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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 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 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Data descriptor」の詳細全文を読む スポンサード リンク
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