Data descriptor

<h2 id="examples">Examples</h2>
<p>The following descriptor is used by IBM <i>Enterprise PL/I</i> to describe a <a href="/facts/String_(computer_science)/4ChJu5WS">character string</a>:<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a>
</p>
<table><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="/facts/EBCDIC/qAhmvgj0">EBCDIC</a> or <a href="/facts/ASCII/vGUI33Qu">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="/facts/Array_data_structure/BKGvF8AM">array</a> descriptor from Multics.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a>  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>
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;

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Burroughs_large_systems_descriptors/aSobs023">Burroughs large systems descriptors</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Holt, Richard C. (July 1987). "Data descriptors: a compile-time model of data and addressing". ACM Transactions on Programming Languages and Systems. 9 (3): 367–389. doi:10.1145/24039.24051. <a href="https://doi.org/10.1145%2F24039.24051" target="_blank">https://doi.org/10.1145%2F24039.24051</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Schonberg, Ed. "Ada Compared with C++". The Advantages of Ada 95. Retrieved January 15, 2013. <a href="http://archive.adaic.com/intro/ada-vs-c/ada-vs-c.html" target="_blank">http://archive.adaic.com/intro/ada-vs-c/ada-vs-c.html</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>Hewlett-Packard. "Chapter 7 OpenVMS Argument Descriptors". HP OpenVMS Systems Documentation. Retrieved July 11, 2020. <a href="http://h30266.www3.hpe.com/odl/axpos/opsys/vmsos84/5973/5973pro_011.html#arg_desc_formats_chap" target="_blank">http://h30266.www3.hpe.com/odl/axpos/opsys/vmsos84/5973/5973pro_011.html#arg_desc_formats_chap</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>Honeywell, Inc. (1979). Multics Programmers' Manual – Subsystem Writers' Guide (PDF). pp. 2–13-2–18. <a href="http://bitsavers.informatik.uni-stuttgart.de/pdf/honeywell/multics/AK92-2_MPM_SubsysWrtGud_Mar79.pdf" target="_blank">http://bitsavers.informatik.uni-stuttgart.de/pdf/honeywell/multics/AK92-2_MPM_SubsysWrtGud_Mar79.pdf</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>IBM Corporation (2006). Enterprise PL/I for z/OSProgramming Guide (PDF). p. 385. <a href="http://pic.dhe.ibm.com/infocenter/pdthelp/v1r1/topic/com.ibm.entpli.doc_3.6/ibm3pg50.pdf" target="_blank">http://pic.dhe.ibm.com/infocenter/pdthelp/v1r1/topic/com.ibm.entpli.doc_3.6/ibm3pg50.pdf</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
<li id="fn:6"><p>MIT/Honeywell. "array.incl.pl1". Multics/ldd/include. Retrieved January 20, 2012. <a href="http://web.mit.edu/multics-history/source/Multics/ldd/include/array.incl.pl1" target="_blank">http://web.mit.edu/multics-history/source/Multics/ldd/include/array.incl.pl1</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li>
</ol>

Data descriptor open-in-new

Data descriptor