Execute Direct Access Program: Difference between revisions

Revision as of 15:47, 31 July 2011

Revision as of 15:49, 31 July 2011

Line 4:

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.

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 block 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.