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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 a true access method, usually BSAM or BPAM, is required. XDAP may coexist with BSAM or BPAM, and several OS components are implemented in this way.
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, for any direct access device type. Later versions of the OS facilitate accessing very large capacity devices by using the TRKADDR macro.
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