The data extent block (DEB) contains one or more extent entries for the associated data set and other control information. An extent defines all or part of the physical boundaries on an I/O device occupied by, or reserved for, a particular data set. Each extent entry contains the address of a unit control block (UCB) that provides information about the type and location of an I/O device. More than one extent entry can contain the same UCB address. For all I/O devices supported by the operating system, the DEB is produced during execution of the OPEN macro instruction for the DCB. The system places the address of the DEB into the DCB. See Data Extent Block (DEB) Fields.