1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<!-- This file documents the GNU Assembler "as". 4 5Copyright (C) 1991-2021 Free Software Foundation, Inc. 6 7Permission is granted to copy, distribute and/or modify this document 8under the terms of the GNU Free Documentation License, Version 1.3 9or any later version published by the Free Software Foundation; 10with no Invariant Sections, with no Front-Cover Texts, and with no 11Back-Cover Texts. A copy of the license is included in the 12section entitled "GNU Free Documentation License". 13 --> 14<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> 15<head> 16<title>Using as: MIPS Small Data</title> 17 18<meta name="description" content="Using as: MIPS Small Data"> 19<meta name="keywords" content="Using as: MIPS Small Data"> 20<meta name="resource-type" content="document"> 21<meta name="distribution" content="global"> 22<meta name="Generator" content="makeinfo"> 23<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 24<link href="index.html#Top" rel="start" title="Top"> 25<link href="AS-Index.html#AS-Index" rel="index" title="AS Index"> 26<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> 27<link href="MIPS_002dDependent.html#MIPS_002dDependent" rel="up" title="MIPS-Dependent"> 28<link href="MIPS-ISA.html#MIPS-ISA" rel="next" title="MIPS ISA"> 29<link href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes" rel="previous" title="MIPS Symbol Sizes"> 30<style type="text/css"> 31<!-- 32a.summary-letter {text-decoration: none} 33blockquote.smallquotation {font-size: smaller} 34div.display {margin-left: 3.2em} 35div.example {margin-left: 3.2em} 36div.indentedblock {margin-left: 3.2em} 37div.lisp {margin-left: 3.2em} 38div.smalldisplay {margin-left: 3.2em} 39div.smallexample {margin-left: 3.2em} 40div.smallindentedblock {margin-left: 3.2em; font-size: smaller} 41div.smalllisp {margin-left: 3.2em} 42kbd {font-style:oblique} 43pre.display {font-family: inherit} 44pre.format {font-family: inherit} 45pre.menu-comment {font-family: serif} 46pre.menu-preformatted {font-family: serif} 47pre.smalldisplay {font-family: inherit; font-size: smaller} 48pre.smallexample {font-size: smaller} 49pre.smallformat {font-family: inherit; font-size: smaller} 50pre.smalllisp {font-size: smaller} 51span.nocodebreak {white-space:nowrap} 52span.nolinebreak {white-space:nowrap} 53span.roman {font-family:serif; font-weight:normal} 54span.sansserif {font-family:sans-serif; font-weight:normal} 55ul.no-bullet {list-style: none} 56--> 57</style> 58 59 60</head> 61 62<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> 63<a name="MIPS-Small-Data"></a> 64<div class="header"> 65<p> 66Next: <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> 67</div> 68<hr> 69<a name="Controlling-the-use-of-small-data-accesses"></a> 70<h4 class="subsection">9.27.4 Controlling the use of small data accesses</h4> 71 72<a name="index-small-data_002c-MIPS"></a> 73<a name="index-gp-register_002c-MIPS"></a> 74<p>It often takes several instructions to load the address of a symbol. 75For example, when ‘<samp>addr</samp>’ is a 32-bit symbol, the non-PIC expansion 76of ‘<samp>dla $4,addr</samp>’ is usually: 77</p> 78<div class="smallexample"> 79<pre class="smallexample">lui $4,%hi(addr) 80daddiu $4,$4,%lo(addr) 81</pre></div> 82 83<p>The sequence is much longer when ‘<samp>addr</samp>’ is a 64-bit symbol. 84See <a href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes">Directives to override the size of symbols</a>. 85</p> 86<p>In order to cut down on this overhead, most embedded MIPS systems 87set aside a 64-kilobyte “small data” area and guarantee that all 88data of size <var>n</var> and smaller will be placed in that area. 89The limit <var>n</var> is passed to both the assembler and the linker 90using 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 91when linking and when assembling all input files to the link; any 92inconsistency could cause a relocation overflow error. 93</p> 94<p>The size of an object in the <code>.bss</code> section is set by the 95<code>.comm</code> or <code>.lcomm</code> directive that defines it. The size of 96an external object may be set with the <code>.extern</code> directive. For 97example, ‘<samp>.extern sym,4</samp>’ declares that the object at <code>sym</code> 98is 4 bytes in length, while leaving <code>sym</code> otherwise undefined. 99</p> 100<p>When no <samp>-G</samp> option is given, the default limit is 8 bytes. 101The option <samp>-G 0</samp> prevents any data from being automatically 102classified as small. 103</p> 104<p>It is also possible to mark specific objects as small by putting them 105in the special sections <code>.sdata</code> and <code>.sbss</code>, which are 106“small” counterparts of <code>.data</code> and <code>.bss</code> respectively. 107The toolchain will treat such data as small regardless of the 108<samp>-G</samp> setting. 109</p> 110<p>On startup, systems that support a small data area are expected to 111initialize register <code>$28</code>, also known as <code>$gp</code>, in such a 112way that small data can be accessed using a 16-bit offset from that 113register. For example, when ‘<samp>addr</samp>’ is small data, 114the ‘<samp>dla $4,addr</samp>’ instruction above is equivalent to: 115</p> 116<div class="smallexample"> 117<pre class="smallexample">daddiu $4,$28,%gp_rel(addr) 118</pre></div> 119 120<p>Small data is not supported for SVR4-style PIC. 121</p> 122<hr> 123<div class="header"> 124<p> 125Next: <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> 126</div> 127 128 129 130</body> 131</html> 132