to a UCB foundation address). Eventually an installation could choose to place UCBs above the 16 MiB line line, although in a process called shadowing the UCB the OS creates a temporary local copy of the UCB when allocating a file without the XTIOT option. Handling parallel I/O operations UCBs were introduced in the 1960s with OS/360. Then a device addressed by UCB was typically a moving head hard disk drive or a tape drive, with no internal cache. Without it, the device was usually grossly outperformed by the mainframe's channel processor. Hence, there was no reason to execute multiple input/output operations to it at the same time, as these would be impossible for a device to physically handle. In 1968, IBM introduced the 2305-1 and 2305-2 fixed-head disks, which had rotational position sensing (RPS) and eight exposures (alias addresses) per disk; the OS/360 support provided a UCB per exposure in order to permit multiple concurrent channel programs. Similarly, later systems derived from OS/360 required an additional UCB for each allocated virtual volume in a IBM 3850 Mass Storage System (MSS), and for each exposure on a 3880-11 and its successors. Parallel Access Volumes (PAVs) Since only one set of channel commands or I/O could be run at one time. This was fine in the 1960s when CPUs were slow and I/O could only be processed as fast as CPUs could process it. As systems matured and CPU speed greatly surpassed I/O input capacity, access to the device that was serialized at the UCB level became a serious bottleneck. Parallel Access Volume (PAV) allow UCBs to clone themselves to allow multiple I/O to run simultaneously. With appropriate support by the DASD hardware, PAV provides support for more than one I/O to a single device at a time. To maintain backward compatibility, operations are still serialized below the UCB level. But PAV allows the definition of additional UCBs to the same logical device, each using an additional alias address. For example, a DASD device at base address 1000, could have alias addresses of 1001, 1002 and 1003. Each of these alias addresses would have their own UCB. Since there are now four UCBs to a single device, four concurrent I/Os are possible. Writes to the same extent, an area of the disk assigned to one contiguous area of a file, are still serialized, but other reads and writes occur simultaneously. The