Interesting question! Today is the first time I've heard of this particular RISC project, but at a conference we recently attended (http://oshug.org/event/42) I actually got talking to someone from Cambridge who's also working on his own open source RISC implementation (shout out to BERI: An open RISC softcore for research and experimentation).

Being Open Source is actually not our main differentiator from what's already out there in the processor market. Our main differentiator is in the very architecture of the processor, and perhaps interestingly the architecture also affects the kinds of businesses and projects we're targeting.

The OpenTransputer uses the Transputer instruction set, which is 30 years old, much like a current Intel or ARM processor use instructions sets that in their original form are also 30 years old.

The Transputer instruction set architecture (ISA) is very unique in that it is a stack machine, much like Java actually, just in hardware! On the microarchitecture level both the original Transputer and OpenTransputer come with inbuilt memory and communication links.

The Transputer is very closely tied to Occam, a language based on CSP, which means it uses message passing for everything. So we're much different from current RISC and CISC processors in that we don't use shared memory but message passing to achieve multicore. It makes concurrency simpler and allows for simple and large scaling (in particular for embarrassingly parallel problems).

The intricate nature of the Transputer ISA actually made us come to the conclusion that we should be targeting the IoT market, you know, all these small devices that both the maker community but also big businesses are interested in. Because what we keep hearing is how hard it is to interface with multiple processors and external devices.

We actually made the way you communicate with peripherals (I/O) the same as how you communicate with processors: Message passing. It takes a line to set a pin high, it could not be easier.

Please keep asking if there's anything unclear in there, because I'm sure I left something relevant out!

There's actually a video of that conference I mentioned somewhere, that would probably be helpful in explaining what's going on regarding the Architecture.