Lookahead control in the IBM System 370 model 165

Speed advances were provided in the third generation of computers. Since then, an order-of-magnitude improvement in speed has been achieved in logic circuitry. A high-speed parallel arithmetic unit can perform a complete operation in 80 nanoseconds; main memory, with the help of high speed buffers, can produce operands or instructions in 80 nanoseconds. Why should the arithmetic unit have to wait for several cycles before it gets each instruction and the operands for it? To improve this situation, lookahead control was developed and performs these functions: • fetches instructions in advance continuously; • validates each instruction as encountered; • obtains operand addresses and operands for preprocessed instructions; • prepares for the alternatives of branching.