109 lines
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109 lines
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<title>Ld Sections - Using as</title>
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<link rel="up" href="Sections.html#Sections" title="Sections">
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<link rel="prev" href="Secs-Background.html#Secs-Background" title="Secs Background">
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<!--
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This file documents the GNU Assembler "as".
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Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001, 2002,
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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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<div class="node">
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<p>
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<a name="Ld-Sections"></a>Next: <a rel="next" accesskey="n" href="As-Sections.html#As-Sections">As Sections</a>,
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Previous: <a rel="previous" accesskey="p" href="Secs-Background.html#Secs-Background">Secs Background</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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<h3 class="section">4.2 Linker Sections</h3>
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<p><code>ld</code> deals with just four kinds of sections, summarized below.
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<a name="index-named-sections-192"></a>
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<a name="index-sections_002c-named-193"></a>
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<dl><dt><strong>named sections</strong><dd><a name="index-text-section-194"></a><a name="index-data-section-195"></a><dt><strong>text section</strong><dt><strong>data section</strong><dd>These sections hold your program. <span class="command">as</span> and <code>ld</code> treat them as
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separate but equal sections. Anything you can say of one section is
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true of another.
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<!-- @ifset aout-bout -->
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When the program is running, however, it is
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customary for the text section to be unalterable. The
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text section is often shared among processes: it contains
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instructions, constants and the like. The data section of a running
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program is usually alterable: for example, C variables would be stored
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in the data section.
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<!-- @end ifset -->
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<p><a name="index-bss-section-196"></a><br><dt><strong>bss section</strong><dd>This section contains zeroed bytes when your program begins running. It
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is used to hold uninitialized variables or common storage. The length of
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each partial program's bss section is important, but because it starts
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out containing zeroed bytes there is no need to store explicit zero
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bytes in the object file. The bss section was invented to eliminate
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those explicit zeros from object files.
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<p><a name="index-absolute-section-197"></a><br><dt><strong>absolute section</strong><dd>Address 0 of this section is always “relocated” to runtime address 0.
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This is useful if you want to refer to an address that <code>ld</code> must
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not change when relocating. In this sense we speak of absolute
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addresses being “unrelocatable”: they do not change during relocation.
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<p><a name="index-undefined-section-198"></a><br><dt><strong>undefined section</strong><dd>This “section” is a catch-all for address references to objects not in
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the preceding sections.
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<!-- FIXME: ref to some other doc on obj-file formats could go here. -->
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</dl>
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<p><a name="index-relocation-example-199"></a>An idealized example of three relocatable sections follows.
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The example uses the traditional section names <span class="samp">.text</span> and <span class="samp">.data</span>.
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Memory addresses are on the horizontal axis.
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<!-- TEXI2ROFF-KILL -->
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<!-- END TEXI2ROFF-KILL -->
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<pre class="smallexample"> +-----+----+--+
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partial program # 1: |ttttt|dddd|00|
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+-----+----+--+
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text data bss
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seg. seg. seg.
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+---+---+---+
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partial program # 2: |TTT|DDD|000|
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+---+---+---+
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+--+---+-----+--+----+---+-----+~~
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linked program: | |TTT|ttttt| |dddd|DDD|00000|
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+--+---+-----+--+----+---+-----+~~
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addresses: 0 ...
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</pre>
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<!-- TEXI2ROFF-KILL -->
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<!-- END TEXI2ROFF-KILL -->
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</body></html>
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