>’, ‘<samp>loopz</samp>’,
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‘<samp>loope</samp>’, ‘<samp>loopnz</samp>’ and ‘<samp>loopne</samp>’ instructions only come in byte
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displacements, so that if you use these instructions (<code>gcc</code> does
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not use them) you may get an error message (and incorrect code). The AT&T
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80386 assembler tries to get around this problem by expanding ‘<samp>jcxz foo</samp>’
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to
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</p>
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<div class="smallexample">
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<pre class="smallexample"> jcxz cx_zero
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jmp cx_nonzero
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cx_zero: jmp foo
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cx_nonzero:
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</pre></div>
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</body>
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</html>
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