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150 lines
7.4 KiB
HTML
150 lines
7.4 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- This file documents the GNU linker LD
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(SiFive Binutils 2.32.0-2020.04.1)
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version 2.32.
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Copyright (C) 1991-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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Back-Cover Texts. A copy of the license is included in the
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section entitled "GNU Free Documentation License". -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Simple Example (LD)</title>
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<meta name="description" content="Simple Example (LD)">
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<meta name="keywords" content="Simple Example (LD)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="LD-Index.html#LD-Index" rel="index" title="LD Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Scripts.html#Scripts" rel="up" title="Scripts">
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<link href="Simple-Commands.html#Simple-Commands" rel="next" title="Simple Commands">
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<link href="Script-Format.html#Script-Format" rel="prev" title="Script Format">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Simple-Example"></a>
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<div class="header">
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<p>
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Next: <a href="Simple-Commands.html#Simple-Commands" accesskey="n" rel="next">Simple Commands</a>, Previous: <a href="Script-Format.html#Script-Format" accesskey="p" rel="prev">Script Format</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>
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</div>
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<hr>
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<a name="Simple-Linker-Script-Example"></a>
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<h3 class="section">3.3 Simple Linker Script Example</h3>
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<a name="index-linker-script-example"></a>
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<a name="index-example-of-linker-script"></a>
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<p>Many linker scripts are fairly simple.
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</p>
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<p>The simplest possible linker script has just one command:
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‘<samp>SECTIONS</samp>’. You use the ‘<samp>SECTIONS</samp>’ command to describe the
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memory layout of the output file.
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</p>
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<p>The ‘<samp>SECTIONS</samp>’ command is a powerful command. Here we will
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describe a simple use of it. Let’s assume your program consists only of
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code, initialized data, and uninitialized data. These will be in the
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‘<samp>.text</samp>’, ‘<samp>.data</samp>’, and ‘<samp>.bss</samp>’ sections, respectively.
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Let’s assume further that these are the only sections which appear in
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your input files.
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</p>
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<p>For this example, let’s say that the code should be loaded at address
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0x10000, and that the data should start at address 0x8000000. Here is a
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linker script which will do that:
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</p><div class="smallexample">
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<pre class="smallexample">SECTIONS
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{
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. = 0x10000;
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.text : { *(.text) }
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. = 0x8000000;
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.data : { *(.data) }
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.bss : { *(.bss) }
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}
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</pre></div>
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<p>You write the ‘<samp>SECTIONS</samp>’ command as the keyword ‘<samp>SECTIONS</samp>’,
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followed by a series of symbol assignments and output section
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descriptions enclosed in curly braces.
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</p>
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<p>The first line inside the ‘<samp>SECTIONS</samp>’ command of the above example
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sets the value of the special symbol ‘<samp>.</samp>’, which is the location
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counter. If you do not specify the address of an output section in some
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other way (other ways are described later), the address is set from the
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current value of the location counter. The location counter is then
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incremented by the size of the output section. At the start of the
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‘<samp>SECTIONS</samp>’ command, the location counter has the value ‘<samp>0</samp>’.
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</p>
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<p>The second line defines an output section, ‘<samp>.text</samp>’. The colon is
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required syntax which may be ignored for now. Within the curly braces
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after the output section name, you list the names of the input sections
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which should be placed into this output section. The ‘<samp>*</samp>’ is a
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wildcard which matches any file name. The expression ‘<samp>*(.text)</samp>’
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means all ‘<samp>.text</samp>’ input sections in all input files.
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</p>
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<p>Since the location counter is ‘<samp>0x10000</samp>’ when the output section
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‘<samp>.text</samp>’ is defined, the linker will set the address of the
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‘<samp>.text</samp>’ section in the output file to be ‘<samp>0x10000</samp>’.
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</p>
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<p>The remaining lines define the ‘<samp>.data</samp>’ and ‘<samp>.bss</samp>’ sections in
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the output file. The linker will place the ‘<samp>.data</samp>’ output section
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at address ‘<samp>0x8000000</samp>’. After the linker places the ‘<samp>.data</samp>’
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output section, the value of the location counter will be
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‘<samp>0x8000000</samp>’ plus the size of the ‘<samp>.data</samp>’ output section. The
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effect is that the linker will place the ‘<samp>.bss</samp>’ output section
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immediately after the ‘<samp>.data</samp>’ output section in memory.
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</p>
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<p>The linker will ensure that each output section has the required
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alignment, by increasing the location counter if necessary. In this
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example, the specified addresses for the ‘<samp>.text</samp>’ and ‘<samp>.data</samp>’
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sections will probably satisfy any alignment constraints, but the linker
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may have to create a small gap between the ‘<samp>.data</samp>’ and ‘<samp>.bss</samp>’
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sections.
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</p>
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<p>That’s it! That’s a simple and complete linker script.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Simple-Commands.html#Simple-Commands" accesskey="n" rel="next">Simple Commands</a>, Previous: <a href="Script-Format.html#Script-Format" accesskey="p" rel="prev">Script Format</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>
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</div>
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</body>
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</html>
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