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<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-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 <samp><span class="command">as</span></samp>
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 <samp><span class="command">as</span></samp> 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,
<samp><span class="command">as</span></samp> 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 <samp><span class="command">as</span></samp> 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,
<samp><span class="command">as</span></samp> can also generate whatever other named sections you specify
using the &lsquo;<samp><span class="samp">.section</span></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><span class="samp">.text</span></samp>&rsquo;
or &lsquo;<samp><span class="samp">.data</span></samp>&rsquo; sections, these sections still exist, but are empty.

   <p>When <samp><span class="command">as</span></samp> generates SOM or ELF output for the HPPA,
<samp><span class="command">as</span></samp> can also generate whatever other named sections you
specify using the &lsquo;<samp><span class="samp">.space</span></samp>&rsquo; and &lsquo;<samp><span class="samp">.subspace</span></samp>&rsquo; directives.  See
<cite>HP9000 Series 800 Assembly Language Reference Manual</cite>
(HP 92432-90001) for details on the &lsquo;<samp><span class="samp">.space</span></samp>&rsquo; and &lsquo;<samp><span class="samp">.subspace</span></samp>&rsquo;
assembler directives.

   <p>Additionally, <samp><span class="command">as</span></samp> uses different names for the standard
text, data, and bss sections when generating SOM output.  Program text
is placed into the &lsquo;<samp><span class="samp">$CODE$</span></samp>&rsquo; section, data into &lsquo;<samp><span class="samp">$DATA$</span></samp>&rsquo;, and
BSS into &lsquo;<samp><span class="samp">$BSS$</span></samp>&rsquo;.

   <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, <samp><span class="command">as</span></samp> 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>) &minus; (<var>start-address of section</var>)?
</pre>
     <li>Is the reference to an address &ldquo;Program-Counter relative&rdquo;? 
</ul>

   <p><a name="index-addresses_002c-format-of-190"></a><a name="index-section_002drelative-addressing-191"></a>In fact, every address <samp><span class="command">as</span></samp> ever uses is expressed as
<pre class="display">     (<var>section</var>) + (<var>offset into section</var>)
</pre>
   <p class="noindent">Further, most expressions <samp><span class="command">as</span></samp> 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 &ldquo;offset
<var>N</var> into section <var>secname</var>.&rdquo;

   <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 &ldquo;relocated&rdquo; 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>}&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>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 <samp><span class="command">as</span></samp> and have no meaning except during assembly.

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