Data descriptor

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

Contents

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. 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
0descriptor typestring type(res)flags
4maximum string length

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

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References

  1. ↑ 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 .
  2. ↑ Schonberg, Ed. "Ada Compared with C++". The Advantages of Ada 95. Retrieved January 15, 2013.
  3. ↑ Hewlett-Packard. "Chapter 7 OpenVMS Argument Descriptors". HP OpenVMS Systems Documentation. Retrieved July 11, 2020.
  4. ↑ Honeywell, Inc. (1979). Multics Programmers' Manual – Subsystem Writers' Guide (PDF). pp. 2–13-2–18.
  5. ↑ IBM Corporation (2006). Enterprise PL/I for z/OSProgramming Guide (PDF). p. 385.
  6. ↑ MIT/Honeywell. "array.incl.pl1". Multics/ldd/include. Retrieved January 20, 2012.


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