>>Typically you make the memory redundant too, and just ensure that input and outputs are common and all code is deterministic. e.g. There's Tandem / HP NonStop

>>Typically you make the memory redundant too, and just ensure that input and outputs are common and all code is deterministic. e.g. There's Tandem / HP NonStop which use these techniques. Tandem was software fault tolerant - the Stratus machines were fully hardware fault tolerant and ran code in parallel on seperate logical cpus to test for faults. Truly neat machines. Also, it was a PL/1 based machine - even the OS.