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3.9 KiB
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77 lines
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<title>Xtensa Opcodes - Using as</title>
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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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<p>
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<a name="Xtensa-Opcodes"></a>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><br>
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</div>
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<h5 class="subsubsection">9.36.2.1 Opcode Names</h5>
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<p><a name="index-Xtensa-opcode-names-1851"></a><a name="index-opcode-names_002c-Xtensa-1852"></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-_005f-opcode-prefix-1853"></a>If an opcode name is prefixed with an underscore character (<span class="samp">_</span>),
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<span class="command">as</span> 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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