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142 lines
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142 lines
6.9 KiB
HTML
<html lang="en">
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<head>
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<title>Overlay Description - Untitled</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="Untitled">
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<meta name="generator" content="makeinfo 4.7">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="SECTIONS.html#SECTIONS" title="SECTIONS">
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<link rel="prev" href="Output-Section-Attributes.html#Output-Section-Attributes" title="Output Section Attributes">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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This file documents the GNU linker LD
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(GNU Binutils)
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version 2.19.
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Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000,
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2001, 2002, 2003, 2004, 2005, 2006, 2007 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 1.1
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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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<meta http-equiv="Content-Style-Type" content="text/css">
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<style type="text/css"><!--
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pre.display { font-family:inherit }
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pre.format { font-family:inherit }
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pre.smalldisplay { font-family:inherit; font-size:smaller }
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pre.smallformat { font-family:inherit; font-size:smaller }
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span.sc { font-variant:small-caps }
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span.roman { font-family: serif; font-weight: normal; }
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--></style>
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</head>
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<body>
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<div class="node">
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<p>
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<a name="Overlay-Description"></a>Previous: <a rel="previous" accesskey="p" href="Output-Section-Attributes.html#Output-Section-Attributes">Output Section Attributes</a>,
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Up: <a rel="up" accesskey="u" href="SECTIONS.html#SECTIONS">SECTIONS</a>
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<hr><br>
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</div>
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<h4 class="subsection">3.6.9 Overlay Description</h4>
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<p><a name="index-OVERLAY-429"></a><a name="index-overlays-430"></a>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>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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<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>
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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> contruct (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>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>If the <code>NOCROSSREFS</code> keyword is used, and there any references
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among the sections, the linker will report an error. Since the sections
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all run at the same address, it normally does not make sense for one
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section to refer directly to another. See <a href="Miscellaneous-Commands.html#Miscellaneous-Commands">NOCROSSREFS</a>.
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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_</code><var>secname</var> is
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defined as the starting load address of the section. The symbol
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<code>__load_stop_</code><var>secname</var> 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>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>Here is an example. Remember that this would appear inside a
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<code>SECTIONS</code> construct.
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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>
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<p class="noindent">This will define both <span class="samp">.text0</span> and <span class="samp">.text1</span> to start at
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address 0x1000. <span class="samp">.text0</span> will be loaded at address 0x4000, and
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<span class="samp">.text1</span> will be loaded immediately after <span class="samp">.text0</span>. 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>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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<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>
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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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<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>
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</body></html>
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