Data descriptor

In computing a data descriptor is a structure containing information that describes data.

Data descriptors may be used in compilers, 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.

Data descriptors are typically used at run-time to pass argument information to called subroutines. HP OpenVMS and Multics have system-wide language independent standards for argument descriptors.

Unlike a dope vector, a data descriptor does not contain address information.

Example

The following descriptor is used by IBM Enterprise PL/I to describe a character string:

      +--------+--------+--------+--------+
      |  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.

See also

• Burroughs large systems descriptors