Abstract: A central processor unit for a digital data processing system that processes prefetched instructions including a conditional branch instruction. The processor includes a fetch unit that has separate portions, one that retrieves operands and the other that retrieves instructions. When the fetch unit fetches a conditional branch instruction, it may continue to prefetch "branch not taken" instructions using the instruction fetch portion. The fetch unit initially uses the operand fetch portion to prefetch "branch taken" instructions. If it is determined that the branch is not taken, the prefetch operation is aborted, otherwise the prefetch operation is allowed to continue to provide the next instruction used by the processor.

Abstract: The method and apparatus for isolating faults in circuitry of a digital computer includes the use of a fault isolation generation program which provides a data base containing a list of possible faulty components for each cycle of the computer's clock for execution by a service processor of the actual computer during testing. The fault isolation generation program is generated by using a simulator loaded with a mathematical model of the actual computer in connection with the execution of the diagnostic program executed on the actual computer during testing. The fault isolation program generates a list of circuit elements capable of generating fault indications, excluding circuit elements not capable of generating such fault indications.