Okay, but what's the purpose of this? Is it merely proof of concept work? Are there software that could utilize a 100 MHz BBC Micro? Or is just something incredibly cool for BBC Micro fans?
It would be nice to see a re-vamped version of it and the 'pi' has all the bits and pieces. Hopefully this is the prelude to something along those lines that no longer needs a bbc micro as an io front-end. (Though, it would seem that doing that right out of the gate would be simpler than to use the original bbc so probably this is to satisfy an itch.)
The Pi is being used to emulate a co-processor communicates via an interface available on the BBC Micro called "Tube". For more info: https://en.wikipedia.org/wiki/Tube_(BBC_Micro)
BBC games tend not to work with a coprocessor - but a lot of business/serious software probably would (not much use these days of course...), and you can always use it for programming.
I've got a 16MHz 6502 coprocessor for my BBC and it works well. The coprocessor system is quite well designed, so the coprocessor has basically nothing to do but run BASIC or whatever, and the main 2MHz CPU only gets used for I/O. I/O-heavy stuff can be a little bit quicker sometimes; calculations and stuff run about as much more quickly as you'd expect from the clock rate. (In fact, they're a little bit quicker than that - because the coprocessor doesn't have to handle I/O, timers, keyboard scanning, etc., all of which can bog down the main CPU a little.)