188 lines
8.6 KiB
HTML
188 lines
8.6 KiB
HTML
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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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(Linaro_Binutils-2017.02)
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version 2.27.0.
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Copyright (C) 1991-2016 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 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>LD: Overlay Description</title>
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<meta name="description" content="LD: Overlay Description">
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<meta name="keywords" content="LD: Overlay Description">
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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="SECTIONS.html#SECTIONS" rel="up" title="SECTIONS">
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<link href="MEMORY.html#MEMORY" rel="next" title="MEMORY">
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<link href="Output-Section-Fill.html#Output-Section-Fill" rel="prev" title="Output Section Fill">
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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.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.indentedblock {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.smallindentedblock {margin-left: 3.2em; font-size: smaller}
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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.nocodebreak {white-space:nowrap}
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span.nolinebreak {white-space:nowrap}
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span.roman {font-family:serif; 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" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Overlay-Description"></a>
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<div class="header">
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<p>
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Previous: <a href="Output-Section-Attributes.html#Output-Section-Attributes" accesskey="p" rel="prev">Output Section Attributes</a>, Up: <a href="SECTIONS.html#SECTIONS" accesskey="u" rel="up">SECTIONS</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="Overlay-Description-1"></a>
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<h4 class="subsection">3.6.9 Overlay Description</h4>
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<a name="index-OVERLAY"></a>
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<a name="index-overlays"></a>
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<p>An overlay description provides an easy way to describe sections which
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are to be loaded as part of a single memory image but are to be run at
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the same memory address. At run time, some sort of overlay manager will
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copy the overlaid sections in and out of the runtime memory address as
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required, perhaps by simply manipulating addressing bits. This approach
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can be useful, for example, when a certain region of memory is faster
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than another.
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</p>
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<p>Overlays are described using the <code>OVERLAY</code> command. The
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<code>OVERLAY</code> command is used within a <code>SECTIONS</code> command, like an
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output section description. The full syntax of the <code>OVERLAY</code>
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command is as follows:
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</p><div class="smallexample">
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<pre class="smallexample">OVERLAY [<var>start</var>] : [NOCROSSREFS] [AT ( <var>ldaddr</var> )]
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{
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<var>secname1</var>
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{
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<var>output-section-command</var>
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<var>output-section-command</var>
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…
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} [:<var>phdr</var>…] [=<var>fill</var>]
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<var>secname2</var>
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{
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<var>output-section-command</var>
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<var>output-section-command</var>
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…
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} [:<var>phdr</var>…] [=<var>fill</var>]
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…
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} [><var>region</var>] [:<var>phdr</var>…] [=<var>fill</var>] [,]
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</pre></div>
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<p>Everything is optional except <code>OVERLAY</code> (a keyword), and each
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section must have a name (<var>secname1</var> and <var>secname2</var> above). The
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section definitions within the <code>OVERLAY</code> construct are identical to
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those within the general <code>SECTIONS</code> construct (see <a href="SECTIONS.html#SECTIONS">SECTIONS</a>),
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except that no addresses and no memory regions may be defined for
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sections within an <code>OVERLAY</code>.
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</p>
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<p>The comma at the end may be required if a <var>fill</var> is used and
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the next <var>sections-command</var> looks like a continuation of the expression.
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</p>
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<p>The sections are all defined with the same starting address. The load
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addresses of the sections are arranged such that they are consecutive in
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memory starting at the load address used for the <code>OVERLAY</code> as a
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whole (as with normal section definitions, the load address is optional,
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and defaults to the start address; the start address is also optional,
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and defaults to the current value of the location counter).
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</p>
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<p>If the <code>NOCROSSREFS</code> keyword is used, and there are any
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references among the sections, the linker will report an error. Since
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the sections all run at the same address, it normally does not make
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sense for one section to refer directly to another.
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See <a href="Miscellaneous-Commands.html#Miscellaneous-Commands">NOCROSSREFS</a>.
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</p>
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<p>For each section within the <code>OVERLAY</code>, the linker automatically
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provides two symbols. The symbol <code>__load_start_<var>secname</var></code> is
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defined as the starting load address of the section. The symbol
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<code>__load_stop_<var>secname</var></code> is defined as the final load address of
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the section. Any characters within <var>secname</var> which are not legal
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within C identifiers are removed. C (or assembler) code may use these
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symbols to move the overlaid sections around as necessary.
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</p>
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<p>At the end of the overlay, the value of the location counter is set to
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the start address of the overlay plus the size of the largest section.
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</p>
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<p>Here is an example. Remember that this would appear inside a
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<code>SECTIONS</code> construct.
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</p><div class="smallexample">
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<pre class="smallexample"> OVERLAY 0x1000 : AT (0x4000)
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{
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.text0 { o1/*.o(.text) }
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.text1 { o2/*.o(.text) }
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}
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</pre></div>
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<p>This will define both ‘<samp>.text0</samp>’ and ‘<samp>.text1</samp>’ to start at
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address 0x1000. ‘<samp>.text0</samp>’ will be loaded at address 0x4000, and
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‘<samp>.text1</samp>’ will be loaded immediately after ‘<samp>.text0</samp>’. The
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following symbols will be defined if referenced: <code>__load_start_text0</code>,
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<code>__load_stop_text0</code>, <code>__load_start_text1</code>,
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<code>__load_stop_text1</code>.
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</p>
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<p>C code to copy overlay <code>.text1</code> into the overlay area might look
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like the following.
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</p>
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<div class="smallexample">
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<pre class="smallexample"> extern char __load_start_text1, __load_stop_text1;
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memcpy ((char *) 0x1000, &__load_start_text1,
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&__load_stop_text1 - &__load_start_text1);
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</pre></div>
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<p>Note that the <code>OVERLAY</code> command is just syntactic sugar, since
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everything it does can be done using the more basic commands. The above
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example could have been written identically as follows.
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</p>
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<div class="smallexample">
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<pre class="smallexample"> .text0 0x1000 : AT (0x4000) { o1/*.o(.text) }
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PROVIDE (__load_start_text0 = LOADADDR (.text0));
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PROVIDE (__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0));
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.text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) { o2/*.o(.text) }
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PROVIDE (__load_start_text1 = LOADADDR (.text1));
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PROVIDE (__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1));
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. = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));
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</pre></div>
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<hr>
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<div class="header">
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<p>
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Previous: <a href="Output-Section-Attributes.html#Output-Section-Attributes" accesskey="p" rel="prev">Output Section Attributes</a>, Up: <a href="SECTIONS.html#SECTIONS" accesskey="u" rel="up">SECTIONS</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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