Data descriptor
In computing a data descriptor is a structure containing information that describes data.
Data descriptors may be used in compilers[1], as a software structure at run time in languages like Ada[2] or PL/I, or as a hardware structure in some computers such as Burroughs large systems.
Data descriptors are typically used at run-time to pass argument information to called subroutines. HP OpenVMS[3] and Multics[4] 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 dynamically allocated array.
Unlike a dope vector, a data descriptor does not contain address information.
Examples
The following descriptor is used by IBM Enterprise PL/I to describe a character string[5]:
+--------+--------+--------+--------+
| desc | string | | flags |
| type | type | (res) | |
+--------+--------+--------+--------+
| maximum string length |
| |
+--------+--------+--------+--------+
byte 0 1 2 3
- 'desc type' is 2 to indicate that this is an element descriptor rather than an array or structure descriptor.
- 'string type' indicates that this is a character or a bit string, with varying or nonvarying length. 2 indicates a nonvarying (fixed-length) character string.
- '(res)' is a reserved byte not used for character strings.
- 'flags' indicate the encoding of the string, EBCDIC or ASCII, and the encoding of the length of varying strings.
- 'maximum string length' is the actual length of the string for nonvarying strings, or the maximum length for varying strings.
Here is the source of an array descriptor from Multics[6]. The definitions include a structure for the base array information and a structure for each dimension. (Multics ran on systems with 36-bit words).
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;See also
References
- ^ Holt, Richard C (July, 1987). "Data descriptors: a compile-time model of data and addressing". ACM Transactions on Programming Languages and Systems (TOPLAS). 9 (3): 367–389. doi:10.1145/24039.24051.
{{cite journal}}: Check date values in:|date=(help) - ^ Schonberg, Ed. "Ada Compared with C++". The Advantages of Ada 95. Retrieved January 15, 2013.
- ^ Hewlett-Packard. "Chapter 7 OpenVMS Argument Descriptors". HP OpenVMS Systems Documentation. Retrieved January 15, 2013.
- ^ Honeywell, Inc. (1979). Multics Programmers' Manual – Subsystem Writers' Guide (PDF). pp. 2–13-2–18.
- ^ IBM Corporation (2006). Enterprise PL/I for z/OSProgramming Guide (PDF). p. 385.
- ^ MIT/Honeywell. "array.incl.pl1". Multics/ldd/include. Retrieved January 20, 2012.
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