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 <a name="MIPS-Small-Data"></a>
 <div class="header">
 <p>
 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="prev">MIPS Symbol Sizes</a>, Up: <a href="MIPS_002dDependent.html#MIPS_002dDependent" accesskey="u" rel="up">MIPS-Dependent</a> &nbsp; [<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>
 </div>
 <hr>
 <a name="Controlling-the-use-of-small-data-accesses"></a>
 <h4 class="subsection">9.27.4 Controlling the use of small data accesses</h4>

 <a name="index-small-data_002c-MIPS"></a>
 <a name="index-gp-register_002c-MIPS"></a>
 <p>It often takes several instructions to load the address of a symbol.
 For example, when &lsquo;<samp>addr</samp>&rsquo; is a 32-bit symbol, the non-PIC expansion
 of &lsquo;<samp>dla $4,addr</samp>&rsquo; is usually:
 </p>
 <div class="smallexample">
 <pre class="smallexample">lui     $4,%hi(addr)
 daddiu  $4,$4,%lo(addr)
 </pre></div>

 <p>The sequence is much longer when &lsquo;<samp>addr</samp>&rsquo; is a 64-bit symbol.
 See <a href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes">Directives to override the size of symbols</a>.
 </p>
 <p>In order to cut down on this overhead, most embedded MIPS systems
 set aside a 64-kilobyte &ldquo;small data&rdquo; area and guarantee that all
 data of size <var>n</var> and smaller will be placed in that area.
 The limit <var>n</var> is passed to both the assembler and the linker
 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
 when linking and when assembling all input files to the link; any
 inconsistency could cause a relocation overflow error.
 </p>
 <p>The size of an object in the <code>.bss</code> section is set by the
 <code>.comm</code> or <code>.lcomm</code> directive that defines it.  The size of
 an external object may be set with the <code>.extern</code> directive.  For
 example, &lsquo;<samp>.extern sym,4</samp>&rsquo; declares that the object at <code>sym</code>
 is 4 bytes in length, while leaving <code>sym</code> otherwise undefined.
 </p>
 <p>When no <samp>-G</samp> option is given, the default limit is 8 bytes.
 The option <samp>-G 0</samp> prevents any data from being automatically
 classified as small.
 </p>
 <p>It is also possible to mark specific objects as small by putting them
 in the special sections <code>.sdata</code> and <code>.sbss</code>, which are
 &ldquo;small&rdquo; counterparts of <code>.data</code> and <code>.bss</code> respectively.
 The toolchain will treat such data as small regardless of the
 <samp>-G</samp> setting.
 </p>
 <p>On startup, systems that support a small data area are expected to
 initialize register <code>$28</code>, also known as <code>$gp</code>, in such a
 way that small data can be accessed using a 16-bit offset from that
 register.  For example, when &lsquo;<samp>addr</samp>&rsquo; is small data,
 the &lsquo;<samp>dla $4,addr</samp>&rsquo; instruction above is equivalent to:
 </p>
 <div class="smallexample">
 <pre class="smallexample">daddiu  $4,$28,%gp_rel(addr)
 </pre></div>

 <p>Small data is not supported for SVR4-style PIC.
 </p>
 <hr>
 <div class="header">
 <p>
 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="prev">MIPS Symbol Sizes</a>, Up: <a href="MIPS_002dDependent.html#MIPS_002dDependent" accesskey="u" rel="up">MIPS-Dependent</a> &nbsp; [<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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	<title>Using as: MIPS Small Data</title>

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	<a name="MIPS-Small-Data"></a>
	<div class="header">
	<p>
	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="prev">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>
	</div>
	<hr>
	<a name="Controlling-the-use-of-small-data-accesses"></a>
	<h4 class="subsection">9.27.4 Controlling the use of small data accesses</h4>

	<a name="index-small-data_002c-MIPS"></a>
	<a name="index-gp-register_002c-MIPS"></a>
	<p>It often takes several instructions to load the address of a symbol.
	For example, when ‘<samp>addr</samp>’ is a 32-bit symbol, the non-PIC expansion
	of ‘<samp>dla $4,addr</samp>’ is usually:
	</p>
	<div class="smallexample">
	<pre class="smallexample">lui $4,%hi(addr)
	daddiu $4,$4,%lo(addr)
	</pre></div>

	<p>The sequence is much longer when ‘<samp>addr</samp>’ is a 64-bit symbol.
	See <a href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes">Directives to override the size of symbols</a>.
	</p>
	<p>In order to cut down on this overhead, most embedded MIPS systems
	set aside a 64-kilobyte “small data” area and guarantee that all
	data of size <var>n</var> and smaller will be placed in that area.
	The limit <var>n</var> is passed to both the assembler and the linker
	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
	when linking and when assembling all input files to the link; any
	inconsistency could cause a relocation overflow error.
	</p>
	<p>The size of an object in the <code>.bss</code> section is set by the
	<code>.comm</code> or <code>.lcomm</code> directive that defines it. The size of
	an external object may be set with the <code>.extern</code> directive. For
	example, ‘<samp>.extern sym,4</samp>’ declares that the object at <code>sym</code>
	is 4 bytes in length, while leaving <code>sym</code> otherwise undefined.
	</p>
	<p>When no <samp>-G</samp> option is given, the default limit is 8 bytes.
	The option <samp>-G 0</samp> prevents any data from being automatically
	classified as small.
	</p>
	<p>It is also possible to mark specific objects as small by putting them
	in the special sections <code>.sdata</code> and <code>.sbss</code>, which are
	“small” counterparts of <code>.data</code> and <code>.bss</code> respectively.
	The toolchain will treat such data as small regardless of the
	<samp>-G</samp> setting.
	</p>
	<p>On startup, systems that support a small data area are expected to
	initialize register <code>$28</code>, also known as <code>$gp</code>, in such a
	way that small data can be accessed using a 16-bit offset from that
	register. For example, when ‘<samp>addr</samp>’ is small data,
	the ‘<samp>dla $4,addr</samp>’ instruction above is equivalent to:
	</p>
	<div class="smallexample">
	<pre class="smallexample">daddiu $4,$28,%gp_rel(addr)
	</pre></div>

	<p>Small data is not supported for SVR4-style PIC.
	</p>
	<hr>
	<div class="header">
	<p>
	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="prev">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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