Execute Direct Access Program: Difference between revisions

Revision as of 15:49, 31 July 2011

Revision as of 18:00, 19 October 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 block by block basis. XDAP is more specifically described in the OS/360 System Programmer's Guide<ref>http://www.bitsavers.org/pdf/ibm/360/os/R01-08/C28-6550-2_OSsysPrmg_Mar67.pdf</ref>.

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.

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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.

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[[Category:IBM Mainframe computer operating systems]]