Execute Direct Access Program: Difference between revisions

Revision as of 18:20, 23 May 2011

Revision as of 19:15, 24 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 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 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.