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 or PL/I, or as a hardware structure in some computers such as Burroughs large systems.
Example
The following descriptor is used by IBM Enterprise PL/I to describe a character string[2]:
+--------+--------+--------+--------+
| 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)' indicates 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.
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) - ^ IBM Corporation (2006). Enterprise PL/I for z/OSProgramming Guide (PDF). p. 385.
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