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91 lines
4.4 KiB
HTML
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<title>Word - Using as</title>
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<link rel="up" href="Pseudo-Ops.html#Pseudo-Ops" title="Pseudo Ops">
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<link rel="prev" href="Weakref.html#Weakref" title="Weakref">
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<!--
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This file documents the GNU Assembler "as".
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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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Permission is granted to copy, distribute and/or modify this document
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<p>
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<a name="Word"></a>Next: <a rel="next" accesskey="n" href="Deprecated.html#Deprecated">Deprecated</a>,
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Previous: <a rel="previous" accesskey="p" href="Weakref.html#Weakref">Weakref</a>,
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Up: <a rel="up" accesskey="u" href="Pseudo-Ops.html#Pseudo-Ops">Pseudo Ops</a>
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<hr><br>
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</div>
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<h3 class="section">7.120 <code>.word </code><var>expressions</var></h3>
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<p><a name="index-_0040code_007bword_007d-directive-499"></a>This directive expects zero or more <var>expressions</var>, of any section,
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separated by commas.
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<p>The size of the number emitted, and its byte order,
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depend on what target computer the assembly is for.
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<!-- on amd29k, i960, sparc the "special treatment to support compilers" doesn't -->
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<!-- happen-32-bit addressability, period; no long/short jumps. -->
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<p><a name="index-difference-tables-altered-500"></a><a name="index-altered-difference-tables-501"></a><blockquote>
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<em>Warning: Special Treatment to support Compilers</em>
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</blockquote>
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<p>Machines with a 32-bit address space, but that do less than 32-bit
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addressing, require the following special treatment. If the machine of
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interest to you does 32-bit addressing (or doesn't require it;
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see <a href="Machine-Dependencies.html#Machine-Dependencies">Machine Dependencies</a>), you can ignore this issue.
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<p>In order to assemble compiler output into something that works,
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<span class="command">as</span> occasionally does strange things to <span class="samp">.word</span> directives.
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Directives of the form <span class="samp">.word sym1-sym2</span> are often emitted by
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compilers as part of jump tables. Therefore, when <span class="command">as</span> assembles a
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directive of the form <span class="samp">.word sym1-sym2</span>, and the difference between
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<code>sym1</code> and <code>sym2</code> does not fit in 16 bits, <span class="command">as</span>
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creates a <dfn>secondary jump table</dfn>, immediately before the next label.
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This secondary jump table is preceded by a short-jump to the
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first byte after the secondary table. This short-jump prevents the flow
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of control from accidentally falling into the new table. Inside the
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table is a long-jump to <code>sym2</code>. The original <span class="samp">.word</span>
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contains <code>sym1</code> minus the address of the long-jump to
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<code>sym2</code>.
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<p>If there were several occurrences of <span class="samp">.word sym1-sym2</span> before the
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secondary jump table, all of them are adjusted. If there was a
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<span class="samp">.word sym3-sym4</span>, that also did not fit in sixteen bits, a
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long-jump to <code>sym4</code> is included in the secondary jump table,
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and the <code>.word</code> directives are adjusted to contain <code>sym3</code>
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minus the address of the long-jump to <code>sym4</code>; and so on, for as many
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entries in the original jump table as necessary.
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