 The main processor complex of the APC, TPC, TCU, RLU, and RSU all consisted of a pair of processor cards, and each of those processor cards contained a pair of processors.The on-card pair of processors executed precisely the same sequence of instructions, and the output of the pair were compared each clock cycle.If the results were not identical, the processors were immediately reset, and the pair of processors on the other card were brought online as the active processor complex.