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<title>Secs Background - Using as</title>
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<!--
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This file documents the GNU Assembler "as".
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Copyright (C) 1991-2015 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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<div class="node">
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<a name="Secs-Background"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Ld-Sections.html#Ld-Sections">Ld 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">4.1 Background</h3>
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<p>Roughly, a section is a range of addresses, with no gaps; all data
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“in” those addresses is treated the same for some particular purpose.
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For example there may be a “read only” section.
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<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
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combines their contents to form a runnable program. When <samp><span class="command">as</span></samp>
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emits an object file, the partial program is assumed to start at address 0.
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<code>ld</code> assigns the final addresses for the partial program, so that
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different partial programs do not overlap. This is actually an
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oversimplification, but it suffices to explain how <samp><span class="command">as</span></samp> uses
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sections.
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<p><code>ld</code> moves blocks of bytes of your program to their run-time
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addresses. These blocks slide to their run-time addresses as rigid
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units; their length does not change and neither does the order of bytes
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within them. Such a rigid unit is called a <em>section</em>. Assigning
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run-time addresses to sections is called <dfn>relocation</dfn>. It includes
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the task of adjusting mentions of object-file addresses so they refer to
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the proper run-time addresses.
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For the H8/300, and for the Renesas / SuperH SH,
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<samp><span class="command">as</span></samp> pads sections if needed to
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ensure they end on a word (sixteen bit) boundary.
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<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
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of which may be empty. These are named <dfn>text</dfn>, <dfn>data</dfn> and
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<dfn>bss</dfn> sections.
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<p>When it generates COFF or ELF output,
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<samp><span class="command">as</span></samp> can also generate whatever other named sections you specify
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using the ‘<samp><span class="samp">.section</span></samp>’ directive (see <a href="Section.html#Section"><code>.section</code></a>).
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If you do not use any directives that place output in the ‘<samp><span class="samp">.text</span></samp>’
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or ‘<samp><span class="samp">.data</span></samp>’ sections, these sections still exist, but are empty.
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<p>When <samp><span class="command">as</span></samp> generates SOM or ELF output for the HPPA,
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<samp><span class="command">as</span></samp> can also generate whatever other named sections you
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specify using the ‘<samp><span class="samp">.space</span></samp>’ and ‘<samp><span class="samp">.subspace</span></samp>’ directives. See
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<cite>HP9000 Series 800 Assembly Language Reference Manual</cite>
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(HP 92432-90001) for details on the ‘<samp><span class="samp">.space</span></samp>’ and ‘<samp><span class="samp">.subspace</span></samp>’
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assembler directives.
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<p>Additionally, <samp><span class="command">as</span></samp> uses different names for the standard
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text, data, and bss sections when generating SOM output. Program text
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is placed into the ‘<samp><span class="samp">$CODE$</span></samp>’ section, data into ‘<samp><span class="samp">$DATA$</span></samp>’, and
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BSS into ‘<samp><span class="samp">$BSS$</span></samp>’.
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<p>Within the object file, the text section starts at address <code>0</code>, the
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data section follows, and the bss section follows the data section.
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<p>When generating either SOM or ELF output files on the HPPA, the text
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section starts at address <code>0</code>, the data section at address
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<code>0x4000000</code>, and the bss section follows the data section.
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<p>To let <code>ld</code> know which data changes when the sections are
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relocated, and how to change that data, <samp><span class="command">as</span></samp> also writes to the
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object file details of the relocation needed. To perform relocation
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<code>ld</code> must know, each time an address in the object
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file is mentioned:
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<ul>
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<li>Where in the object file is the beginning of this reference to
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an address?
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<li>How long (in bytes) is this reference?
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<li>Which section does the address refer to? What is the numeric value of
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<pre class="display"> (<var>address</var>) − (<var>start-address of section</var>)?
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</pre>
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<li>Is the reference to an address “Program-Counter relative”?
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</ul>
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<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
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<pre class="display"> (<var>section</var>) + (<var>offset into section</var>)
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</pre>
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<p class="noindent">Further, most expressions <samp><span class="command">as</span></samp> computes have this section-relative
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nature.
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(For some object formats, such as SOM for the HPPA, some expressions are
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symbol-relative instead.)
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<p>In this manual we use the notation {<var>secname</var> <var>N</var>} to mean “offset
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<var>N</var> into section <var>secname</var>.”
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<p>Apart from text, data and bss sections you need to know about the
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<dfn>absolute</dfn> section. When <code>ld</code> mixes partial programs,
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addresses in the absolute section remain unchanged. For example, address
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<code>{absolute 0}</code> is “relocated” to run-time address 0 by
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<code>ld</code>. Although the linker never arranges two partial programs'
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data sections with overlapping addresses after linking, <em>by definition</em>
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their absolute sections must overlap. Address <code>{absolute 239}</code> in one
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part of a program is always the same address when the program is running as
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address <code>{absolute 239}</code> in any other part of the program.
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<p>The idea of sections is extended to the <dfn>undefined</dfn> section. Any
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address whose section is unknown at assembly time is by definition
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rendered {undefined <var>U</var>}—where <var>U</var> is filled in later.
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Since numbers are always defined, the only way to generate an undefined
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address is to mention an undefined symbol. A reference to a named
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common block would be such a symbol: its value is unknown at assembly
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time so it has section <em>undefined</em>.
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<p>By analogy the word <em>section</em> is used to describe groups of sections in
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the linked program. <code>ld</code> puts all partial programs' text
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sections in contiguous addresses in the linked program. It is
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customary to refer to the <em>text section</em> of a program, meaning all
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the addresses of all partial programs' text sections. Likewise for
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data and bss sections.
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<p>Some sections are manipulated by <code>ld</code>; others are invented for
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use of <samp><span class="command">as</span></samp> and have no meaning except during assembly.
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</body></html>
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