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128 lines
5.0 KiB
HTML
128 lines
5.0 KiB
HTML
4 years ago
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<!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>MIPS Symbol Sizes (Using as)</title>
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<meta name="description" content="MIPS Symbol Sizes (Using as)">
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<meta name="keywords" content="MIPS Symbol Sizes (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="MIPS_002dDependent.html#MIPS_002dDependent" rel="up" title="MIPS-Dependent">
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<link href="MIPS-Small-Data.html#MIPS-Small-Data" rel="next" title="MIPS Small Data">
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<link href="MIPS-Macros.html#MIPS-Macros" rel="prev" title="MIPS Macros">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="MIPS-Symbol-Sizes"></a>
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<div class="header">
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<p>
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Next: <a href="MIPS-Small-Data.html#MIPS-Small-Data" accesskey="n" rel="next">MIPS Small Data</a>, Previous: <a href="MIPS-Macros.html#MIPS-Macros" accesskey="p" rel="prev">MIPS Macros</a>, Up: <a href="MIPS_002dDependent.html#MIPS_002dDependent" accesskey="u" rel="up">MIPS-Dependent</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="Directives-to-override-the-size-of-symbols"></a>
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<h4 class="subsection">9.26.3 Directives to override the size of symbols</h4>
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<a name="index-_002eset-sym32"></a>
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<a name="index-_002eset-nosym32"></a>
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<p>The n64 ABI allows symbols to have any 64-bit value. Although this
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provides a great deal of flexibility, it means that some macros have
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much longer expansions than their 32-bit counterparts. For example,
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the non-PIC expansion of ‘<samp>dla $4,sym</samp>’ is usually:
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</p>
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<div class="smallexample">
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<pre class="smallexample">lui $4,%highest(sym)
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lui $1,%hi(sym)
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daddiu $4,$4,%higher(sym)
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daddiu $1,$1,%lo(sym)
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dsll32 $4,$4,0
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daddu $4,$4,$1
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</pre></div>
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<p>whereas the 32-bit expansion is simply:
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</p>
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<div class="smallexample">
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<pre class="smallexample">lui $4,%hi(sym)
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daddiu $4,$4,%lo(sym)
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</pre></div>
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<p>n64 code is sometimes constructed in such a way that all symbolic
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constants are known to have 32-bit values, and in such cases, it’s
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preferable to use the 32-bit expansion instead of the 64-bit
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expansion.
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</p>
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<p>You can use the <code>.set sym32</code> directive to tell the assembler
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that, from this point on, all expressions of the form
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‘<samp><var>symbol</var></samp>’ or ‘<samp><var>symbol</var> + <var>offset</var></samp>’
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have 32-bit values. For example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">.set sym32
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dla $4,sym
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lw $4,sym+16
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sw $4,sym+0x8000($4)
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</pre></div>
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<p>will cause the assembler to treat ‘<samp>sym</samp>’, <code>sym+16</code> and
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<code>sym+0x8000</code> as 32-bit values. The handling of non-symbolic
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addresses is not affected.
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</p>
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<p>The directive <code>.set nosym32</code> ends a <code>.set sym32</code> block and
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reverts to the normal behavior. It is also possible to change the
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symbol size using the command-line options <samp>-msym32</samp> and
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<samp>-mno-sym32</samp>.
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</p>
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<p>These options and directives are always accepted, but at present,
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they have no effect for anything other than n64.
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</p>
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</body>
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</html>
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