From his reddit post:
More technical (read:mindnumbing, but hopefully redditor-friendly) info about my plans, if anyone's interested:
Several things will have to be more limited than the Hack specs in the book. First of all, that Hack machine expects 48KB of RAM and 64KB of ROM. From what I've seen with my experiments in making bits of memory with DFFs in minecraft, to say that I won't get that much is an understatement. I'm limited to a 300x300 block patch of land due to minecraft not simulating what happens past that distance. Plus, whatever storage space I manage to put in will not involve me making sixty-four thousand of something. The Atari 2600 had 128 bytes of RAM, that should be enough for anybody.
Also, the Hack platform expects a 512x256 screen and a keyboard. Forget that (it would require 16KB for the memory map alone, plus the screen could never be any bigger than 300x128 due to minecraft's limitations), but there are options: a two line or so high terminal of limited resolution seems the most realistic for actually getting pixels as output. Maybe a ticker can be used (something that has already been designed in minecraft). In any case, whatever's used will be what takes up the least amount of space and RAM. I'll barely have enough as it is.
The compiler (or, rather, the standard library for the language) expects those memory maps and screen sizes to be fixed, but since part of the book was coding the compiler and library, I have the sources, and can change those to whatever I end up putting together.
I'm getting ahead of myself though. Next step is looking at clocks speeds and figuring out how to make a bit of memory in as few blocks as possible...
An awesome Redstone Simulator that helped me figure out the design (fun fact: redstone is weird, and redstone torches will light things unplanned directions, ignore leads because they aren't bent the right way, and power stuff through solid rock that you want left well enough alone.
Without MCEdit, just clearing out the space for this project would have been a job in itself.
And The Elements of Computing Systems is just about the best book on the "everything" of the computer that I've ever read (another really good one is Code) It's amazingly approachable; if you want to learn how PCs work, and then make one that works yourself, pick it up.