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: Output Section LMA</title> 18 19<meta name="description" content="LD: Output Section LMA"> 20<meta name="keywords" content="LD: Output Section LMA"> 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="Output-Section-Attributes.html#Output-Section-Attributes" rel="up" title="Output Section Attributes"> 29<link href="Forced-Output-Alignment.html#Forced-Output-Alignment" rel="next" title="Forced Output Alignment"> 30<link href="Output-Section-Type.html#Output-Section-Type" rel="previous" title="Output Section Type"> 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="Output-Section-LMA"></a> 65<div class="header"> 66<p> 67Next: <a href="Forced-Output-Alignment.html#Forced-Output-Alignment" accesskey="n" rel="next">Forced Output Alignment</a>, Previous: <a href="Output-Section-Type.html#Output-Section-Type" accesskey="p" rel="previous">Output Section Type</a>, Up: <a href="Output-Section-Attributes.html#Output-Section-Attributes" accesskey="u" rel="up">Output Section Attributes</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="Output-Section-LMA-1"></a> 71<h4 class="subsubsection">3.6.8.2 Output Section LMA</h4> 72<a name="index-AT_003elma_005fregion"></a> 73<a name="index-AT_0028lma_0029"></a> 74<a name="index-load-address"></a> 75<a name="index-section-load-address"></a> 76<p>Every section has a virtual address (VMA) and a load address (LMA); see 77<a href="Basic-Script-Concepts.html#Basic-Script-Concepts">Basic Script Concepts</a>. The virtual address is specified by the 78see <a href="Output-Section-Address.html#Output-Section-Address">Output Section Address</a> described earlier. The load address is 79specified by the <code>AT</code> or <code>AT></code> keywords. Specifying a load 80address is optional. 81</p> 82<p>The <code>AT</code> keyword takes an expression as an argument. This 83specifies the exact load address of the section. The <code>AT></code> keyword 84takes the name of a memory region as an argument. See <a href="MEMORY.html#MEMORY">MEMORY</a>. The 85load address of the section is set to the next free address in the 86region, aligned to the section’s alignment requirements. 87</p> 88<p>If neither <code>AT</code> nor <code>AT></code> is specified for an allocatable 89section, the linker will use the following heuristic to determine the 90load address: 91</p> 92<ul> 93<li> If the section has a specific VMA address, then this is used as 94the LMA address as well. 95 96</li><li> If the section is not allocatable then its LMA is set to its VMA. 97 98</li><li> Otherwise if a memory region can be found that is compatible 99with the current section, and this region contains at least one 100section, then the LMA is set so the difference between the 101VMA and LMA is the same as the difference between the VMA and LMA of 102the last section in the located region. 103 104</li><li> If no memory regions have been declared then a default region 105that covers the entire address space is used in the previous step. 106 107</li><li> If no suitable region could be found, or there was no previous 108section then the LMA is set equal to the VMA. 109</li></ul> 110 111<a name="index-ROM-initialized-data"></a> 112<a name="index-initialized-data-in-ROM"></a> 113<p>This feature is designed to make it easy to build a ROM image. For 114example, the following linker script creates three output sections: one 115called ‘<samp>.text</samp>’, which starts at <code>0x1000</code>, one called 116‘<samp>.mdata</samp>’, which is loaded at the end of the ‘<samp>.text</samp>’ section 117even though its VMA is <code>0x2000</code>, and one called ‘<samp>.bss</samp>’ to hold 118uninitialized data at address <code>0x3000</code>. The symbol <code>_data</code> is 119defined with the value <code>0x2000</code>, which shows that the location 120counter holds the VMA value, not the LMA value. 121</p> 122<div class="smallexample"> 123<pre class="smallexample">SECTIONS 124 { 125 .text 0x1000 : { *(.text) _etext = . ; } 126 .mdata 0x2000 : 127 AT ( ADDR (.text) + SIZEOF (.text) ) 128 { _data = . ; *(.data); _edata = . ; } 129 .bss 0x3000 : 130 { _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;} 131} 132</pre></div> 133 134<p>The run-time initialization code for use with a program generated with 135this linker script would include something like the following, to copy 136the initialized data from the ROM image to its runtime address. Notice 137how this code takes advantage of the symbols defined by the linker 138script. 139</p> 140<div class="smallexample"> 141<pre class="smallexample">extern char _etext, _data, _edata, _bstart, _bend; 142char *src = &_etext; 143char *dst = &_data; 144 145/* ROM has data at end of text; copy it. */ 146while (dst < &_edata) 147 *dst++ = *src++; 148 149/* Zero bss. */ 150for (dst = &_bstart; dst< &_bend; dst++) 151 *dst = 0; 152</pre></div> 153 154<hr> 155<div class="header"> 156<p> 157Next: <a href="Forced-Output-Alignment.html#Forced-Output-Alignment" accesskey="n" rel="next">Forced Output Alignment</a>, Previous: <a href="Output-Section-Type.html#Output-Section-Type" accesskey="p" rel="previous">Output Section Type</a>, Up: <a href="Output-Section-Attributes.html#Output-Section-Attributes" accesskey="u" rel="up">Output Section Attributes</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> 158</div> 159 160 161 162</body> 163</html> 164