6.5 Mapping for access to devices This document does not cover the internal operation of the z/OS platform. However, it simplifies the following topics to briefly mention three elements, referred to as control blocks . These are the Unit Control Block (UCB), the Unit Control Word (UCW) and the Control Unit Control Block (CUCB), as explained in more detail here: UCB The UCB is used by z/OS to represent every device that may be used for an I/O operation. It contains state information and queues of requests to access the device. The UCW is equivalent to the UCB in representing a device, but it is used by the hardware. It contains state indicators and points to one or more CUCBs. As mentioned, a device may be connected to several control units, which in turn may be connected to several channels. A CUCB is present to represent each control unit defined to the system. It contains the pointers to the channels it is connected to. UCW CUCB ESCD CU