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122 lines
5.6 KiB
HTML
122 lines
5.6 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- This file documents the GNU Assembler "as".
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Copyright (C) 1991-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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Back-Cover Texts. A copy of the license is included in the
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section entitled "GNU Free Documentation License".
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-->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Word (Using as)</title>
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<meta name="description" content="Word (Using as)">
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<meta name="keywords" content="Word (Using as)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="AS-Index.html#AS-Index" rel="index" title="AS Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Pseudo-Ops.html#Pseudo-Ops" rel="up" title="Pseudo Ops">
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<link href="Zero.html#Zero" rel="next" title="Zero">
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<link href="Weakref.html#Weakref" rel="prev" title="Weakref">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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</style>
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</head>
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<body lang="en">
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<a name="Word"></a>
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<div class="header">
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<p>
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Next: <a href="Zero.html#Zero" accesskey="n" rel="next">Zero</a>, Previous: <a href="Weakref.html#Weakref" accesskey="p" rel="prev">Weakref</a>, Up: <a href="Pseudo-Ops.html#Pseudo-Ops" accesskey="u" rel="up">Pseudo Ops</a> [<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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</div>
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<hr>
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<a name="g_t_002eword-expressions"></a>
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<h3 class="section">7.107 <code>.word <var>expressions</var></code></h3>
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<a name="index-word-directive"></a>
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<p>This directive expects zero or more <var>expressions</var>, of any section,
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separated by commas.
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</p>
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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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</p>
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<a name="index-difference-tables-altered"></a>
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<a name="index-altered-difference-tables"></a>
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<blockquote>
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<p><em>Warning: Special Treatment to support Compilers</em>
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</p></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>
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<p>In order to assemble compiler output into something that works,
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<code>as</code> occasionally does strange things to ‘<samp>.word</samp>’ directives.
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Directives of the form ‘<samp>.word sym1-sym2</samp>’ are often emitted by
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compilers as part of jump tables. Therefore, when <code>as</code> assembles a
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directive of the form ‘<samp>.word sym1-sym2</samp>’, and the difference between
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<code>sym1</code> and <code>sym2</code> does not fit in 16 bits, <code>as</code>
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creates a <em>secondary jump table</em>, 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 ‘<samp>.word</samp>’
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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>
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<p>If there were several occurrences of ‘<samp>.word sym1-sym2</samp>’ before the
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secondary jump table, all of them are adjusted. If there was a
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‘<samp>.word sym3-sym4</samp>’, 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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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Zero.html#Zero" accesskey="n" rel="next">Zero</a>, Previous: <a href="Weakref.html#Weakref" accesskey="p" rel="prev">Weakref</a>, Up: <a href="Pseudo-Ops.html#Pseudo-Ops" accesskey="u" rel="up">Pseudo Ops</a> [<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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</div>
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</body>
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</html>
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