Execute Direct Access Program: Difference between revisions

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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>

{{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 record by record basis.

==Existing blocks==

Existing recordsblocks may be read or updated. The dataset may not be created nor extended using XDAP; for that purpose BSAMa ortrue access BPAMmethod is required. XDAP may coexist with BSAM or BPAM, and several OS components are implemented in this way.

==DASD address computation==

The programmer is responsible for computing the full direct access recordblock 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==

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 formconventionally, 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.

==XDAP implicitly invokes EXCP==

XDAP results in anthe SVC[[Execute 0 instruction, theChannel 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''' ('''CCW'''s).

==OPENing an XDAP dataset==

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 ("INOUT"), 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, i.e. the ''initial'' or an ''additional'' block, and, particularly, the ''final'' block).

==IOBSPSVC flag for XDAP and SAM and PAM coexistence==

The IOBSPSVC flag (logically, the '''IOB''''s '''S'''AM and '''P'''AM '''SVC''' flag) is available, and which causes Sequential Access Methods appendages<ref>{{cite manual|publisher=IBM|title=IBM System/360 Operating System Sequential Access Methods Program Logic Manual|id=Y28-6604-1|date=January 1967|url=http://www.bitsavers.org/pdf/ibm/360/os/plm_1966-67/Y28-6604-1_Sequential_Access_Methods_PLM_Jan67.pdf}}</ref> 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 pre-SVS SAM and PAM appendages could cause random supervisor storage overlays when presented with certain XDAP channel programs. As IBM did not guarantee system security and data integrity on pre-SVS systems, an obvious circumvention was to utilize only those XDAP channel programs which did not also utilize RPS as it was the RPS-type XDAP channel programs which were confusing the SAM and PAM appendages, thereby leading to random supervisor storage overlays.

==References==

<references />

{{Mainframe I/O access methods}}

[[Category:IBM Mainframe computermainframe operating systems]]

[[Category:Computer file formats]]