Samiksha Jaiswal (Editor)

Data descriptor

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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. 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:

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

    References

    Data descriptor Wikipedia
    (Text) CC BY-SA