You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
98 lines
5.0 KiB
HTML
98 lines
5.0 KiB
HTML
4 years ago
|
<html lang="en">
|
||
|
|
<head>
|
||
|
|
<title>MIPS Small Data - Using as</title>
|
||
|
|
<meta http-equiv="Content-Type" content="text/html">
|
||
|
|
<meta name="description" content="Using as">
|
||
|
|
<meta name="generator" content="makeinfo 4.13">
|
||
|
|
<link title="Top" rel="start" href="index.html#Top">
|
||
|
|
<link rel="up" href="MIPS_002dDependent.html#MIPS_002dDependent" title="MIPS-Dependent">
|
||
|
|
<link rel="prev" href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes" title="MIPS Symbol Sizes">
|
||
|
|
<link rel="next" href="MIPS-ISA.html#MIPS-ISA" title="MIPS ISA">
|
||
|
|
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
||
|
|
<!--
|
||
|
|
This file documents the GNU Assembler "as".
|
||
|
|
|
||
|
|
Copyright (C) 1991-2015 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''.
|
||
|
|
|
||
|
|
-->
|
||
|
|
<meta http-equiv="Content-Style-Type" content="text/css">
|
||
|
|
<style type="text/css"><!--
|
||
|
|
pre.display { font-family:inherit }
|
||
|
|
pre.format { font-family:inherit }
|
||
|
|
pre.smalldisplay { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallformat { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallexample { font-size:smaller }
|
||
|
|
pre.smalllisp { font-size:smaller }
|
||
|
|
span.sc { font-variant:small-caps }
|
||
|
|
span.roman { font-family:serif; font-weight:normal; }
|
||
|
|
span.sansserif { font-family:sans-serif; font-weight:normal; }
|
||
|
|
--></style>
|
||
|
|
</head>
|
||
|
|
<body>
|
||
|
|
<div class="node">
|
||
|
|
<a name="MIPS-Small-Data"></a>
|
||
|
|
<p>
|
||
|
|
Next: <a rel="next" accesskey="n" href="MIPS-ISA.html#MIPS-ISA">MIPS ISA</a>,
|
||
|
|
Previous: <a rel="previous" accesskey="p" href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes">MIPS Symbol Sizes</a>,
|
||
|
|
Up: <a rel="up" accesskey="u" href="MIPS_002dDependent.html#MIPS_002dDependent">MIPS-Dependent</a>
|
||
|
|
<hr>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
<h4 class="subsection">9.27.4 Controlling the use of small data accesses</h4>
|
||
|
|
|
||
|
|
<!-- This section deliberately glosses over the possibility of using -G -->
|
||
|
|
<!-- in SVR4-style PIC, as could be done on IRIX. We don't support that. -->
|
||
|
|
<p><a name="index-small-data_002c-MIPS-1494"></a><a name="index-g_t_0040code_007bgp_007d-register_002c-MIPS-1495"></a>It often takes several instructions to load the address of a symbol.
|
||
|
|
For example, when ‘<samp><span class="samp">addr</span></samp>’ is a 32-bit symbol, the non-PIC expansion
|
||
|
|
of ‘<samp><span class="samp">dla $4,addr</span></samp>’ is usually:
|
||
|
|
|
||
|
|
<pre class="smallexample"> lui $4,%hi(addr)
|
||
|
|
daddiu $4,$4,%lo(addr)
|
||
|
|
</pre>
|
||
|
|
<p>The sequence is much longer when ‘<samp><span class="samp">addr</span></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>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><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
|
||
|
|
when linking and when assembling all input files to the link; any
|
||
|
|
inconsistency could cause a relocation overflow error.
|
||
|
|
|
||
|
|
<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><span class="samp">.extern sym,4</span></samp>’ declares that the object at <code>sym</code>
|
||
|
|
is 4 bytes in length, while leaving <code>sym</code> otherwise undefined.
|
||
|
|
|
||
|
|
<p>When no <samp><span class="option">-G</span></samp> option is given, the default limit is 8 bytes.
|
||
|
|
The option <samp><span class="option">-G 0</span></samp> prevents any data from being automatically
|
||
|
|
classified as small.
|
||
|
|
|
||
|
|
<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><span class="option">-G</span></samp> setting.
|
||
|
|
|
||
|
|
<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><span class="samp">addr</span></samp>’ is small data,
|
||
|
|
the ‘<samp><span class="samp">dla $4,addr</span></samp>’ instruction above is equivalent to:
|
||
|
|
|
||
|
|
<pre class="smallexample"> daddiu $4,$28,%gp_rel(addr)
|
||
|
|
</pre>
|
||
|
|
<p>Small data is not supported for SVR4-style PIC.
|
||
|
|
|
||
|
|
</body></html>
|
||
|
|
|