149 lines
7.1 KiB
HTML
149 lines
7.1 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 Assembler "as".
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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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-->
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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>Using as: Sub-Sections</title>
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<meta name="description" content="Using as: Sub-Sections">
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<meta name="keywords" content="Using as: Sub-Sections">
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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="bss.html#bss" rel="next" title="bss">
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<link href="As-Sections.html#As-Sections" rel="prev" title="As Sections">
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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="Sub_002dSections"></a>
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<div class="header">
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<p>
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Next: <a href="bss.html#bss" accesskey="n" rel="next">bss</a>, Previous: <a href="As-Sections.html#As-Sections" accesskey="p" rel="prev">As Sections</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="Sub_002dSections-1"></a>
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<h3 class="section">4.4 Sub-Sections</h3>
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<a name="index-numbered-subsections"></a>
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<a name="index-grouping-data"></a>
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<p>Assembled bytes
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conventionally
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fall into two sections: text and data.
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You may have separate groups of
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data in named sections
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that you want to end up near to each other in the object file, even though they
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are not contiguous in the assembler source. <code>as</code> allows you to
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use <em>subsections</em> for this purpose. Within each section, there can be
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numbered subsections with values from 0 to 8192. Objects assembled into the
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same subsection go into the object file together with other objects in the same
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subsection. For example, a compiler might want to store constants in the text
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section, but might not want to have them interspersed with the program being
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assembled. In this case, the compiler could issue a ‘<samp>.text 0</samp>’ before each
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section of code being output, and a ‘<samp>.text 1</samp>’ before each group of
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constants being output.
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</p>
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<p>Subsections are optional. If you do not use subsections, everything
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goes in subsection number zero.
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</p>
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<p>Each subsection is zero-padded up to a multiple of four bytes.
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(Subsections may be padded a different amount on different flavors
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of <code>as</code>.)
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</p>
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<p>Subsections appear in your object file in numeric order, lowest numbered
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to highest. (All this to be compatible with other people’s assemblers.)
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The object file contains no representation of subsections; <code>ld</code> and
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other programs that manipulate object files see no trace of them.
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They just see all your text subsections as a text section, and all your
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data subsections as a data section.
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</p>
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<p>To specify which subsection you want subsequent statements assembled
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into, use a numeric argument to specify it, in a ‘<samp>.text
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<var>expression</var></samp>’ or a ‘<samp>.data <var>expression</var></samp>’ statement.
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When generating COFF output, you
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can also use an extra subsection
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argument with arbitrary named sections: ‘<samp>.section <var>name</var>,
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<var>expression</var></samp>’.
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When generating ELF output, you
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can also use the <code>.subsection</code> directive (see <a href="SubSection.html#SubSection">SubSection</a>)
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to specify a subsection: ‘<samp>.subsection <var>expression</var></samp>’.
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<var>Expression</var> should be an absolute expression
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(see <a href="Expressions.html#Expressions">Expressions</a>). If you just say ‘<samp>.text</samp>’ then ‘<samp>.text 0</samp>’
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is assumed. Likewise ‘<samp>.data</samp>’ means ‘<samp>.data 0</samp>’. Assembly
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begins in <code>text 0</code>. For instance:
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</p><div class="smallexample">
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<pre class="smallexample">.text 0 # The default subsection is text 0 anyway.
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.ascii "This lives in the first text subsection. *"
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.text 1
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.ascii "But this lives in the second text subsection."
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.data 0
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.ascii "This lives in the data section,"
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.ascii "in the first data subsection."
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.text 0
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.ascii "This lives in the first text section,"
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.ascii "immediately following the asterisk (*)."
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</pre></div>
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<p>Each section has a <em>location counter</em> incremented by one for every byte
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assembled into that section. Because subsections are merely a convenience
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restricted to <code>as</code> there is no concept of a subsection location
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counter. There is no way to directly manipulate a location counter—but the
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<code>.align</code> directive changes it, and any label definition captures its
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current value. The location counter of the section where statements are being
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assembled is said to be the <em>active</em> location counter.
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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="bss.html#bss" accesskey="n" rel="next">bss</a>, Previous: <a href="As-Sections.html#As-Sections" accesskey="p" rel="prev">As Sections</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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