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

• Burroughs large systems descriptors

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. Archived from the original on July 13, 2020. 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.