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<title>Using as: MIPS Small Data</title>
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<a name="MIPS-Small-Data"></a>
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<div class="header">
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<p>
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Next: <a href="MIPS-ISA.html#MIPS-ISA" accesskey="n" rel="next">MIPS ISA</a>, Previous: <a href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes" accesskey="p" rel="previous">MIPS Symbol Sizes</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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<hr>
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<a name="Controlling-the-use-of-small-data-accesses"></a>
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<h4 class="subsection">9.27.4 Controlling the use of small data accesses</h4>
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<a name="index-small-data_002c-MIPS"></a>
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<a name="index-gp-register_002c-MIPS"></a>
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<p>It often takes several instructions to load the address of a symbol.
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For example, when ‘<samp>addr</samp>’ is a 32-bit symbol, the non-PIC expansion
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of ‘<samp>dla $4,addr</samp>’ is usually:
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</p>
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<div class="smallexample">
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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></div>
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<p>The sequence is much longer when ‘<samp>addr</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>
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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>-G <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>
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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>.extern sym,4</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>
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<p>When no <samp>-G</samp> option is given, the default limit is 8 bytes.
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The option <samp>-G 0</samp> prevents any data from being automatically
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classified as small.
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</p>
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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>-G</samp> setting.
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</p>
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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>addr</samp>’ is small data,
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the ‘<samp>dla $4,addr</samp>’ instruction above is equivalent to:
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</p>
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<div class="smallexample">
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<pre class="smallexample">daddiu $4,$28,%gp_rel(addr)
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</pre></div>
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<p>Small data is not supported for SVR4-style PIC.
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</p>
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<p>
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Next: <a href="MIPS-ISA.html#MIPS-ISA" accesskey="n" rel="next">MIPS ISA</a>, Previous: <a href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes" accesskey="p" rel="previous">MIPS Symbol Sizes</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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