Allison wrote: The best processor is going to be different based on the person you are talking to. Everyone has their own idea as to what features are important, and fond old memories of working on an ancient (wasn't then) OS tend to sway their decision. If you go back to one of those old systems, you often think to yourself "did I really put up with all this crap?" If you just want to play around with a lot of different CPU architectures to see what they were like, the 'simh' series of emulators might be a place to start. Most of them are sturdy enough to run a real OS, and they have the ability to work at the instruction level. Includes a simple assembler/ disassembler for each CPU. Lets you play with a lot of hardware before deciding you want to spend actual money on them.