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<a name="Secs-Background"></a>
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<p>
Next: <a href="Ld-Sections.html#Ld-Sections" accesskey="n" rel="next">Ld Sections</a>, Up: <a href="Sections.html#Sections" accesskey="u" rel="up">Sections</a> &nbsp; [<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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<hr>
<a name="Background"></a>
<h3 class="section">4.1 Background</h3>

<p>Roughly, a section is a range of addresses, with no gaps; all data
&ldquo;in&rdquo; those addresses is treated the same for some particular purpose.
For example there may be a &ldquo;read only&rdquo; section.
</p>
<a name="index-linker_002c-and-assembler"></a>
<a name="index-assembler_002c-and-linker"></a>
<p>The linker <code>ld</code> reads many object files (partial programs) and
combines their contents to form a runnable program.  When <code>as</code>
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 <code>as</code> uses
sections.
</p>
<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 <em>relocation</em>.  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,
<code>as</code> pads sections if needed to
ensure they end on a word (sixteen bit) boundary.
</p>
<a name="index-standard-assembler-sections"></a>
<p>An object file written by <code>as</code> has at least three sections, any
of which may be empty.  These are named <em>text</em>, <em>data</em> and
<em>bss</em> sections.
</p>
<p>When it generates COFF or ELF output,
<code>as</code> can also generate whatever other named sections you specify
using the &lsquo;<samp>.section</samp>&rsquo; directive (see <a href="Section.html#Section"><code>.section</code></a>).
If you do not use any directives that place output in the &lsquo;<samp>.text</samp>&rsquo;
or &lsquo;<samp>.data</samp>&rsquo; sections, these sections still exist, but are empty.
</p>
<p>When <code>as</code> generates SOM or ELF output for the HPPA,
<code>as</code> can also generate whatever other named sections you
specify using the &lsquo;<samp>.space</samp>&rsquo; and &lsquo;<samp>.subspace</samp>&rsquo; directives.  See
<cite>HP9000 Series 800 Assembly Language Reference Manual</cite>
(HP 92432-90001) for details on the &lsquo;<samp>.space</samp>&rsquo; and &lsquo;<samp>.subspace</samp>&rsquo;
assembler directives.
</p>
<p>Additionally, <code>as</code> uses different names for the standard
text, data, and bss sections when generating SOM output.  Program text
is placed into the &lsquo;<samp>$CODE$</samp>&rsquo; section, data into &lsquo;<samp>$DATA$</samp>&rsquo;, and
BSS into &lsquo;<samp>$BSS$</samp>&rsquo;.
</p>
<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>
<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>
<p>To let <code>ld</code> know which data changes when the sections are
relocated, and how to change that data, <code>as</code> 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:
</p><ul>
<li> Where in the object file is the beginning of this reference to
an address?
</li><li> How long (in bytes) is this reference?
</li><li> Which section does the address refer to?  What is the numeric value of
<div class="display">
<pre class="display">(<var>address</var>) - (<var>start-address of section</var>)?
</pre></div>
</li><li> Is the reference to an address &ldquo;Program-Counter relative&rdquo;?
</li></ul>

<a name="index-addresses_002c-format-of"></a>
<a name="index-section_002drelative-addressing"></a>
<p>In fact, every address <code>as</code> ever uses is expressed as
</p><div class="display">
<pre class="display">(<var>section</var>) + (<var>offset into section</var>)
</pre></div>
<p>Further, most expressions <code>as</code> computes have this section-relative
nature.
(For some object formats, such as SOM for the HPPA, some expressions are
symbol-relative instead.)
</p>
<p>In this manual we use the notation {<var>secname</var> <var>N</var>} to mean &ldquo;offset
<var>N</var> into section <var>secname</var>.&rdquo;
</p>
<p>Apart from text, data and bss sections you need to know about the
<em>absolute</em> section.  When <code>ld</code> mixes partial programs,
addresses in the absolute section remain unchanged.  For example, address
<code>{absolute 0}</code> is &ldquo;relocated&rdquo; to run-time address 0 by
<code>ld</code>.  Although the linker never arranges two partial programs&rsquo;
data sections with overlapping addresses after linking, <em>by definition</em>
their absolute sections must overlap.  Address <code>{absolute&nbsp;239}</code> in one
part of a program is always the same address when the program is running as
address <code>{absolute&nbsp;239}</code> in any other part of the program.
</p>
<p>The idea of sections is extended to the <em>undefined</em> section.  Any
address whose section is unknown at assembly time is by definition
rendered {undefined <var>U</var>}&mdash;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>
<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&rsquo; 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&rsquo; text sections.  Likewise for
data and bss sections.
</p>
<p>Some sections are manipulated by <code>ld</code>; others are invented for
use of <code>as</code> and have no meaning except during assembly.
</p>
<hr>
<div class="header">
<p>
Next: <a href="Ld-Sections.html#Ld-Sections" accesskey="n" rel="next">Ld Sections</a>, Up: <a href="Sections.html#Sections" accesskey="u" rel="up">Sections</a> &nbsp; [<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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