Execute Direct Access Program: Difference between revisions

Revision as of 22:42, 8 May 2011

Revision as of 20:36, 10 May 2011

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The programmer is responsible for computing the full direct access record identifier, MBBCCHHR. System algorithms are available for calculating the MBBCCHHR from a TTRN. System data, in the form of "track capacity tables", are available for calculating the TTRN from a record number.

List and Execute forms of XDAP are supported, whereby a prototype form, called the List Form, is expanded as data, and a functional form, called the Execute Form, is expanded as instructions. XDAP is also supported in a conventional form, in which case the two forms are combined, and is expanded as inline data and instructions. The use of List and Execute Forms facilitate the development of re-enterable programs.

XDAP results in the EXCP supervisor call being executed, hence '''Execute Channel Program''' ('''EXCP''') is implicitly embedded within XDAP. The IOB, which is the sole parameter to EXCP, is included as a data structure within the XDAP macro expansion as are the '''channel command words''' ('''CCW'''s).

XDAP must reference an OPENed DCB and a full direct access record identifier. The DCB may be OPENed for input, for output or for input/ and output, but all outputs must be update writes of existing records as XDAP does not support format writes. BSAM and BPAM are available for executing format writes (writes which add new records to the dataset).

[[Category:IBM Mainframe computer operating systems]]