If OPEN or EOV mounted a tape, then it has done all processing of an existing volume label before calling the file validation exit. If the call is for OPEN, the forward DCB merge has not been done and the DCB OPEN exit has not been called. If it is an SL or AL tape, OPEN or EOV has read the first 80 bytes of the header or trailer label (if reading backward) but has not examined it yet except to translate it from ASCII if the first four characters are ASCII, HDR1, EOV1, or EOF1. If open for output, then no writing has been done yet on the volume. If it is not SL or AL, then OPEN has not yet positioned the tape. This is so the exit can reject the tape if the user is trying to read past the last file or writing more than one file past the last file. This exit can verify that the specified 44-character data set name matches the tape management system's database.

This call is before RACDEF is called and after RACHECK.

This exit does not replace any checking done by OPEN or EOV except when the exit specifies that the operator is not to be asked about expiration dates on the volume and/or that password security processing should be bypassed during tape SL output processing to file sequence one.

The file validation 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 but TIOEFSRT will contain zero if the actual UCB is above the line and allocation did not capture the UCB address. TEPMUCB 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. It is best not to use the DCBTIOT field to find the TIOT entry address. This is because DCBTIOT will contain zeroes if the user program caused an XTIOT to be created instead of a TIOT entry. You always can use TEPMDSAB to point to the DSAB, which points to the TIOT entry or XTIOT.