Xtensa-Opcodes.html 5.0 KB

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  14. <title>Using as: Xtensa Opcodes</title>
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  60. 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> &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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  63. <a name="Opcode-Names-2"></a>
  64. <h4 class="subsubsection">9.53.2.1 Opcode Names</h4>
  65. <a name="index-Xtensa-opcode-names"></a>
  66. <a name="index-opcode-names_002c-Xtensa"></a>
  67. <p>See the <cite>Xtensa Instruction Set Architecture (ISA) Reference
  68. Manual</cite> for a complete list of opcodes and descriptions of their
  69. semantics.
  70. </p>
  71. <a name="index-_005f-opcode-prefix"></a>
  72. <p>If an opcode name is prefixed with an underscore character (&lsquo;<samp>_</samp>&rsquo;),
  73. <code>as</code> will not transform that instruction in any way. The
  74. 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
  75. use the underscore prefix when it is essential to select the exact
  76. opcode produced by the assembler. Using this feature unnecessarily
  77. makes the code less efficient by disabling assembler optimization and
  78. less flexible by disabling relaxation.
  79. </p>
  80. <p>Note that this special handling of underscore prefixes only applies to
  81. Xtensa opcodes, not to either built-in macros or user-defined macros.
  82. When an underscore prefix is used with a macro (e.g., <code>_MOV</code>), it
  83. refers to a different macro. The assembler generally provides built-in
  84. macros both with and without the underscore prefix, where the underscore
  85. versions behave as if the underscore carries through to the instructions
  86. in the macros. For example, <code>_MOV</code> may expand to <code>_MOV.N</code>.
  87. </p>
  88. <p>The underscore prefix only applies to individual instructions, not to
  89. series of instructions. For example, if a series of instructions have
  90. underscore prefixes, the assembler will not transform the individual
  91. instructions, but it may insert other instructions between them (e.g.,
  92. to align a <code>LOOP</code> instruction). To prevent the assembler from
  93. modifying a series of instructions as a whole, use the
  94. <code>no-transform</code> directive. See <a href="Transform-Directive.html#Transform-Directive">transform</a>.
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