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153 lines
6.3 KiB
HTML
153 lines
6.3 KiB
HTML
<!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 Assembler "as".
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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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-->
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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>Ld Sections (Using as)</title>
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<meta name="description" content="Ld Sections (Using as)">
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<meta name="keywords" content="Ld Sections (Using as)">
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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="AS-Index.html#AS-Index" rel="index" title="AS 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="As-Sections.html#As-Sections" rel="next" title="As Sections">
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<link href="Secs-Background.html#Secs-Background" rel="prev" title="Secs Background">
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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="Ld-Sections"></a>
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<div class="header">
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<p>
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Next: <a href="As-Sections.html#As-Sections" accesskey="n" rel="next">As Sections</a>, Previous: <a href="Secs-Background.html#Secs-Background" accesskey="p" rel="prev">Secs Background</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="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Linker-Sections"></a>
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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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</p>
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<dl compact="compact">
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<dd>
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<a name="index-named-sections"></a>
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<a name="index-sections_002c-named"></a>
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</dd>
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<dt><strong>named sections</strong></dt>
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<dd><a name="index-text-section"></a>
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<a name="index-data-section"></a>
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</dd>
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<dt><strong>text section</strong></dt>
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<dt><strong>data section</strong></dt>
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<dd><p>These sections hold your program. <code>as</code> 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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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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</p>
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<a name="index-bss-section"></a>
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</dd>
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<dt><strong>bss section</strong></dt>
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<dd><p>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>
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<a name="index-absolute-section"></a>
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</dd>
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<dt><strong>absolute section</strong></dt>
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<dd><p>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>
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<a name="index-undefined-section"></a>
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</dd>
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<dt><strong>undefined section</strong></dt>
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<dd><p>This “section” is a catch-all for address references to objects not in
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the preceding sections.
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</p></dd>
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</dl>
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<a name="index-relocation-example"></a>
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<p>An idealized example of three relocatable sections follows.
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The example uses the traditional section names ‘<samp>.text</samp>’ and ‘<samp>.data</samp>’.
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Memory addresses are on the horizontal axis.
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</p>
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<div class="smallexample">
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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></div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="As-Sections.html#As-Sections" accesskey="n" rel="next">As Sections</a>, Previous: <a href="Secs-Background.html#Secs-Background" accesskey="p" rel="prev">Secs Background</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="AS-Index.html#AS-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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