Execute Direct Access Program: Difference between revisions

Revision as of 17:39, 17 June 2011

Revision as of 15:47, 31 July 2011

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{{Mainframe I/O access methods}}

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

Existing recordsblocks 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 recordblock number, for any direct access device type. Later versions of the OS facilitate accessing very large capacity devices by using the TRKADDR macro.

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 conventionally, in which case the two forms are combined and is expanded as inline data and instructions. The use of List and Execute Forms facilitates the development of re-enterable programs.

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XDAP results in the [[Execute Channel Program|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.

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 recordsblocks as XDAP does not support format writes. BSAM and BPAM are available for executing format writes (writes which add new recordsblocks to the dataset).

The IOBSPSVC flag (logically, the '''IOB''''s '''S'''AM and '''P'''AM '''SVC''' flag) is available, and which causes SAM and PAM appendages to be bypassed. This enhancement, which was first implemented with SVS, but was not retrofitted to previous versions of the OS, significantly facilitated processing a dataset already OPENed for BSAM or BPAM, while using XDAP. Otherwise, certain SAM and PAM appendages could cause random supervisor storage overlays when presented with certain XDAP channel programs. An obvious circumvention, for pre-SVS systems as IBM did not guarantee system security and data integrity on these systems, was to utilize only those XDAP channel programs which did not also utilize RPS as the RPS-type XDAP channel programs were confusing the SAM and PAM appendages, thereby leading to the supervisor storage overlays.

[[Category:IBM Mainframe computer operating systems]]