huangmeng
/
Embedded_AI


			
							123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131
							<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- This file documents the GNU Assembler "as".

Copyright (C) 1991-2017 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, with no Front-Cover Texts, and with no
Back-Cover Texts.  A copy of the license is included in the
section entitled "GNU Free Documentation License".
 -->
<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
<head>
<title>Using as: MIPS Small Data</title>

<meta name="description" content="Using as: MIPS Small Data">
<meta name="keywords" content="Using as: MIPS Small Data">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<link href="index.html#Top" rel="start" title="Top">
<link href="AS-Index.html#AS-Index" rel="index" title="AS Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="MIPS_002dDependent.html#MIPS_002dDependent" rel="up" title="MIPS-Dependent">
<link href="MIPS-ISA.html#MIPS-ISA" rel="next" title="MIPS ISA">
<link href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes" rel="prev" title="MIPS Symbol Sizes">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.smallquotation {font-size: smaller}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.indentedblock {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
div.smalldisplay {margin-left: 3.2em}
div.smallexample {margin-left: 3.2em}
div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
div.smalllisp {margin-left: 3.2em}
kbd {font-style:oblique}
pre.display {font-family: inherit}
pre.format {font-family: inherit}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
pre.smalldisplay {font-family: inherit; font-size: smaller}
pre.smallexample {font-size: smaller}
pre.smallformat {font-family: inherit; font-size: smaller}
pre.smalllisp {font-size: smaller}
span.nocodebreak {white-space:nowrap}
span.nolinebreak {white-space:nowrap}
span.roman {font-family:serif; font-weight:normal}
span.sansserif {font-family:sans-serif; font-weight:normal}
ul.no-bullet {list-style: none}
-->
</style>


</head>

<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
<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>
</div>


</body>
</html>