Check out Minix. It's an OS designed to be part of an educational course in OS design (and, incidentally, the precursor to Unix). The course books (by Andrew Tannenbaum) are available - if not free/online, at least via Amazon & ebay. And they are highly recommended by people who appear to know what they're talking about.

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Yeah, Minix! I remember Bill Buzbee got it running on his wire-wrapped TTL CPU "Magic-1." Seems like a good option.

Lately, I've been trying to rein in my brainfarts by writing them down someplace, forgetting about them, and getting back to the "having a functional workshop again" project with will, in theory, make all the other projects possible. Though even that has been severely backburnered due to Life and Stuff[tm].

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Hah, "life and stuff," always getting in the way. Big things (good things) are happening with me at work these days, which keeps me quite busy, but in a couple weeks my project will finally end and I will have some significant time off. That sounds like my chance!

a 500k Xilinx Spartan-3E board, so I'm going to see if I can put some sort of soft core on there.

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Pardon me for not remembering or googling for names, but there are some CPU softcores out there which, rather than being direct copies of known hard-CPUs, are original designs optimized for synthesis on FGPAs in the least gate-space. Poking around opencores.org should turn them up.

Still, there should be plenty of space on a 3E for a core or two of some description.

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Yeah, there are a lot of soft core options. I see that the OpenRISC minsoc (minimal system on chip) runs successfully on the spartan 3e, so I think I'll give that a shot tonight.