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.
155 lines
8.0 KiB
HTML
155 lines
8.0 KiB
HTML
<html lang="en">
|
|
<head>
|
|
<title>Secs Background - Using as</title>
|
|
<meta http-equiv="Content-Type" content="text/html">
|
|
<meta name="description" content="Using as">
|
|
<meta name="generator" content="makeinfo 4.7">
|
|
<link title="Top" rel="start" href="index.html#Top">
|
|
<link rel="up" href="Sections.html#Sections" title="Sections">
|
|
<link rel="next" href="Ld-Sections.html#Ld-Sections" title="Ld Sections">
|
|
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
|
<!--
|
|
This file documents the GNU Assembler "as".
|
|
|
|
Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000, 2001, 2002,
|
|
2006, 2007 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 1.1
|
|
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''.
|
|
|
|
man end-->
|
|
<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; }
|
|
--></style>
|
|
</head>
|
|
<body>
|
|
<div class="node">
|
|
<p>
|
|
<a name="Secs-Background"></a>Next: <a rel="next" accesskey="n" href="Ld-Sections.html#Ld-Sections">Ld Sections</a>,
|
|
Up: <a rel="up" accesskey="u" href="Sections.html#Sections">Sections</a>
|
|
<hr><br>
|
|
</div>
|
|
|
|
<h3 class="section">4.1 Background</h3>
|
|
|
|
<p>Roughly, a section is a range of addresses, with no gaps; all data
|
|
“in” those addresses is treated the same for some particular purpose.
|
|
For example there may be a “read only” section.
|
|
|
|
<p><a name="index-linker_002c-and-assembler-187"></a><a name="index-assembler_002c-and-linker-188"></a>The linker <code>ld</code> reads many object files (partial programs) and
|
|
combines their contents to form a runnable program. When <span class="command">as</span>
|
|
emits an object file, the partial program is assumed to start at address 0.
|
|
<code>ld</code> assigns the final addresses for the partial program, so that
|
|
different partial programs do not overlap. This is actually an
|
|
oversimplification, but it suffices to explain how <span class="command">as</span> uses
|
|
sections.
|
|
|
|
<p><code>ld</code> moves blocks of bytes of your program to their run-time
|
|
addresses. These blocks slide to their run-time addresses as rigid
|
|
units; their length does not change and neither does the order of bytes
|
|
within them. Such a rigid unit is called a <em>section</em>. Assigning
|
|
run-time addresses to sections is called <dfn>relocation</dfn>. It includes
|
|
the task of adjusting mentions of object-file addresses so they refer to
|
|
the proper run-time addresses.
|
|
For the H8/300, and for the Renesas / SuperH SH,
|
|
<span class="command">as</span> pads sections if needed to
|
|
ensure they end on a word (sixteen bit) boundary.
|
|
|
|
<p><a name="index-standard-assembler-sections-189"></a>An object file written by <span class="command">as</span> has at least three sections, any
|
|
of which may be empty. These are named <dfn>text</dfn>, <dfn>data</dfn> and
|
|
<dfn>bss</dfn> sections.
|
|
|
|
<p>When it generates COFF or ELF output,
|
|
<span class="command">as</span> can also generate whatever other named sections you specify
|
|
using the <span class="samp">.section</span> directive (see <a href="Section.html#Section"><code>.section</code></a>).
|
|
If you do not use any directives that place output in the <span class="samp">.text</span>
|
|
or <span class="samp">.data</span> sections, these sections still exist, but are empty.
|
|
|
|
<p>When <span class="command">as</span> generates SOM or ELF output for the HPPA,
|
|
<span class="command">as</span> can also generate whatever other named sections you
|
|
specify using the <span class="samp">.space</span> and <span class="samp">.subspace</span> directives. See
|
|
<cite>HP9000 Series 800 Assembly Language Reference Manual</cite>
|
|
(HP 92432-90001) for details on the <span class="samp">.space</span> and <span class="samp">.subspace</span>
|
|
assembler directives.
|
|
|
|
<p>Additionally, <span class="command">as</span> uses different names for the standard
|
|
text, data, and bss sections when generating SOM output. Program text
|
|
is placed into the <span class="samp">$CODE$</span> section, data into <span class="samp">$DATA$</span>, and
|
|
BSS into <span class="samp">$BSS$</span>.
|
|
|
|
<p>Within the object file, the text section starts at address <code>0</code>, the
|
|
data section follows, and the bss section follows the data section.
|
|
|
|
<p>When generating either SOM or ELF output files on the HPPA, the text
|
|
section starts at address <code>0</code>, the data section at address
|
|
<code>0x4000000</code>, and the bss section follows the data section.
|
|
|
|
<p>To let <code>ld</code> know which data changes when the sections are
|
|
relocated, and how to change that data, <span class="command">as</span> also writes to the
|
|
object file details of the relocation needed. To perform relocation
|
|
<code>ld</code> must know, each time an address in the object
|
|
file is mentioned:
|
|
<ul>
|
|
<li>Where in the object file is the beginning of this reference to
|
|
an address?
|
|
<li>How long (in bytes) is this reference?
|
|
<li>Which section does the address refer to? What is the numeric value of
|
|
<pre class="display"> (<var>address</var>) − (<var>start-address of section</var>)?
|
|
</pre>
|
|
<li>Is the reference to an address “Program-Counter relative”?
|
|
</ul>
|
|
|
|
<p><a name="index-addresses_002c-format-of-190"></a><a name="index-section_002drelative-addressing-191"></a>In fact, every address <span class="command">as</span> ever uses is expressed as
|
|
<pre class="display"> (<var>section</var>) + (<var>offset into section</var>)
|
|
</pre>
|
|
<p class="noindent">Further, most expressions <span class="command">as</span> computes have this section-relative
|
|
nature.
|
|
(For some object formats, such as SOM for the HPPA, some expressions are
|
|
symbol-relative instead.)
|
|
|
|
<p>In this manual we use the notation {<var>secname</var> <var>N</var>} to mean “offset
|
|
<var>N</var> into section <var>secname</var>.”
|
|
|
|
<p>Apart from text, data and bss sections you need to know about the
|
|
<dfn>absolute</dfn> section. When <code>ld</code> mixes partial programs,
|
|
addresses in the absolute section remain unchanged. For example, address
|
|
<code>{absolute 0}</code> is “relocated” to run-time address 0 by
|
|
<code>ld</code>. Although the linker never arranges two partial programs'
|
|
data sections with overlapping addresses after linking, <em>by definition</em>
|
|
their absolute sections must overlap. Address <code>{absolute 239}</code> in one
|
|
part of a program is always the same address when the program is running as
|
|
address <code>{absolute 239}</code> in any other part of the program.
|
|
|
|
<p>The idea of sections is extended to the <dfn>undefined</dfn> section. Any
|
|
address whose section is unknown at assembly time is by definition
|
|
rendered {undefined <var>U</var>}—where <var>U</var> is filled in later.
|
|
Since numbers are always defined, the only way to generate an undefined
|
|
address is to mention an undefined symbol. A reference to a named
|
|
common block would be such a symbol: its value is unknown at assembly
|
|
time so it has section <em>undefined</em>.
|
|
|
|
<p>By analogy the word <em>section</em> is used to describe groups of sections in
|
|
the linked program. <code>ld</code> puts all partial programs' text
|
|
sections in contiguous addresses in the linked program. It is
|
|
customary to refer to the <em>text section</em> of a program, meaning all
|
|
the addresses of all partial programs' text sections. Likewise for
|
|
data and bss sections.
|
|
|
|
<p>Some sections are manipulated by <code>ld</code>; others are invented for
|
|
use of <span class="command">as</span> and have no meaning except during assembly.
|
|
|
|
</body></html>
|
|
|