3-Instruction Target Forth
Frank Sergent demonstrates (and provides code for) a 3 instruction Forth. Using a Motorola MC68HC11 he manages to get the whole Forth written in 66 bytes. And from that humble 66 byte origin you can bootstrap any Forth program you want into the system. Provided you have enough memory :). It looks really cool. I think I’m gonna try it out with a PIC18. Though getting the serial port to work will take some more code, so I bet its gonna be larger than 66 bytes. But hey, the PIC18s have much more memory so its all good.
Frank also has a version Pygmy Forth that he runs on an ARM processor. I have been meaning to get an ARM and try them out. Look real cool. Much beefier than the PICs i use now.