71 lines
3.5 KiB
HTML
71 lines
3.5 KiB
HTML
<html lang="en">
|
|
<head>
|
|
<title>i386-Jumps - Using as</title>
|
|
<meta http-equiv="Content-Type" content="text/html">
|
|
<meta name="description" content="Using as">
|
|
<meta name="generator" content="makeinfo 4.7">
|
|
<link title="Top" rel="start" href="index.html#Top">
|
|
<link rel="up" href="i386_002dDependent.html#i386_002dDependent" title="i386-Dependent">
|
|
<link rel="prev" href="i386_002dMemory.html#i386_002dMemory" title="i386-Memory">
|
|
<link rel="next" href="i386_002dFloat.html#i386_002dFloat" title="i386-Float">
|
|
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
|
<!--
|
|
This file documents the GNU Assembler "as".
|
|
|
|
Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001, 2002,
|
|
2006, 2007 Free Software Foundation, Inc.
|
|
|
|
Permission is granted to copy, distribute and/or modify this document
|
|
under the terms of the GNU Free Documentation License, Version 1.1
|
|
or any later version published by the Free Software Foundation;
|
|
with no Invariant Sections, with no Front-Cover Texts, and with no
|
|
Back-Cover Texts. A copy of the license is included in the
|
|
section entitled ``GNU Free Documentation License''.
|
|
|
|
man end-->
|
|
<meta http-equiv="Content-Style-Type" content="text/css">
|
|
<style type="text/css"><!--
|
|
pre.display { font-family:inherit }
|
|
pre.format { font-family:inherit }
|
|
pre.smalldisplay { font-family:inherit; font-size:smaller }
|
|
pre.smallformat { font-family:inherit; font-size:smaller }
|
|
pre.smallexample { font-size:smaller }
|
|
pre.smalllisp { font-size:smaller }
|
|
span.sc { font-variant:small-caps }
|
|
span.roman { font-family: serif; font-weight: normal; }
|
|
--></style>
|
|
</head>
|
|
<body>
|
|
<div class="node">
|
|
<p>
|
|
<a name="i386_002dJumps"></a>Next: <a rel="next" accesskey="n" href="i386_002dFloat.html#i386_002dFloat">i386-Float</a>,
|
|
Previous: <a rel="previous" accesskey="p" href="i386_002dMemory.html#i386_002dMemory">i386-Memory</a>,
|
|
Up: <a rel="up" accesskey="u" href="i386_002dDependent.html#i386_002dDependent">i386-Dependent</a>
|
|
<hr><br>
|
|
</div>
|
|
|
|
<h4 class="subsection">9.13.9 Handling of Jump Instructions</h4>
|
|
|
|
<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
|
|
displacements. This is accomplished by using byte (8-bit) displacement
|
|
jumps whenever the target is sufficiently close. If a byte displacement
|
|
is insufficient a long displacement is used. We do not support
|
|
word (16-bit) displacement jumps in 32-bit mode (i.e. prefixing the jump
|
|
instruction with the <span class="samp">data16</span> instruction prefix), since the 80386
|
|
insists upon masking <span class="samp">%eip</span> to 16 bits after the word displacement
|
|
is added. (See also see <a href="i386_002dArch.html#i386_002dArch">i386-Arch</a>)
|
|
|
|
<p>Note that the <span class="samp">jcxz</span>, <span class="samp">jecxz</span>, <span class="samp">loop</span>, <span class="samp">loopz</span>,
|
|
<span class="samp">loope</span>, <span class="samp">loopnz</span> and <span class="samp">loopne</span> instructions only come in byte
|
|
displacements, so that if you use these instructions (<code>gcc</code> does
|
|
not use them) you may get an error message (and incorrect code). The AT&T
|
|
80386 assembler tries to get around this problem by expanding <span class="samp">jcxz foo</span>
|
|
to
|
|
|
|
<pre class="smallexample"> jcxz cx_zero
|
|
jmp cx_nonzero
|
|
cx_zero: jmp foo
|
|
cx_nonzero:
|
|
</pre>
|
|
</body></html>
|
|
|