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Literal-Position-Directive.html 5.1KB

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  15. <title>Literal Position Directive (Using as)</title>
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  59. Next: <a href="Literal-Prefix-Directive.html#Literal-Prefix-Directive" accesskey="n" rel="next">Literal Prefix Directive</a>, Previous: <a href="Literal-Directive.html#Literal-Directive" accesskey="p" rel="prev">Literal Directive</a>, Up: <a href="Xtensa-Directives.html#Xtensa-Directives" accesskey="u" rel="up">Xtensa Directives</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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  62. <a name="literal_005fposition"></a>
  63. <h4 class="subsubsection">9.55.5.5 literal_position</h4>
  64. <a name="index-literal_005fposition-directive"></a>
  65. <p>When using &lsquo;<samp>--text-section-literals</samp>&rsquo; to place literals inline
  66. in the section being assembled, the <code>.literal_position</code> directive
  67. can be used to mark a potential location for a literal pool.
  68. </p>
  69. <div class="smallexample">
  70. <pre class="smallexample"> .literal_position
  71. </pre></div>
  72. <p>The <code>.literal_position</code> directive is ignored when the
  73. &lsquo;<samp>--text-section-literals</samp>&rsquo; option is not used or when
  74. <code>L32R</code> instructions use the absolute addressing mode.
  75. </p>
  76. <p>The assembler will automatically place text section literal pools
  77. before <code>ENTRY</code> instructions, so the <code>.literal_position</code>
  78. directive is only needed to specify some other location for a literal
  79. pool. You may need to add an explicit jump instruction to skip over an
  80. inline literal pool.
  81. </p>
  82. <p>For example, an interrupt vector does not begin with an <code>ENTRY</code>
  83. instruction so the assembler will be unable to automatically find a good
  84. place to put a literal pool. Moreover, the code for the interrupt
  85. vector must be at a specific starting address, so the literal pool
  86. cannot come before the start of the code. The literal pool for the
  87. vector must be explicitly positioned in the middle of the vector (before
  88. any uses of the literals, due to the negative offsets used by
  89. PC-relative <code>L32R</code> instructions). The <code>.literal_position</code>
  90. directive can be used to do this. In the following code, the literal
  91. for &lsquo;<samp>M</samp>&rsquo; will automatically be aligned correctly and is placed after
  92. the unconditional jump.
  93. </p>
  94. <div class="smallexample">
  95. <pre class="smallexample"> .global M
  96. code_start:
  97. </pre><pre class="smallexample"> j continue
  98. .literal_position
  99. .align 4
  100. </pre><pre class="smallexample">continue:
  101. movi a4, M
  102. </pre></div>
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