The error occurred during processing of the graphic attention service routine for the IBM® 2250 Display Unit. The routine referred to a unit control block (UCB) that was associated with a device other than a graphic device. The routine obtained the reference to the unit control block through a data control block (DCB). The user gives the routine the addresses of data control blocks through the poll list address in the first operand of the ANALYZ macro instruction or the address of one data control block through the list item address in the first operand of the GSERV macro instruction. The DCBDEBAD field in the data control block points to a data extent block (DEB), which in turn points to the unit control block. The unit control block is used by the control program to obtain information about an I/O; each unit control block is associated with one I/O device.