Execute Direct Access Program: Difference between revisions

Revision as of 16:09, 19 May 2011

Revision as of 02:59, 22 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, 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 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 facilitatefacilitates the development of re-enterable programs.

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.

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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).

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 OPENed for BSAM or BPAM, butwhile using XDAP. Otherwise, certain SAM and PAM appendages could cause random supervisor storage overlays when presented with certain XDAP channel programs. An obvious circumvention, mainly for pre-SVS systems, was to utilize only those XDAP channel programs which did not also utilize RPS.

[[Category:IBM Mainframe computer operating systems]]