Unlabeled tapes

To process or create a tape with no labels, specify NL or LTM (for VSE tapes) in the LABEL parameter of the DD statement. An unlabeled tape contains only data records and tape marks. The organization of data sets on one or more volumes is shown in Figure 1. The system automatically writes the tape marks on output and expects to find a similar placement of tape marks on input.
  • A tape mark does not precede the first data set on any volume if you specify NL. A tape mark can precede the first data set if you specify LTM. If you specify NL for a tape with a tape mark as the first record, it means the first data set contains no data. If you specify LTM for an unlabeled tape that does not contain a leading tape mark, the system treats the request the same as for NL. See Unlabeled tapes for more information.
  • A tape mark follows each data set.
  • Two tape marks follow a data set if it is the last or only data set on the volume.

An unlabeled tape can be read backward even though there is no tape mark preceding the first data set. In this case, the end-of-data-set condition is signaled by the reflective strip or other indicator at the beginning of the tape.

Open/Close/EOV lookahead mounting does not accept any unlabeled volume that is mounted before the system requests it (UCBNRY=0 and UCBVOLI=zeros). A demount message with a blank vol ser is issued.

Note: AVR automatically recognizes mounts of labeled volumes. If JCL-specific requests are used and a volume that is not specified is mounted at allocation time, the system requests a demount of the incorrect volume and a mount of the specified volume. Avoid mounting unlabeled tapes before the system requests them.

Bypassing label processing (BLP) is identical to NL processing except that the check for an existing label is bypassed. For more information about BLP, see Bypassing label processing.

Figure 1. Organization for unlabeled tapes
Organization for unlabeled tapes