Unit control block (UCB)

A unit control block (UCB) holds information about an I/O device, such as:

State information for the device
Features of the device.

You can access information in UCBs using UCB services, such as UCBSCAN and UCBLOOK. At IPL or dynamic configuration, UCBs for all I/O devices connected to the current operating system configuration are built from HCD device definition information in the IODF and UIMs. There is a UCB for each I/O device in an operating system configuration.