Using the DEB and the DCB to find the UCB address

You can also obtain the UCB address via the data extent block (DEB) and the data control block (DCB). The DCB contains data pertinent to the current use of the data set. After the DCB has been opened, field DCBDEBA contains the DEB address. The DEB contains an extension of the information in the DCB. Each DEB is associated with a DCB and the two point to each other.

The DEB contains information about the physical characteristics of the data set and other information that the control program uses. A device-dependent section for each extent is included as part of the DEB. Each such extent entry contains the UCB address of the device to which that portion of the data set has been allocated. In order to find the UCB address, you must locate the extent entry in the DEB for which you intend to issue the RESERVE macro. (In disk addresses of the form MBBCCHHR, the M indicates the extent number starting with 0.)