PURPOSE: To simplify the circuit constitution by having only to supervise the state of two bits being a sign bit and the most significant bit(MSB) of a count of a counter to detect overflow and underflow of the counter.

CONSTITUTION: With a pulse signal 2 fed to a counter 1, a count 4 is incremented by 1 to be '1000' and both a sign bit 10 and the MSB 11 are changed. Since level '0' of the sign bit 10 before the change and level '1' of the sign bit 10 after the change are applied to an exclusive OR gate 13 in this case, its output is inverted to a level 1. Since two inputs '10' after the change are fed to an exclusive OR gate 14, its output goes to the level '1'. Thus, a detection signal of level 1 being the result of detecting overflow is outputted from an AND gate 15. Thus, the circuit constitution is simplified.