In IBM mainframe operating systems OS/360 and its successors, a Unit Control Block (UCB) is a memory structure, or a control block, that describes any single input/output peripheral device (unit), or an exposure (alias), to the operating system. Certain data within the UCB also instructs the Input/Output Supervisor (IOS) to use certain closed subroutines in addition to normal IOS processing for additional physical device control. Some other operating systems have similar structures. Overview During initial program load (IPL) of current MVS systems, the Nucleus Initialization Program (NIP) reads necessary information from the I/O Definition File (IODF) and uses it to build the UCBs. The UCBs are stored in system-owned memory, in the Extended System Queue Area (ESQA). After IPL completes, UCBs are owned by Input/Output Support. Some of the data stored in the UCB are: device type (e.g. disk, tape, printer, terminal), address of the device (such as 1002), subchannel identifier and device number, channel path ID (CHPID) which defines the path to the device, for some devices the volume serial number (VOLSER), and a large amount of other information, including OS Job Management data. While the contents of the UCB has changed as MVS evolved, the concept has not. It is a representation to the operating system of an external device. Inside every UCB are the UCBIOQ pointer to the current IOS Queue Element (IOQ), UCBIOQF and UCBIOQL pointers to a queue of IOQs (IOQs) and a subchannel number for the subchannel-identification word used in the start subchannel (SSCH) instruction to start a channel program (chain of channel command words (CCWs)). The UCB evolved to be an anchor to hold information and states about the device. The UCB currently has five areas used for an external interface: Device Class Extension, UCB Common Extension, UCB Prefix Stub, UCB Common Segment and the UCB Device Dependent Segment. Other areas are internal use only. This information can be read and used to determine information about the device. In the earliest implementations of OS/360, the UCBs (foundations and extensions) were assembled during SYSGEN, and were located within the first 64 KB of the system area, as the I/O device lookup table consisted of 16-bit addresses. Subsequent enhancements allowed the extensions to be above the 64-kilobyte (65,536 bytes) line, thereby saving space for additional UCB foundations below the 64-kilobyte line and also thereby preserving the architecture of the UCB lookup table (converting a CUu