a method and apparatus pertaining to a firmware control unit for detecting when such control unit is not behaving properly . the control unit is organized to include in each location of the unit &# 39 ; s control store , to which control is not expected to transfer , a predetermined type of pattern containing an address specifying the address of that location , a suitable tag identifying the probable reason for the unexplained jump , and a transfer of control to the appropriate entry point in a reporting firmware routine within the control store . the reporting firmware routine has a number of entry points for collecting all the executions of unexpected locations and for storing the appropriate address and tag information in a predetermined register file for later referencing by an unusual event handler routine .