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 with their descriptors.
Data descriptors are typically used at run-time to pass argument information to called subroutines. 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.
Examples
The following descriptor is used by IBM Enterprise PL/I to describe a character string:[5]
| byte offset | +0 | +1 | +2 | +3 |
|---|---|---|---|---|
| 0 | descriptor type | string type | (res) | flags |
| 4 | maximum string length | |||
- '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 non varying length. 2 indicates a non varying (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 non varying 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. 9 (3): 367–389. doi:10.1145/24039.24051.
- ^ 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. Archived from the original on July 13, 2020. Retrieved July 11, 2020.
- ^ 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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