Opening a data set

The open routine is entered when the processing program issues an OPEN call that is generated by a macro or by a compiler. The open routine completes the specified DCB, and prepares and positions the data set for processing. It analyzes input header labels (or trailer labels if the tape is read backwards), or creates output header labels.

The open routines also resolve many possible conflicts, such as specifying a label type that differs from the type on the tape or the tape drive not being able to read due to the tape technology being incompatible. Tape conflict resolution can be handled by DFSMSrmm, tape management installation exit and by Open and EOV label editor installation exits. See IBM standard volume label editor routines and ISO/ANSI volume label verification and editor routines.

If DFSMSrmm is in use, additional tape volume validation is performed and information recorded in the DFSMSrmm control data set. For multifile tapes, if using DFSMSrmm, the positioning of standard labeled tape volumes is much faster than normal label positioning during OPEN processing. For more information on high-speed-positioning, see z/OS DFSMS Installation Exits.