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<title>Data descriptor</title>
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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Data descriptor</span></span>
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<div id="mw-content-text" class="mw-body-content mw-content-ltr" lang="en" dir="ltr"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><p>In <a href="Computing" title="Computing">computing</a>, a <b>data descriptor</b> is a structure containing information that describes data.
</p><p>Data descriptors may be used in <a href="Compiler" title="Compiler">compilers</a>,<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> as a software structure at run time in languages like <a href="Ada_(programming_language)" title="Ada (programming language)">Ada</a><sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> or <a href="PL/I" title="PL/I">PL/I</a>, or as a hardware structure in some computers such as <a href="Burroughs_large_systems" class="mw-redirect" title="Burroughs large systems">Burroughs large systems</a> with their <a href="Burroughs_large_systems_descriptors" title="Burroughs large systems descriptors">descriptors</a>.
</p><p>Data descriptors are typically used at run-time to pass argument information to called <a href="Subroutines" class="mw-redirect" title="Subroutines">subroutines</a>. <a href="OpenVMS" title="OpenVMS">OpenVMS</a><sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> and <a href="Multics" title="Multics">Multics</a><sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> have system-wide language-independent standards for argument descriptors. Descriptors are also used to hold information about data that is only fully known at run-time, such as a <a href="Dynamic_memory_allocation" class="mw-redirect" title="Dynamic memory allocation">dynamically allocated array</a>.
</p>
<div class="mw-heading mw-heading2"><h2 id="Examples">Examples</h2></div>
<p>The following descriptor is used by IBM <i>Enterprise PL/I</i> to describe a <a href="String_(computer_science)" title="String (computer science)">character string</a>:<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup>
</p>
<table class="wikitable" style="text-align:center;">
<tbody><tr>
<th>byte offset</th>
<th>+0</th>
<th>+1</th>
<th>+2</th>
<th>+3
</th></tr>
<tr>
<th>0
</th>
<td>descriptor type</td>
<td>string type</td>
<td>(res)</td>
<td>flags
</td></tr>
<tr>
<th>4
</th>
<td colspan="4">maximum string length
</td></tr></tbody></table>
<ul><li>'desc type' is 2 to indicate that this is an element descriptor rather than an array or structure descriptor.</li>
<li>'string type' indicates that this is a character or a bit string, with varying or non varying length. 2 indicates a non varying (fixed-length) character string.</li>
<li>'(res)' is a reserved byte not used for character strings.</li>
<li>'flags' indicate the encoding of the string, <a href="EBCDIC" title="EBCDIC">EBCDIC</a> or <a href="ASCII" title="ASCII">ASCII</a>, and the encoding of the length of varying strings.</li>
<li>'maximum string length' is the actual length of the string for non varying strings, or the maximum length for varying strings.</li></ul>
<p>Here is the source of an <a href="Array_data_structure" class="mw-redirect" title="Array data structure">array</a> descriptor from Multics.<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> The definitions include a structure for the base array information and a structure for each dimension. (Multics ran on systems with 36-bit words).
</p>
<pre>dcl 1 array based aligned,
2 node_type bit(9) unaligned,
2 reserved bit(34) unaligned,
2 number_of_dimensions fixed(7) unaligned,
2 own_number_of_dimensions fixed(7) unaligned,
2 element_boundary fixed(3) unaligned,
2 size_units fixed(3) unaligned,
2 offset_units fixed(3) unaligned,
2 interleaved bit(1) unaligned,
2 c_element_size fixed(24),
2 c_element_size_bits fixed(24),
2 c_virtual_origin fixed(24),
2 element_size ptr unaligned,
2 element_size_bits ptr unaligned,
2 virtual_origin ptr unaligned,
2 symtab_virtual_origin ptr unaligned,
2 symtab_element_size ptr unaligned,
2 bounds ptr unaligned,
2 element_descriptor ptr unaligned;
dcl 1 bound based aligned,
2 node_type bit(9),
2 c_lower fixed(24),
2 c_upper fixed(24),
2 c_multiplier fixed(24),
2 c_desc_multiplier fixed(24),
2 lower ptr unaligned,
2 upper ptr unaligned,
2 multiplier ptr unaligned,
2 desc_multiplier ptr unaligned,
2 symtab_lower ptr unaligned,
2 symtab_upper ptr unaligned,
2 symtab_multiplier ptr unaligned,
2 next ptr unaligned;
</pre>
<div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2></div>
<ul><li><a href="Burroughs_large_systems_descriptors" title="Burroughs large systems descriptors">Burroughs large systems descriptors</a></li></ul>
<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
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