I/O operation, before returning it to the pool. Thus, a smaller number of UCBs are required to service the same workload, compared to Dynamic PAVs. There is no delay waiting for WLM to react. In other operating systems Digital's VMS operating system uses an identically named structure, the UCB, for similar purposes. A UCB is created for each I/O device. The data in the UCB includes the device's unit number (a part of the device name) and a listhead to which pending I/O requests may be queued. The UCB may have a device-driver defined extension in which the driver can keep driver-defined data that is instantiated for each device. Notes References IBM mainframe operating systems