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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 block by block basis. XDAP is more specifically described in the OS System Programmer's Guide.<ref>{{cite manual|publisher=IBM|title=IBM System/360 Operating System System Programmer's Guide|id=C28-6550-2|date=March 1967|url=http://www.bitsavers.org/pdf/ibm/360/os/R01-08/C28-6550-2_OSsysPrmg_Mar67.pdf}}</ref>
==Existing
Existing blocks may be read or updated. The dataset may not be created nor extended using XDAP; for that purpose a true access method is required. XDAP may coexist with BSAM or BPAM, and several OS components are implemented in this way.
==DASD
The programmer is responsible for computing the full direct access block 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 block 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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