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 linker LD 4(GNU Toolchain for the A-profile Architecture 10.3-2021.07 (arm-10.29)) 5version 2.36.1. 6 7Copyright (C) 1991-2021 Free Software Foundation, Inc. 8 9Permission is granted to copy, distribute and/or modify this document 10under the terms of the GNU Free Documentation License, Version 1.3 11or any later version published by the Free Software Foundation; 12with no Invariant Sections, with no Front-Cover Texts, and with no 13Back-Cover Texts. A copy of the license is included in the 14section entitled "GNU Free Documentation License". --> 15<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> 16<head> 17<title>LD: MEMORY</title> 18 19<meta name="description" content="LD: MEMORY"> 20<meta name="keywords" content="LD: MEMORY"> 21<meta name="resource-type" content="document"> 22<meta name="distribution" content="global"> 23<meta name="Generator" content="makeinfo"> 24<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 25<link href="index.html#Top" rel="start" title="Top"> 26<link href="LD-Index.html#LD-Index" rel="index" title="LD Index"> 27<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> 28<link href="Scripts.html#Scripts" rel="up" title="Scripts"> 29<link href="PHDRS.html#PHDRS" rel="next" title="PHDRS"> 30<link href="Overlay-Description.html#Overlay-Description" rel="previous" title="Overlay Description"> 31<style type="text/css"> 32<!-- 33a.summary-letter {text-decoration: none} 34blockquote.smallquotation {font-size: smaller} 35div.display {margin-left: 3.2em} 36div.example {margin-left: 3.2em} 37div.indentedblock {margin-left: 3.2em} 38div.lisp {margin-left: 3.2em} 39div.smalldisplay {margin-left: 3.2em} 40div.smallexample {margin-left: 3.2em} 41div.smallindentedblock {margin-left: 3.2em; font-size: smaller} 42div.smalllisp {margin-left: 3.2em} 43kbd {font-style:oblique} 44pre.display {font-family: inherit} 45pre.format {font-family: inherit} 46pre.menu-comment {font-family: serif} 47pre.menu-preformatted {font-family: serif} 48pre.smalldisplay {font-family: inherit; font-size: smaller} 49pre.smallexample {font-size: smaller} 50pre.smallformat {font-family: inherit; font-size: smaller} 51pre.smalllisp {font-size: smaller} 52span.nocodebreak {white-space:nowrap} 53span.nolinebreak {white-space:nowrap} 54span.roman {font-family:serif; font-weight:normal} 55span.sansserif {font-family:sans-serif; font-weight:normal} 56ul.no-bullet {list-style: none} 57--> 58</style> 59 60 61</head> 62 63<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> 64<a name="MEMORY"></a> 65<div class="header"> 66<p> 67Next: <a href="PHDRS.html#PHDRS" accesskey="n" rel="next">PHDRS</a>, Previous: <a href="SECTIONS.html#SECTIONS" accesskey="p" rel="previous">SECTIONS</a>, Up: <a href="Scripts.html#Scripts" accesskey="u" rel="up">Scripts</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p> 68</div> 69<hr> 70<a name="MEMORY-Command"></a> 71<h3 class="section">3.7 MEMORY Command</h3> 72<a name="index-MEMORY"></a> 73<a name="index-memory-regions"></a> 74<a name="index-regions-of-memory"></a> 75<a name="index-allocating-memory"></a> 76<a name="index-discontinuous-memory"></a> 77<p>The linker’s default configuration permits allocation of all available 78memory. You can override this by using the <code>MEMORY</code> command. 79</p> 80<p>The <code>MEMORY</code> command describes the location and size of blocks of 81memory in the target. You can use it to describe which memory regions 82may be used by the linker, and which memory regions it must avoid. You 83can then assign sections to particular memory regions. The linker will 84set section addresses based on the memory regions, and will warn about 85regions that become too full. The linker will not shuffle sections 86around to fit into the available regions. 87</p> 88<p>A linker script may contain many uses of the <code>MEMORY</code> command, 89however, all memory blocks defined are treated as if they were 90specified inside a single <code>MEMORY</code> command. The syntax for 91<code>MEMORY</code> is: 92</p><div class="smallexample"> 93<pre class="smallexample">MEMORY 94 { 95 <var>name</var> [(<var>attr</var>)] : ORIGIN = <var>origin</var>, LENGTH = <var>len</var> 96 … 97 } 98</pre></div> 99 100<p>The <var>name</var> is a name used in the linker script to refer to the 101region. The region name has no meaning outside of the linker script. 102Region names are stored in a separate name space, and will not conflict 103with symbol names, file names, or section names. Each memory region 104must have a distinct name within the <code>MEMORY</code> command. However you can 105add later alias names to existing memory regions with the <a href="REGION_005fALIAS.html#REGION_005fALIAS">REGION_ALIAS</a> 106command. 107</p> 108<a name="index-memory-region-attributes"></a> 109<p>The <var>attr</var> string is an optional list of attributes that specify 110whether to use a particular memory region for an input section which is 111not explicitly mapped in the linker script. As described in 112<a href="SECTIONS.html#SECTIONS">SECTIONS</a>, if you do not specify an output section for some input 113section, the linker will create an output section with the same name as 114the input section. If you define region attributes, the linker will use 115them to select the memory region for the output section that it creates. 116</p> 117<p>The <var>attr</var> string must consist only of the following characters: 118</p><dl compact="compact"> 119<dt>‘<samp>R</samp>’</dt> 120<dd><p>Read-only section 121</p></dd> 122<dt>‘<samp>W</samp>’</dt> 123<dd><p>Read/write section 124</p></dd> 125<dt>‘<samp>X</samp>’</dt> 126<dd><p>Executable section 127</p></dd> 128<dt>‘<samp>A</samp>’</dt> 129<dd><p>Allocatable section 130</p></dd> 131<dt>‘<samp>I</samp>’</dt> 132<dd><p>Initialized section 133</p></dd> 134<dt>‘<samp>L</samp>’</dt> 135<dd><p>Same as ‘<samp>I</samp>’ 136</p></dd> 137<dt>‘<samp>!</samp>’</dt> 138<dd><p>Invert the sense of any of the attributes that follow 139</p></dd> 140</dl> 141 142<p>If an unmapped section matches any of the listed attributes other than 143‘<samp>!</samp>’, it will be placed in the memory region. The ‘<samp>!</samp>’ 144attribute reverses the test for the characters that follow, so that an 145unmapped section will be placed in the memory region only if it does 146not match any of the attributes listed afterwards. Thus an attribute 147string of ‘<samp>RW!X</samp>’ will match any unmapped section that has either 148or both of the ‘<samp>R</samp>’ and ‘<samp>W</samp>’ attributes, but only as long as 149the section does not also have the ‘<samp>X</samp>’ attribute. 150</p> 151<a name="index-ORIGIN-_003d"></a> 152<a name="index-o-_003d"></a> 153<a name="index-org-_003d"></a> 154<p>The <var>origin</var> is an numerical expression for the start address of 155the memory region. The expression must evaluate to a constant and it 156cannot involve any symbols. The keyword <code>ORIGIN</code> may be 157abbreviated to <code>org</code> or <code>o</code> (but not, for example, 158<code>ORG</code>). 159</p> 160<a name="index-LENGTH-_003d"></a> 161<a name="index-len-_003d"></a> 162<a name="index-l-_003d"></a> 163<p>The <var>len</var> is an expression for the size in bytes of the memory 164region. As with the <var>origin</var> expression, the expression must 165be numerical only and must evaluate to a constant. The keyword 166<code>LENGTH</code> may be abbreviated to <code>len</code> or <code>l</code>. 167</p> 168<p>In the following example, we specify that there are two memory regions 169available for allocation: one starting at ‘<samp>0</samp>’ for 256 kilobytes, 170and the other starting at ‘<samp>0x40000000</samp>’ for four megabytes. The 171linker will place into the ‘<samp>rom</samp>’ memory region every section which 172is not explicitly mapped into a memory region, and is either read-only 173or executable. The linker will place other sections which are not 174explicitly mapped into a memory region into the ‘<samp>ram</samp>’ memory 175region. 176</p> 177<div class="smallexample"> 178<pre class="smallexample">MEMORY 179 { 180 rom (rx) : ORIGIN = 0, LENGTH = 256K 181 ram (!rx) : org = 0x40000000, l = 4M 182 } 183</pre></div> 184 185<p>Once you define a memory region, you can direct the linker to place 186specific output sections into that memory region by using the 187‘<samp>><var>region</var></samp>’ output section attribute. For example, if you have 188a memory region named ‘<samp>mem</samp>’, you would use ‘<samp>>mem</samp>’ in the 189output section definition. See <a href="Output-Section-Region.html#Output-Section-Region">Output Section Region</a>. If no address 190was specified for the output section, the linker will set the address to 191the next available address within the memory region. If the combined 192output sections directed to a memory region are too large for the 193region, the linker will issue an error message. 194</p> 195<p>It is possible to access the origin and length of a memory in an 196expression via the <code>ORIGIN(<var>memory</var>)</code> and 197<code>LENGTH(<var>memory</var>)</code> functions: 198</p> 199<div class="smallexample"> 200<pre class="smallexample"> _fstack = ORIGIN(ram) + LENGTH(ram) - 4; 201</pre></div> 202 203<hr> 204<div class="header"> 205<p> 206Next: <a href="PHDRS.html#PHDRS" accesskey="n" rel="next">PHDRS</a>, Previous: <a href="SECTIONS.html#SECTIONS" accesskey="p" rel="previous">SECTIONS</a>, Up: <a href="Scripts.html#Scripts" accesskey="u" rel="up">Scripts</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p> 207</div> 208 209 210 211</body> 212</html> 213