Positioning the volume to the data set

When the tape is accepted for output, it is positioned to receive the new data set. Usually, the new data set is the first or only data set on the volume, so the tape is positioned at load point.

To create a data set that follows another data set already stored on the volume, specify a data set sequence number in the LABEL parameter of the DD statement.

The sequence number can be from 1 to 65535, with 1 representing the first data set on the volume. If you specify a sequence number that is 2 or more greater than the number of data sets existing on the volume, one of three things may happen:
- the tape will be spaced through and removed from its volume, or
- the data set will be written but separated from the preceding data set by unusable (old) data, or
- the cartridge probably will return a positioning error, causing OPEN to issue an ABEND.
If you do not specify a sequence number, or if you specify 0, the system assumes that the data set is to be written as the first one on the volume.

To position the tape, the system maintains a logical data set sequence number in the UCB. The method of positioning is the same as that previously explained for opening an input data set.

No more than one data set on a tape volume can be open at a time. Processing is abnormally terminated if you attempt to open a second data set on the same volume.