IBM standard label processing

Label processing is handled by the open/close/end-of-volume routines. Label processing consists of the following basic functions:

Checking the labels on input tapes to ensure that the correct volume is mounted, and to identify, describe, and protect the data set being processed
Checking the existing labels on output tapes to ensure that the correct volume is mounted, and to prevent overwriting of vital data
Creating and writing new labels on output tapes
For volumes in an automated tape library, comparing the external volume serial number to the magnetically recorded internal label

These processing functions are summarized in Table 1. The table shows the specific labels that are processed for each function and which routines perform the functions.

Table 1. IBM standard label processing by Open/Close/EOV routines
Processing	Volume Label VOL1	Header Labels¹			Trailer Labels¹
Processing	Volume Label VOL1	HDR1	HDR2	UHL1-8	EOF1 or EOV1	EOF2 or EOV2	UTL1-8
First or only volume²
Checks labels on input tape	Open	Open	Open	Open	EOV	Bypassed	EOV
Checks existing labels on output tape before overwriting	Open	Open	Open	Not read	Not read	Not read	Open³
Writes new labels on output tape	Open or user⁴	Open	Open	Open	Close or EOV	Close or EOV	Close or EOV
Compares the volume serial number on the external volume label to the serial number in the tape volume label⁶	Open	Open	N/A⁷	N/A⁷	N/A⁷	N/A⁷	N/A⁷
Second or subsequent volumes⁵
Checks labels on input tape.	EOV	EOV	Bypassed	EOV	EOV	Bypassed	EOV
Checks existing labels on output tape before overwriting.	EOV	EOV	Not read	Not read	Not read	Not read	Not read
Writes new labels on output tape.	EOV or user⁴	EOV	EOV	EOV	Close or EOV	Close or EOV	Close or EOV
Compares the volume serial number on the external volume label to the serial number in the tape volume label⁶	EOV	N/A⁷	N/A⁷	N/A⁷	N/A⁷	N/A⁷	N/A⁷
Notes: For read backward operations, the action on header and trailer labels is reversed. Includes the first volume of concatenated data sets with unlike characteristics when the user program has turned on the DCB unlike attributes bit (DCBOFPPC). See z/OS DFSMS Using Data Sets. If DISP=MOD is specified on the DD statement, or the OPEN macro specifies EXTEND or OUTINX, then the open routine positions the tape at the end of the existing data set and allows an input user trailer label routine to process user trailer labels (prior to overwriting the existing labels). User can create the label with the EDGINERS or IEHINITT utility programs, or a user program. Subsequently, the label may be rewritten by the open and EOV routines. Includes the first volume of concatenated data sets when the user has not turned on the DCB unlike attributes bit. Applies only to tape library. Not Available.

When a data set is opened for input, the volume label and HDR1 are processed. HDR2 is processed if it exists. For an input end-of-data condition, the trailer labels are processed, unless deferred user input trailer label processing is specified in the DCB exit list. For an input end-of-volume condition, the trailer labels on the current volume are processed, and then the volume label and header labels on the next volume are processed. When the FEOV macro is issued for an input tape, the trailer labels on the current volume are not processed, but the volume labels and header labels on the next volume are processed. No label processing is performed when an input data set is closed, unless deferred user input trailer label processing is specified in the DCB. If deferred user input trailer label processing is specified, the processing otherwise performed for an input end-of-data condition is performed when an input data set is closed.

When a data set is opened for output, the existing volume label and HDR1 label are checked, and a new volume label and new header labels are written. For an output end-of-volume condition (including FEOV), trailer labels are written on the current volume, the existing volume and header labels on the next volume are checked, and then a new volume label and new header labels are written on the next volume. When an output data set is closed, trailer labels are written.