116 lines
5.9 KiB
HTML
116 lines
5.9 KiB
HTML
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<html lang="en">
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<head>
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<title>Sub-Sections - Using as</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="Using as">
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<meta name="generator" content="makeinfo 4.13">
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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="As-Sections.html#As-Sections" title="As Sections">
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<link rel="next" href="bss.html#bss" title="bss">
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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 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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<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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pre.smallexample { font-size:smaller }
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pre.smalllisp { 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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span.sansserif { font-family:sans-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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<a name="Sub-Sections"></a>
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<a name="Sub_002dSections"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="bss.html#bss">bss</a>,
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Previous: <a rel="previous" accesskey="p" href="As-Sections.html#As-Sections">As Sections</a>,
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Up: <a rel="up" accesskey="u" href="Sections.html#Sections">Sections</a>
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<hr>
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</div>
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<h3 class="section">4.4 Sub-Sections</h3>
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<p><a name="index-numbered-subsections-206"></a><a name="index-grouping-data-207"></a>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. <samp><span class="command">as</span></samp> allows you to
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use <dfn>subsections</dfn> 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><span class="samp">.text 0</span></samp>’ before each
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section of code being output, and a ‘<samp><span class="samp">.text 1</span></samp>’ before each group of
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constants being output.
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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>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 <samp><span class="command">as</span></samp>.)
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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>To specify which subsection you want subsequent statements assembled
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into, use a numeric argument to specify it, in a ‘<samp><span class="samp">.text
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</span><var>expression</var></samp>’ or a ‘<samp><span class="samp">.data </span><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><span class="samp">.section </span><var>name</var><span class="samp">,
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</span><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><span class="samp">.subsection </span><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><span class="samp">.text</span></samp>’ then ‘<samp><span class="samp">.text 0</span></samp>’
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is assumed. Likewise ‘<samp><span class="samp">.data</span></samp>’ means ‘<samp><span class="samp">.data 0</span></samp>’. Assembly
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begins in <code>text 0</code>. For instance:
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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>
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<p>Each section has a <dfn>location counter</dfn> 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 <samp><span class="command">as</span></samp> 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 <dfn>active</dfn> location counter.
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</body></html>
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