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79 lines
3.9 KiB
HTML
79 lines
3.9 KiB
HTML
4 years ago
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<html lang="en">
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<title>Xtensa Opcodes - Using as</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="Using as">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Xtensa-Syntax.html#Xtensa-Syntax" title="Xtensa Syntax">
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<link rel="next" href="Xtensa-Registers.html#Xtensa-Registers" title="Xtensa Registers">
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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) 1991-2015 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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<style type="text/css"><!--
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span.sansserif { font-family:sans-serif; font-weight:normal; }
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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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<a name="Xtensa-Opcodes"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Xtensa-Registers.html#Xtensa-Registers">Xtensa Registers</a>,
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Up: <a rel="up" accesskey="u" href="Xtensa-Syntax.html#Xtensa-Syntax">Xtensa Syntax</a>
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<hr>
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</div>
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<h5 class="subsubsection">9.52.2.1 Opcode Names</h5>
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<p><a name="index-Xtensa-opcode-names-2453"></a><a name="index-opcode-names_002c-Xtensa-2454"></a>
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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><a name="index-g_t_005f-opcode-prefix-2455"></a>If an opcode name is prefixed with an underscore character (‘<samp><span class="samp">_</span></samp>’),
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<samp><span class="command">as</span></samp> 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>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>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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</body></html>
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