A processor supervisory unit for monitoring the program flow executed by a processor, the supervisory unit being arranged to store a set of values representing locations to which the program flow is expected to return after jumps in the program flow, the unit being capable of: in a first mode, on detecting a jump in the program flow to store a location value representing a location to which the program flow is expected to return from that jump; and in a second mode, on detecting a jump in the program flow to increment a counter associated with a location value representing a location to which the program flow is expected to return from that jump.