Apparatus for checking and detecting erroneous start signals is provided in the arithmetic section of a high speed instruction processor and may be embodied in other types of processors. The novel logic circuits include circuits for detecting an attempted start signal while a previous instruction is still in process; logic circuits for detecting when an even arithmetic sequence and an odd arithmetic sequence other than the first sequence are being concurrently processed; and logic circuits for detecting when an AR start instruction is being attempted during a wrong minor cycle.