XDAP must reference an OPENed DCB and a full direct access 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 blocks as XDAP does not support format writes. BSAM and BPAM are available for executing format writes (writes which add new blocks to the dataset).
The IOBSPSVC flag (logically, the '''IOB''''s '''S'''AM and '''P'''AM '''SVC''' flag) is available, and which causes SAMSequential andAccess PAMMethods appendages<ref>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 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.
