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216 lines
8.0 KiB
HTML
216 lines
8.0 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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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Linker Scripts (Embed with GNU)</title>
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<meta name="description" content="Linker Scripts (Embed with GNU)">
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<meta name="keywords" content="Linker Scripts (Embed with GNU)">
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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="leds_002ec.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Libraries.html#Libraries" rel="up" title="Libraries">
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<link href="What-to-do-now.html#What-to-do-now" rel="next" title="What to do now">
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<link href="Crt0.html#Crt0" rel="prev" title="Crt0">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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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="Linker-Scripts"></a>
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<div class="header">
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<p>
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Next: <a href="What-to-do-now.html#What-to-do-now" accesskey="n" rel="next">What to do now</a>, Previous: <a href="Crt0.html#Crt0" accesskey="p" rel="prev">Crt0</a>, Up: <a href="Libraries.html#Libraries" accesskey="u" rel="up">Libraries</a> [<a href="leds_002ec.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p>
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</div>
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<hr>
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<a name="Linker-scripts-for-memory-management"></a>
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<h3 class="section">3.2 Linker scripts for memory management</h3>
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<p>The linker script sets up the memory map of an application. It also
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sets up default values for variables used elsewhere by sbrk() and the
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crt0. These default variables are typically called <code>_bss_start</code> and
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<code>_end</code>.
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</p>
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<p>For G++, the constructor and destructor tables must also be setup here.
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The actual section names vary depending on the object file format. For
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<code>a.out</code> and <code>coff</code>, the three main sections are <code>.text</code>,
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<code>.data</code>, and <code>.bss</code>.
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</p>
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<p>Now that you have an image, you can test to make sure it got the
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memory map right. You can do this by having the linker create a memory
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map (by using the <code>-Map</code> option), or afterwards by using <code>nm</code> to
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check a few critical addresses like <code>start</code>, <code>bss_end</code>, and
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<code>_etext</code>.
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</p>
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<p>Here’s a breakdown of a linker script for a m68k based target board.
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See the file <code>libgloss/m68k/idp.ld</code>, or go to the appendixes in
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the end of the manual. <a href="idp_002eld.html#idp_002eld">Example Linker Script</a>.
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</p>
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<div class="smallexample">
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<pre class="smallexample">STARTUP(crt0.o)
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OUTPUT_ARCH(m68k)
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INPUT(idp.o)
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SEARCH_DIR(.)
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__DYNAMIC = 0;
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</pre></div>
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<p>The <code>STARTUP</code> command loads the file specified so that it’s
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first. In this case it also doubles to load the file as well, because
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the m68k-coff configuration defaults to not linking in the crt0.o by
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default. It assumes that the developer probably has their own crt0.o.
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This behavior is controlled in the config file for each architecture.
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It’s a macro called <code>STARTFILE_SPEC</code>, and if it’s set to
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<code>null</code>, then when <code>gcc</code> formats it’s command line, it doesn’t
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add <code>crto.o</code>. Any file name can be specified here, but the default
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is always <code>crt0.o</code>.
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</p>
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<p>Course if you only use <code>ld</code> to link, then the control of whether or
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not to link in <code>crt0.o</code> is done on the command line. If you have
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multiple crto files, then you can leave this out all together, and link
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in the <code>crt0.o</code> in the makefile, or by having different linker
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scripts. Sometimes this is done for initializing floating point
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optionally, or to add device support.
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</p>
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<p>The <code>OUTPUT_ARCH</code> sets architecture the output file is for.
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</p>
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<p><code>INPUT</code> loads in the file specified. In this case, it’s a relocated
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library that contains the definitions for the low-level functions need
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by libc.a. This could have also been specified on the command line, but
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as it’s always needed, it might as well be here as a default.
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<code>SEARCH_DIR</code> specifies the path to look for files, and
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<code>_DYNAMIC</code> means in this case there are no shared libraries.
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</p>
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<div class="smallexample">
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<pre class="smallexample">/*
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* Setup the memory map of the MC68ec0x0 Board (IDP)
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* stack grows up towards high memory. This works for
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* both the rom68k and the mon68k monitors.
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*/
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MEMORY
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{
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ram : ORIGIN = 0x10000, LENGTH = 2M
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}
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</pre></div>
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<p>This specifies a name for a section that can be referred to later in the
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script. In this case, it’s only a pointer to the beginning of free RAM
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space, with an upper limit at 2M. If the output file exceeds the upper
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limit, it will produce an error message.
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</p>
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<div class="smallexample">
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<pre class="smallexample">/*
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* stick everything in ram (of course)
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*/
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SECTIONS
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{
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.text :
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{
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CREATE_OBJECT_SYMBOLS
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*(.text)
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etext = .;
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__CTOR_LIST__ = .;
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LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)
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*(.ctors)
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LONG(0)
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__CTOR_END__ = .;
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__DTOR_LIST__ = .;
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LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)
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*(.dtors)
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LONG(0)
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__DTOR_END__ = .;
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*(.lit)
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*(.shdata)
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} > ram
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.shbss SIZEOF(.text) + ADDR(.text) : {
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*(.shbss)
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}
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</pre></div>
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<p>Set up the <code>.text</code> section. In a <code>COFF</code> file, .text is where
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all the actual instructions are. This also sets up the <em>CONTRUCTOR</em>
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and the <em>DESTRUCTOR</em> tables for <code>G++</code>. Notice that the section
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description redirects itself to the <em>ram</em> variable setup earlier.
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</p>
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<div class="smallexample">
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<pre class="smallexample"> .talias : { } > ram
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.data : {
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*(.data)
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CONSTRUCTORS
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_edata = .;
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} > ram
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</pre></div>
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<p>Setup the <code>.data</code> section. In a <code>coff</code> file, this is where all
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he initialized data goes. <code>CONSTRUCTORS</code> is a special command used
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by <code>ld</code>.
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</p>
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<div class="smallexample">
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<pre class="smallexample"> .bss SIZEOF(.data) + ADDR(.data) :
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{
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__bss_start = ALIGN(0x8);
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*(.bss)
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*(COMMON)
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end = ALIGN(0x8);
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_end = ALIGN(0x8);
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__end = ALIGN(0x8);
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}
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.mstack : { } > ram
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.rstack : { } > ram
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.stab . (NOLOAD) :
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{
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[ .stab ]
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}
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.stabstr . (NOLOAD) :
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{
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[ .stabstr ]
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}
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}
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</pre></div>
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<p>Setup the <code>.bss</code> section. In a <code>COFF</code> file, this is where
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unitialized data goes. The symbols <code>_bss_start</code> and <code>_end</code>
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are setup here for use by the <code>crt0.o</code> when it zero’s the
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<code>.bss</code> section.
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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="What-to-do-now.html#What-to-do-now" accesskey="n" rel="next">What to do now</a>, Previous: <a href="Crt0.html#Crt0" accesskey="p" rel="prev">Crt0</a>, Up: <a href="Libraries.html#Libraries" accesskey="u" rel="up">Libraries</a> [<a href="leds_002ec.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p>
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</div>
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</body>
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</html>
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