A little-known fact is that the entire preboot environment in Sun's UltraSPARC machines was really just Forth. It was possible to modify that Forth code to do more or different things than what the factory intended, and Sun actually had documentation on how to do that, IIRC.

I really miss having an innovative, open-source friendly hardware manufacturer like Sun around - they were an extraordinary group of people. (I worked for them for a while in the late 90s, and it really was a great company, with by far the best tech-company culture I've ever encountered...)