The system has read or written all file labels (as appropriate), has called all user and installation exits, issued RACDEF if needed and has positioned the tape for reading or writing user data. It is too late to reject the volume. OPEN or EOV has checked for most or all of the expected ABEND conditions.

No tape label is reliably available to this exit because user labels might have been processed. The exit is expected to gather data from other places.

The caller is OPEN or EOV. The exit is called for the beginning of the file on each volume.

The file start on volume 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.