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16<title>Using as: Xtensa Opcodes</title>
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65<p>
66Next: <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> &nbsp; [<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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69<a name="Opcode-Names-2"></a>
70<h4 class="subsubsection">9.55.2.1 Opcode Names</h4>
71<a name="index-Xtensa-opcode-names"></a>
72<a name="index-opcode-names_002c-Xtensa"></a>
73
74<p>See the <cite>Xtensa Instruction Set Architecture (ISA) Reference
75Manual</cite> for a complete list of opcodes and descriptions of their
76semantics.
77</p>
78<a name="index-_005f-opcode-prefix"></a>
79<p>If an opcode name is prefixed with an underscore character (&lsquo;<samp>_</samp>&rsquo;),
80<code>as</code> will not transform that instruction in any way.  The
81underscore 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
82use the underscore prefix when it is essential to select the exact
83opcode produced by the assembler.  Using this feature unnecessarily
84makes the code less efficient by disabling assembler optimization and
85less flexible by disabling relaxation.
86</p>
87<p>Note that this special handling of underscore prefixes only applies to
88Xtensa opcodes, not to either built-in macros or user-defined macros.
89When an underscore prefix is used with a macro (e.g., <code>_MOV</code>), it
90refers to a different macro.  The assembler generally provides built-in
91macros both with and without the underscore prefix, where the underscore
92versions behave as if the underscore carries through to the instructions
93in the macros.  For example, <code>_MOV</code> may expand to <code>_MOV.N</code>.
94</p>
95<p>The underscore prefix only applies to individual instructions, not to
96series of instructions.  For example, if a series of instructions have
97underscore prefixes, the assembler will not transform the individual
98instructions, but it may insert other instructions between them (e.g.,
99to align a <code>LOOP</code> instruction).  To prevent the assembler from
100modifying a series of instructions as a whole, use the
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