Volume Mount Exit

The volume mount exit has three functions. During a particular OPEN or EOV, this exit can be called for any or all of the following reasons:

Verifying the volume (Verify that the mounted volume is acceptable.)
Writing the volume label
Processing volume security.

Each invocation of the exit is for only one of these reasons.

Volume verification.

During the course of opening and processing a tape data set, OPEN and EOV calls the volume mount exit when OPEN or EOV is verifying that the correct volume was mounted. The OPEN option can be any valid option and the label type can be any (SL, SUL, AL, AUL, NL, LTM, BLP, NSL). The system has just mounted the tape. When the system is planning to write file one header labels, this exit is called once for label verification and once prior to rewriting the volume label. UCBTFL1 indicates the type of volume mounted.

For volume verification the exit can change the user's request to be for a different volume or to be a scratch request. Independent of this the exit can cause the mounted SL or AL volume to appear to have a different volume label of the same type or to be unlabeled.

When the exit is called, open or EOV is processing a tape volume mount that has been satisfied by an operator, an automated cartridge loader or a tape library dataserver. The system has not previously verified the volume.

The label anomaly exit or OMODVOL1 or EMODVOL1 handles any input or output error or other problems when reading the label. The error might have been that the tape is 36-track and was read on an 18-track drive. In this case, the sense bytes or label anomaly exit provided the volume serial number, since the real label is not available.

This call is before RACF is called (for RACHECK or RACDEF).

The volume mount exit uses the block ID that is supplied to it by the tape management system. When DFSMSrmm issues an OPEN, the exit writes the block ID that is associated with the data set into JFCB field, JFCRBIDO, and turns on JFCB bit, JFCPOSID. When DFSMSrmm issues a CLOSE, the exit writes the block ID that is associated with the data set into JFCB field, JFCRBIDC. The tape management system can be DFSMSrmm or a functionally equivalent product. The block ID is the one associated with the data set's HDR1 or EOF2 label. For information about how information is written into the JFCB see z/OS DFSMS Using Magnetic Tapes.

The following table shows results from the volume mount exit and OPEN or CLOSE processing. The results depend on the disposition that is specified by the program.

OPEN or CLOSE Processing Method	block ID	Volume Mount Exit	OPEN or CLOSE Processing
OPEN OUTPUT NEW	Associated with data set's HDR1 label	Writes block ID into JFCB field, JFCRBIDO Turns on JFCB bit, JFCPOSID	CLOSE fast positions to block identified by block ID
OPEN OUTPUT MOD	Associated with data set's EOF2 label	Writes block ID into JFCB field, JFCPOSID Turns on JFCB bit, JFCPOSID	OPEN fast positions to block identified by block ID OPEN positions tape in front of tape mark that preceeds the data set's EOF1 label
OPEN OUTPUT OLD	Associated with data set's HDR1 label	Writes block ID into JFCB field, JFCRBIDO Turns on JFCB bit, JFCPOSID	OPEN fast positions to data set's HDR1 label
OPEN INPUT	Associated with data set's HDR1 label	Writes block ID into JFCB field, JFCRBIDO Turns on JFCB bit, JFCPOSID	OPEN fast positions to data set's HDR1 label
OPEN RDBACK	Associated with data set's EOF2 label	Writes block ID into JFCB field, JFCRBIDO Turns on JFCB bit, JFCPOSID	OPEN fast positions tape in front of tape mark that preceeds data set's EOF1 label
CLOSE OUTPUT	Associated with data set's EOF2 label	Writes block ID into JFCB field, JFCRBIDC	CLOSE passes control to DFSMSrmm File End on Volume Exit

Volume write function.
The system is about to rewrite the volume label because the user is open for output to the first file. When the system is writing file one header labels, this exit will be called once prior to label verification and once prior to rewriting the volume label. UCBTFL1 indicates the type of volume mounted.

If the request was for a scratch tape, the system has determined that the mounted tape is acceptable. If the request was for a specific tape, the system has determined that the mounted tape is the correct one.
Volume security function.
The system has just issued the RACROUTE macro to check the user's authority to the volume. The caller of this exit has not tested the SAF return code which is in the exit's parameter list. The exit can change the results of the security processing. You can return details for the first tape data set on the volume. Subject to the DEVSUPxx TAPEAUTHF1 installation option, the tape data set details can be used through RACROUTE as an additional authorization check.

Note: Certain authorized programs can bypass issuing the RACROUTE macro which also bypasses calling this security function.

The volume mount exit will receive a UCB address from TEPMUCB in IFGTEP, from DEBUCBA and from TIOEFSRT. All three sources will allow a 31–bit UCB address. TEPMICB will sometimes be an uncaptured 31–bit address. If the DEB31UCB bit is on, the UCB address and modeset byte will be different as described in IEZDEB. If they use the DCBTIOT field to find the TIOT entry address, they should be changed to use TEPMDSAB to point to the DSAB, which points to the TIOT entry or XTIOT.

This exit does not replace any checking done by open or EOV but it can replace the SAF return code before open or EOV tests it.