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152 lines
7.3 KiB
HTML
152 lines
7.3 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-Prefixes (Using as)</title>
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<meta name="description" content="i386-Prefixes (Using as)">
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<meta name="keywords" content="i386-Prefixes (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="i386_002dMemory.html#i386_002dMemory" rel="next" title="i386-Memory">
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<link href="i386_002dRegs.html#i386_002dRegs" rel="prev" title="i386-Regs">
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</head>
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<body lang="en">
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<a name="i386_002dPrefixes"></a>
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<div class="header">
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<p>
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Next: <a href="i386_002dMemory.html#i386_002dMemory" accesskey="n" rel="next">i386-Memory</a>, Previous: <a href="i386_002dRegs.html#i386_002dRegs" accesskey="p" rel="prev">i386-Regs</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="Instruction-Prefixes"></a>
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<h4 class="subsection">9.15.6 Instruction Prefixes</h4>
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<a name="index-i386-instruction-prefixes"></a>
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<a name="index-instruction-prefixes_002c-i386"></a>
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<a name="index-prefixes_002c-i386"></a>
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<p>Instruction prefixes are used to modify the following instruction. They
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are used to repeat string instructions, to provide section overrides, to
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perform bus lock operations, and to change operand and address sizes.
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(Most instructions that normally operate on 32-bit operands will use
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16-bit operands if the instruction has an “operand size” prefix.)
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Instruction prefixes are best written on the same line as the instruction
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they act upon. For example, the ‘<samp>scas</samp>’ (scan string) instruction is
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repeated with:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> repne scas %es:(%edi),%al
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</pre></div>
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<p>You may also place prefixes on the lines immediately preceding the
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instruction, but this circumvents checks that <code>as</code> does
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with prefixes, and will not work with all prefixes.
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</p>
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<p>Here is a list of instruction prefixes:
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</p>
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<a name="index-section-override-prefixes_002c-i386"></a>
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<ul>
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<li> Section override prefixes ‘<samp>cs</samp>’, ‘<samp>ds</samp>’, ‘<samp>ss</samp>’, ‘<samp>es</samp>’,
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‘<samp>fs</samp>’, ‘<samp>gs</samp>’. These are automatically added by specifying
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using the <var>section</var>:<var>memory-operand</var> form for memory references.
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</li><li> <a name="index-size-prefixes_002c-i386"></a>
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Operand/Address size prefixes ‘<samp>data16</samp>’ and ‘<samp>addr16</samp>’
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change 32-bit operands/addresses into 16-bit operands/addresses,
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while ‘<samp>data32</samp>’ and ‘<samp>addr32</samp>’ change 16-bit ones (in a
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<code>.code16</code> section) into 32-bit operands/addresses. These prefixes
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<em>must</em> appear on the same line of code as the instruction they
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modify. For example, in a 16-bit <code>.code16</code> section, you might
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write:
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<div class="smallexample">
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<pre class="smallexample"> addr32 jmpl *(%ebx)
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</pre></div>
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</li><li> <a name="index-bus-lock-prefixes_002c-i386"></a>
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<a name="index-inhibiting-interrupts_002c-i386"></a>
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The bus lock prefix ‘<samp>lock</samp>’ inhibits interrupts during execution of
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the instruction it precedes. (This is only valid with certain
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instructions; see a 80386 manual for details).
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</li><li> <a name="index-coprocessor-wait_002c-i386"></a>
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The wait for coprocessor prefix ‘<samp>wait</samp>’ waits for the coprocessor to
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complete the current instruction. This should never be needed for the
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80386/80387 combination.
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</li><li> <a name="index-repeat-prefixes_002c-i386"></a>
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The ‘<samp>rep</samp>’, ‘<samp>repe</samp>’, and ‘<samp>repne</samp>’ prefixes are added
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to string instructions to make them repeat ‘<samp>%ecx</samp>’ times (‘<samp>%cx</samp>’
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times if the current address size is 16-bits).
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</li><li> <a name="index-REX-prefixes_002c-i386"></a>
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The ‘<samp>rex</samp>’ family of prefixes is used by x86-64 to encode
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extensions to i386 instruction set. The ‘<samp>rex</samp>’ prefix has four
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bits — an operand size overwrite (<code>64</code>) used to change operand size
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from 32-bit to 64-bit and X, Y and Z extensions bits used to extend the
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register set.
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<p>You may write the ‘<samp>rex</samp>’ prefixes directly. The ‘<samp>rex64xyz</samp>’
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instruction emits ‘<samp>rex</samp>’ prefix with all the bits set. By omitting
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the <code>64</code>, <code>x</code>, <code>y</code> or <code>z</code> you may write other
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prefixes as well. Normally, there is no need to write the prefixes
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explicitly, since gas will automatically generate them based on the
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instruction operands.
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</p></li></ul>
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<hr>
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<div class="header">
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<p>
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Next: <a href="i386_002dMemory.html#i386_002dMemory" accesskey="n" rel="next">i386-Memory</a>, Previous: <a href="i386_002dRegs.html#i386_002dRegs" accesskey="p" rel="prev">i386-Regs</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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