You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
116 lines
5.9 KiB
HTML
116 lines
5.9 KiB
HTML
4 years ago
|
<html lang="en">
|
||
|
|
<head>
|
||
|
|
<title>Sub-Sections - Using as</title>
|
||
|
|
<meta http-equiv="Content-Type" content="text/html">
|
||
|
|
<meta name="description" content="Using as">
|
||
|
|
<meta name="generator" content="makeinfo 4.13">
|
||
|
|
<link title="Top" rel="start" href="index.html#Top">
|
||
|
|
<link rel="up" href="Sections.html#Sections" title="Sections">
|
||
|
|
<link rel="prev" href="As-Sections.html#As-Sections" title="As Sections">
|
||
|
|
<link rel="next" href="bss.html#bss" title="bss">
|
||
|
|
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
||
|
|
<!--
|
||
|
|
This file documents the GNU Assembler "as".
|
||
|
|
|
||
|
|
Copyright (C) 1991-2015 Free Software Foundation, Inc.
|
||
|
|
|
||
|
|
Permission is granted to copy, distribute and/or modify this document
|
||
|
|
under the terms of the GNU Free Documentation License, Version 1.3
|
||
|
|
or any later version published by the Free Software Foundation;
|
||
|
|
with no Invariant Sections, with no Front-Cover Texts, and with no
|
||
|
|
Back-Cover Texts. A copy of the license is included in the
|
||
|
|
section entitled ``GNU Free Documentation License''.
|
||
|
|
|
||
|
|
-->
|
||
|
|
<meta http-equiv="Content-Style-Type" content="text/css">
|
||
|
|
<style type="text/css"><!--
|
||
|
|
pre.display { font-family:inherit }
|
||
|
|
pre.format { font-family:inherit }
|
||
|
|
pre.smalldisplay { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallformat { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallexample { font-size:smaller }
|
||
|
|
pre.smalllisp { font-size:smaller }
|
||
|
|
span.sc { font-variant:small-caps }
|
||
|
|
span.roman { font-family:serif; font-weight:normal; }
|
||
|
|
span.sansserif { font-family:sans-serif; font-weight:normal; }
|
||
|
|
--></style>
|
||
|
|
</head>
|
||
|
|
<body>
|
||
|
|
<div class="node">
|
||
|
|
<a name="Sub-Sections"></a>
|
||
|
|
<a name="Sub_002dSections"></a>
|
||
|
|
<p>
|
||
|
|
Next: <a rel="next" accesskey="n" href="bss.html#bss">bss</a>,
|
||
|
|
Previous: <a rel="previous" accesskey="p" href="As-Sections.html#As-Sections">As Sections</a>,
|
||
|
|
Up: <a rel="up" accesskey="u" href="Sections.html#Sections">Sections</a>
|
||
|
|
<hr>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
<h3 class="section">4.4 Sub-Sections</h3>
|
||
|
|
|
||
|
|
<p><a name="index-numbered-subsections-204"></a><a name="index-grouping-data-205"></a>Assembled bytes
|
||
|
|
conventionally
|
||
|
|
fall into two sections: text and data.
|
||
|
|
You may have separate groups of
|
||
|
|
data in named sections
|
||
|
|
that you want to end up near to each other in the object file, even though they
|
||
|
|
are not contiguous in the assembler source. <samp><span class="command">as</span></samp> allows you to
|
||
|
|
use <dfn>subsections</dfn> for this purpose. Within each section, there can be
|
||
|
|
numbered subsections with values from 0 to 8192. Objects assembled into the
|
||
|
|
same subsection go into the object file together with other objects in the same
|
||
|
|
subsection. For example, a compiler might want to store constants in the text
|
||
|
|
section, but might not want to have them interspersed with the program being
|
||
|
|
assembled. In this case, the compiler could issue a ‘<samp><span class="samp">.text 0</span></samp>’ before each
|
||
|
|
section of code being output, and a ‘<samp><span class="samp">.text 1</span></samp>’ before each group of
|
||
|
|
constants being output.
|
||
|
|
|
||
|
|
<p>Subsections are optional. If you do not use subsections, everything
|
||
|
|
goes in subsection number zero.
|
||
|
|
|
||
|
|
<p>Each subsection is zero-padded up to a multiple of four bytes.
|
||
|
|
(Subsections may be padded a different amount on different flavors
|
||
|
|
of <samp><span class="command">as</span></samp>.)
|
||
|
|
|
||
|
|
<p>Subsections appear in your object file in numeric order, lowest numbered
|
||
|
|
to highest. (All this to be compatible with other people's assemblers.)
|
||
|
|
The object file contains no representation of subsections; <code>ld</code> and
|
||
|
|
other programs that manipulate object files see no trace of them.
|
||
|
|
They just see all your text subsections as a text section, and all your
|
||
|
|
data subsections as a data section.
|
||
|
|
|
||
|
|
<p>To specify which subsection you want subsequent statements assembled
|
||
|
|
into, use a numeric argument to specify it, in a ‘<samp><span class="samp">.text
|
||
|
|
</span><var>expression</var></samp>’ or a ‘<samp><span class="samp">.data </span><var>expression</var></samp>’ statement.
|
||
|
|
When generating COFF output, you
|
||
|
|
can also use an extra subsection
|
||
|
|
argument with arbitrary named sections: ‘<samp><span class="samp">.section </span><var>name</var><span class="samp">,
|
||
|
|
</span><var>expression</var></samp>’.
|
||
|
|
When generating ELF output, you
|
||
|
|
can also use the <code>.subsection</code> directive (see <a href="SubSection.html#SubSection">SubSection</a>)
|
||
|
|
to specify a subsection: ‘<samp><span class="samp">.subsection </span><var>expression</var></samp>’.
|
||
|
|
<var>Expression</var> should be an absolute expression
|
||
|
|
(see <a href="Expressions.html#Expressions">Expressions</a>). If you just say ‘<samp><span class="samp">.text</span></samp>’ then ‘<samp><span class="samp">.text 0</span></samp>’
|
||
|
|
is assumed. Likewise ‘<samp><span class="samp">.data</span></samp>’ means ‘<samp><span class="samp">.data 0</span></samp>’. Assembly
|
||
|
|
begins in <code>text 0</code>. For instance:
|
||
|
|
<pre class="smallexample"> .text 0 # The default subsection is text 0 anyway.
|
||
|
|
.ascii "This lives in the first text subsection. *"
|
||
|
|
.text 1
|
||
|
|
.ascii "But this lives in the second text subsection."
|
||
|
|
.data 0
|
||
|
|
.ascii "This lives in the data section,"
|
||
|
|
.ascii "in the first data subsection."
|
||
|
|
.text 0
|
||
|
|
.ascii "This lives in the first text section,"
|
||
|
|
.ascii "immediately following the asterisk (*)."
|
||
|
|
</pre>
|
||
|
|
<p>Each section has a <dfn>location counter</dfn> incremented by one for every byte
|
||
|
|
assembled into that section. Because subsections are merely a convenience
|
||
|
|
restricted to <samp><span class="command">as</span></samp> there is no concept of a subsection location
|
||
|
|
counter. There is no way to directly manipulate a location counter—but the
|
||
|
|
<code>.align</code> directive changes it, and any label definition captures its
|
||
|
|
current value. The location counter of the section where statements are being
|
||
|
|
assembled is said to be the <dfn>active</dfn> location counter.
|
||
|
|
|
||
|
|
</body></html>
|
||
|
|
|