Execute Direct Access Program: Difference between revisions

Revision as of 04:31, 8 May 2011

Revision as of 06:24, 8 May 2011

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In [[IBM mainframe]] [[operating system]]s, '''Execute Direct Access Program''' ('''XDAP''') is a pseudo access method for accessing direct access datasets on a record by record basis.

Existing records may be read or updated. The dataset may not be created nor extended using XDAP; for that purpose BDAM or BPAM is required. XDAP may coexist with BDAM or BPAM, and several OS components are implemented in this way.

The programmer is responsible for computing the full direct access record addressidentifier, MBBCCHHR. System algorithms are available for calculating the MBBRRHHR from a TTRN. Additional system algorithms are available for calculating the TTRN from a record number.

List and Execute forms of XDAP are provided, 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 provided in a conventional form, in which case the two forms are combined, and is expanded as data and instructions.

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XDAP results in an SVC 0 instruction, 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 addressidentifier. The DCB may be OPENed for input, for output or for input/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]]