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72 lines
3.5 KiB
HTML
72 lines
3.5 KiB
HTML
15 years ago
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<title>i386-Jumps - 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_002dMemory.html#i386_002dMemory" title="i386-Memory">
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<link rel="next" href="i386_002dFloat.html#i386_002dFloat" title="i386-Float">
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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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</head>
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<body>
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<div class="node">
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<p>
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<a name="i386_002dJumps"></a>Next: <a rel="next" accesskey="n" href="i386_002dFloat.html#i386_002dFloat">i386-Float</a>,
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Previous: <a rel="previous" accesskey="p" href="i386_002dMemory.html#i386_002dMemory">i386-Memory</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">9.13.9 Handling of Jump Instructions</h4>
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<p><a name="index-jump-optimization_002c-i386-919"></a><a name="index-i386-jump-optimization-920"></a><a name="index-jump-optimization_002c-x86_002d64-921"></a><a name="index-x86_002d64-jump-optimization-922"></a>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 <span class="samp">data16</span> instruction prefix), since the 80386
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insists upon masking <span class="samp">%eip</span> 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>Note that the <span class="samp">jcxz</span>, <span class="samp">jecxz</span>, <span class="samp">loop</span>, <span class="samp">loopz</span>,
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<span class="samp">loope</span>, <span class="samp">loopnz</span> and <span class="samp">loopne</span> 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 <span class="samp">jcxz foo</span>
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to
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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>
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</body></html>
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