Leading tape marks

To position a tape to the desired data set, you must specify the correct data set sequence number in the DD statement. If a tape mark precedes the first data set and the LABEL subparameter LTM is specified, then the system tests for and bypasses, if present, a leading tape mark. If a tape mark should precede the first data set and you do not specify LTM in the LABEL parameter field, you must add 1 to the data set sequence number.

If a multivolume data set from another system has a leading tape mark on one or more of the volumes, the operating system can process it as an unlabeled multivolume data set if the LABEL subparameter LTM is specified. Otherwise, the operating system cannot process it as an unlabeled multivolume data set.

The presence of a leading tape mark, that is, a tape mark that precedes the first data set on the tape, makes each data set the second in sequence on the tape. However, the operating system always assumes that continued data sets are first in sequence on the tape. By specifying LTM in the LABEL parameter field, the first data set on a tape can be accessed whether or not it is preceded by a leading tape mark.

The specification of LTM in the LABEL parameter field does not make allowances for any other excess tape marks. You must make those adjustments in the data set sequence number.