Greg Lehey said:

>IIRC Stratus had 4 processors per CPU, organized as two pairs which
>kept checks on each other.  If they disagreed, they both died and let
>the other pair get on with it.

And if you had the right support contract, the system would dial up
Stratus and order a new part. You would receive it by next day air.

I used to support five of these system. Every now and then I'd come
into work and find a package from Stratus sitting on my chair.

>Tandem's Integrity series had a thing called triple modular
>redundancy: they saved one processor and voted three ways.  Each
>processor was on a separate hot-pluggable board, so you could repair
>the CPU while it was running.  The Tandem proprietary NonStop
>processors were not as sophisticated: they checkpointed data across a
>high-speed bus, but they only had a single processor per CPU.  They
>were also hot-pluggable.

When I worked for Progress Software, we had to crash systems to test
the Progress database recovery tools. Tandem systems are very hard to
crash.

-- 
Rick Kelly  rmk@rmkhome.com  www.rmkhome.com