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107 lines
5.0 KiB
HTML
107 lines
5.0 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>Xtensa Opcodes (Using as)</title>
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<meta name="description" content="Xtensa Opcodes (Using as)">
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<meta name="keywords" content="Xtensa Opcodes (Using as)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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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="Xtensa-Syntax.html#Xtensa-Syntax" rel="up" title="Xtensa Syntax">
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<link href="Xtensa-Registers.html#Xtensa-Registers" rel="next" title="Xtensa Registers">
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<link href="Xtensa-Syntax.html#Xtensa-Syntax" rel="prev" title="Xtensa Syntax">
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<style type="text/css">
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Xtensa-Opcodes"></a>
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<div class="header">
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<p>
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Next: <a href="Xtensa-Registers.html#Xtensa-Registers" accesskey="n" rel="next">Xtensa Registers</a>, Up: <a href="Xtensa-Syntax.html#Xtensa-Syntax" accesskey="u" rel="up">Xtensa Syntax</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="Opcode-Names-2"></a>
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<h4 class="subsubsection">9.54.2.1 Opcode Names</h4>
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<a name="index-Xtensa-opcode-names"></a>
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<a name="index-opcode-names_002c-Xtensa"></a>
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<p>See the <cite>Xtensa Instruction Set Architecture (ISA) Reference
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Manual</cite> for a complete list of opcodes and descriptions of their
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semantics.
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</p>
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<a name="index-_005f-opcode-prefix"></a>
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<p>If an opcode name is prefixed with an underscore character (‘<samp>_</samp>’),
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<code>as</code> will not transform that instruction in any way. The
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underscore prefix disables both optimization (see <a href="Xtensa-Optimizations.html#Xtensa-Optimizations">Xtensa Optimizations</a>) and relaxation (see <a href="Xtensa-Relaxation.html#Xtensa-Relaxation">Xtensa Relaxation</a>) for that particular instruction. Only
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use the underscore prefix when it is essential to select the exact
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opcode produced by the assembler. Using this feature unnecessarily
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makes the code less efficient by disabling assembler optimization and
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less flexible by disabling relaxation.
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</p>
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<p>Note that this special handling of underscore prefixes only applies to
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Xtensa opcodes, not to either built-in macros or user-defined macros.
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When an underscore prefix is used with a macro (e.g., <code>_MOV</code>), it
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refers to a different macro. The assembler generally provides built-in
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macros both with and without the underscore prefix, where the underscore
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versions behave as if the underscore carries through to the instructions
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in the macros. For example, <code>_MOV</code> may expand to <code>_MOV.N</code>.
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</p>
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<p>The underscore prefix only applies to individual instructions, not to
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series of instructions. For example, if a series of instructions have
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underscore prefixes, the assembler will not transform the individual
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instructions, but it may insert other instructions between them (e.g.,
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to align a <code>LOOP</code> instruction). To prevent the assembler from
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modifying a series of instructions as a whole, use the
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<code>no-transform</code> directive. See <a href="Transform-Directive.html#Transform-Directive">transform</a>.
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</p>
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</body>
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</html>
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