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.
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.
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.