Abstract: A circuit for maintaining copies of original index registers and copies of modified index registers such that in the event original copies of the index registers are needed, they may be restored. The circuit is for use in a fault tolerant system wherein in the event of a rollback, original copies of the index registers may be restored to a processor, even if some of the index registers were modified in the previous execution of a command by the processor.

Abstract: A fault tolerant data processing system includes a pair of processors for simultaneously executing commands for processing data, a memory, and a data transmission bus between the processors and the memory voer which the processors may fetch data from and write data to the memory. A comparison circuit is included between the processors for comparing the data fetched and written by the processors. A rollback module is responsive to the comparison circuit for rolling back the operation of the processors to the beginning of a presently executing command in the event of a miscomparison by the comparison circuit.