Input Header Label Routines (NSLOHDRI, NSLEHDRI)

When nonstandard labels are specified, the control program checks the input tape to ensure that the first record is not a standard volume label. If the first record contains the identifier VOL1 in the first 4 bytes, is recorded in EBCDIC, and is 80 bytes long, or it is recorded in ASCII and is 80 bytes long or more, a message from the control program rejects the tape and directs the operator to remove the current volume and mount the correct volume. The various error conditions that can occur during verification of the first record are explained under Label Anomaly Exit (IFG019LA).

If the tape does not contain a standard volume label, the open or EOV routine gives control to your routine for processing input header labels. Control comes from the open routine for the first or only volume of a data set, or for a concatenated data set with unlike characteristics. (Data sets with like characteristics can be processed correctly using the same DCB, IOB, and channel program. Any exception in processing makes the data sets unlike.) This is not the same as unlike attributes concatenation that is described in z/OS V2R2 DFSMS Using Data Sets. Control comes from the EOV routine for the second and subsequent volumes of a data set, or for a concatenated data set with like characteristics. When your routine receives control, the tape has been rewound and is positioned at the inter-record gap preceding the nonstandard label.

Note: If the control program finds that the tape volume has been previously verified in the job, the tape has not been rewound.

If your routine determines that the wrong volume is mounted, you must place a 1 in the high-order bit position of the UCBDMCT field of the UCB, and return control to the control program. The control program then issues a message directing the operator to mount the correct volume. When the volume is mounted, the control program again checks the initial label on the tape before giving control to your routine.

Before returning control to the control program, your input header label routine must position the tape at the appropriate data set:

For forward read operations, position the tape at the inter-record gap that precedes the initial record of the data set.
For backward read operations, position the tape after the last record of the data set.

The modeset byte is moved from a 24–bit UCB address to a 31–bit UCB address if you request the XTIOT UCB NOCAPTURE option and the NON-VSAM_XTIOT option is set to yes. OPEN, EOV, and CLOSE will always turn DEB31UCB on in the work area to signify that DXDEBUCB is a 31–bit UCB address. Although the UCB address field will be four bytes, it typically will contain a three-byte address.