Many computer applications have stringent requirements for continued correct operation of the computer in the presence of internal faults. The subject of design of such highly reliable computers has been extensively studied, and numerous techniques have been developed to achieve this high reliability. Such computers are termed "fault tolerant"; examples of applications are found in the aerospace industry, communication systems, and computer networks. Several designs of such systems have been proposed and some have been implemented. In general, these designs contain extensive hard-wired logic for such functions as fault masking, comparison, switching, and encoding-decoding.