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5.0 KiB
HTML
98 lines
5.0 KiB
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<html lang="en">
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<head>
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<title>MIPS Small Data - Using as</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="Using as">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="MIPS_002dDependent.html#MIPS_002dDependent" title="MIPS-Dependent">
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<link rel="prev" href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes" title="MIPS Symbol Sizes">
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<link rel="next" href="MIPS-ISA.html#MIPS-ISA" title="MIPS ISA">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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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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<style type="text/css"><!--
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</head>
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<body>
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<div class="node">
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<a name="MIPS-Small-Data"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="MIPS-ISA.html#MIPS-ISA">MIPS ISA</a>,
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Previous: <a rel="previous" accesskey="p" href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes">MIPS Symbol Sizes</a>,
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Up: <a rel="up" accesskey="u" href="MIPS_002dDependent.html#MIPS_002dDependent">MIPS-Dependent</a>
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<hr>
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</div>
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<h4 class="subsection">9.27.4 Controlling the use of small data accesses</h4>
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<!-- This section deliberately glosses over the possibility of using -G -->
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<!-- in SVR4-style PIC, as could be done on IRIX. We don't support that. -->
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<p><a name="index-small-data_002c-MIPS-1578"></a><a name="index-g_t_0040code_007bgp_007d-register_002c-MIPS-1579"></a>It often takes several instructions to load the address of a symbol.
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For example, when ‘<samp><span class="samp">addr</span></samp>’ is a 32-bit symbol, the non-PIC expansion
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of ‘<samp><span class="samp">dla $4,addr</span></samp>’ is usually:
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<pre class="smallexample"> lui $4,%hi(addr)
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daddiu $4,$4,%lo(addr)
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</pre>
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<p>The sequence is much longer when ‘<samp><span class="samp">addr</span></samp>’ is a 64-bit symbol.
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See <a href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes">Directives to override the size of symbols</a>.
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<p>In order to cut down on this overhead, most embedded MIPS systems
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set aside a 64-kilobyte “small data” area and guarantee that all
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data of size <var>n</var> and smaller will be placed in that area.
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The limit <var>n</var> is passed to both the assembler and the linker
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using the command-line option <samp><span class="option">-G </span><var>n</var></samp>, see <a href="MIPS-Options.html#MIPS-Options">Assembler options</a>. Note that the same value of <var>n</var> must be used
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when linking and when assembling all input files to the link; any
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inconsistency could cause a relocation overflow error.
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<p>The size of an object in the <code>.bss</code> section is set by the
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<code>.comm</code> or <code>.lcomm</code> directive that defines it. The size of
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an external object may be set with the <code>.extern</code> directive. For
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example, ‘<samp><span class="samp">.extern sym,4</span></samp>’ declares that the object at <code>sym</code>
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is 4 bytes in length, while leaving <code>sym</code> otherwise undefined.
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<p>When no <samp><span class="option">-G</span></samp> option is given, the default limit is 8 bytes.
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The option <samp><span class="option">-G 0</span></samp> prevents any data from being automatically
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classified as small.
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<p>It is also possible to mark specific objects as small by putting them
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in the special sections <code>.sdata</code> and <code>.sbss</code>, which are
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“small” counterparts of <code>.data</code> and <code>.bss</code> respectively.
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The toolchain will treat such data as small regardless of the
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<samp><span class="option">-G</span></samp> setting.
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<p>On startup, systems that support a small data area are expected to
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initialize register <code>$28</code>, also known as <code>$gp</code>, in such a
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way that small data can be accessed using a 16-bit offset from that
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register. For example, when ‘<samp><span class="samp">addr</span></samp>’ is small data,
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the ‘<samp><span class="samp">dla $4,addr</span></samp>’ instruction above is equivalent to:
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<pre class="smallexample"> daddiu $4,$28,%gp_rel(addr)
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</pre>
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<p>Small data is not supported for SVR4-style PIC.
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</body></html>
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