When VTAM® invokes the USERVAR exit routine to process a USERVAR, the exit attempts to allocate a USERVAR control block (UVCB) in VTAM private storage to represent the USERVAR, if one has not already been allocated for that USERVAR. If the exit allocates the UVCB successfully, the exit initializes the UVCB and adds it to the UVCB queue. The EXCB contains the addresses of the beginning and end of the UVCB queue. Therefore, if a UVCB is the first entry in the queue, its address is stored in the EXCB. If a UVCB follows another entry in the queue, its address is stored in the preceding UVCB. When the exit is called for termination, it releases the storage for any UVCBs.

Sample USERVAR exit processing can continue even when the exit is unable to allocate a UVCB because a UVCB is a local copy of information and is not vital to processing. Although USERVARs can be added and updated without a UVCB, the exit attempts to build a UVCB for the USERVAR at the next opportunity. USERVAR translation does not use a UVCB at all.