The Master-checker reference article from the English Wikipedia on 24-Apr-2004
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Master-checker

For people who check facts
A master-checker is a hardware-supported fault tolerance method for multiprocessor systems.

Two processors calculate in parallel the same functions to increase the probability that the result is exact.

In technical terms: the checker-cpu is synchronised at clock level with the master-cpu and is processing the very same programs as the master. Whenever the master-cpu is generating an output, the checker compares it to its own calculation and in the event of a difference a warning is raised.