Format of ISO/ANSI user header and Trailer labels (UHL/UTL)

Figure 1 shows the format of UHL and UTL labels.

Figure 1. Format of ISO/ANSI user labels

1—Label Identifier (3 bytes)

• Contents: Three characters that identify the label are as follows:

  • UHL

    User header label (at the beginning of a data set)

    UTL

    User trailer label (at the end-of-volume or end-of-data set).

• Processing: This field is read to verify that the record is a user label. The system accepts either UHL or UTL.

2—Label Number (1 byte)

• Contents: Any valid Version 3 or Version 4 character.
• Processing: This field is checked by the operating system for a valid Version 3 or Version 4 character during creation of a user header label (UHL) during open/EOV if validation has not been suppressed. If an invalid character is detected, the label validation exit is entered.
• Difference from IBM Field: This field can contain only numeric 1 to 8 for IBM standard user labels. A maximum of 8 user header or trailer labels is supported for conventional IBM standard user labels, but any number of user labels can be written for Version 3 or Version 4 tapes, and they may be lettered or numbered in any order.

3—User Specified (76 bytes)

• Contents: Specified by the user, but must be valid Version 3 or Version 4 characters.
• Processing: Specified in the DCB exit list. This field is checked as explained for Field 2.