PURPOSE: To correctly detect an OF cycle even against a special instruction of an MC6809 made by Motorola, Inc., by providing a clock controller, means for obtaining discriminating codes and executing cycle numbers, discriminating means, counter, comparator, address tracing means, reference OF cycle detecting circuit, and output control circuit.

CONSTITUTION: An instruction code-to-cycle number converter 2 finds a discriminating code and executing cycle number against an instruction code given from a latch 1. An instruction code controller 4 generates a clock to a latch 3, enable signal (EN) to a cycle number comparator 5, and signals informing an address tracing circuit controller 9 that an instruction is a long branch instruction or RTI instruction, in accordance with discriminating codes given from the converter 2 through the latch 3. A reference OF cycle detecting circuit 8 detects the first OF cycle after interruption vector fetching and leads the detected cycle to an output control circuit 12. The output control circuit 12 receives signals from the comparator 5, detecting circuit 8, and controller 9 and outputs OF cycle signals synchronously to the clock of a clock controller 6.