Determining volume switch

The volume serial numbers of the data set to be processed must be specified by the user at execution time. The serial numbers are specified either directly in the DD statement or indirectly through the catalog. You specify the serial numbers in forward sequence, regardless of whether the tapes are to be read forward or backward.

For noncataloged data sets, you specify the volume serial numbers in the VOLUME parameter of the DD statement. The system processes the group of volumes in whatever order you specify and processes only the volumes you specify.
For cataloged data sets, the system processes the group of volumes in sequential order. However, you can begin processing at any volume of the group by specifying a sequence number in the VOLUME parameter of the DD statement.
For input, the volume serial numbers specified by the user are the basis for determining whether a volume switch is required. The system does not consider whether the data set on the current volume is followed by one or two tape marks.

If another volume is required, the system obtains the next volume serial number from the JFCB and switches volumes. The system checks the initial record of the new volume to ensure that it is not a standard volume label and positions the tape to the data set. For a multivolume data set, the tape is positioned to the first record on the new volume. If another data set is concatenated after this one, the system processes it. If the user has not set on the unlike-attributes bit (the DCBOFPPC bit), then the system treats the concatenated data set as if it began another volume of the current data set. If the user defined an EOV exit, the system calls it (not a DCB open exit). If the unlike-attributes bit is on, then the system closes the current data set and opens the concatenated data set. If the user defined a DCB open exit, the system calls it (not an EOV exit). For more information on using concatenated data sets, see z/OS DFSMS Using Data Sets. For a concatenated data set, the tape is positioned according to the specified data set sequence number.

If another volume is not available in the JFCB, control is given to the user's end-of-data-set routine that is specified in the data control block. Subsequently, the processing program or the operating system closes the data set.

The user's end-of-data-set routine is not entered until the last specified volume, or the last concatenated data set is processed.
If an input data set is closed before it reaches the end of the data set, the user's end-of-data-set routine is not entered.