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A New Old Processor

02 Mar 2010

aka “Lisp all the way down”

Last time I was talking about the my new-found excitement for designing a processor from scratch.

Clearly there’s a lot of previous research and art to draw on in this area. I started going through some ancient, and some newer.

ZPU

There’s a few decent processors on OpenCores: cycle accurate version of Z80s, 6502s, simple x86s, etc.

There’s also a neat processor called ZPU that’s entirely stack based. It’s sort of assuming a pre-2000s model of computing in that RAM access isn’t (relatively) abysmally slow. It’s got a very nice opcode encoding where every opcode is 8 bits, including the immediate instruction. The immediate opcode is “high bit set” and that pushes the other 7 bits as a signed value. If you put 2+ immediates in the opcode stream, then the previous value on the stack is shifted left and the new 7 bits are ORd in, rather than creating a new stack value. The data bus is 32 bits, and the rest of the 128 available opcodes are assigned to the various standard operations, mostly operating on the stack items, but a few include a small immediate value (again, only in the 8 bits) to allow a displacement. This is used in the SP-relative indexers to allow local variables to be accessed without a lot of pain. I poked around this design for a while, and it’s really nifty. I might try something like this one, though it’s kind of already “been done” so I’m not sure how interesting it’d be just to reimplement that design.

Lisp Machines and SIMPLE

I didn’t get too far in my pseudo-literature review before I started looking at Lisp Machines. It turns out the actual Lisp Machines were not really fundamentally anything that was specifically designed to run Lisp. They did have custom microcode, and extra pointer bits for helping on type dispatch, or helping the GC, but fundamentally, they were “regular” machines that ran simple microcode, had flags, PC, stacks, registers, conditional branching, etc.

An interesting exception to this is SIMPLE which at the very lowest level is actually an interpreter for a Lisp. car, cdr, cons, etc. are all primitive opcodes, and computation is modelled on eval/apply’ing a recursive tree structure.

That paper is very interesting. Looking back with 35 years future vantage point on someone’s University project is pretty humbling (well, it’s not just anyone’s University project, but still). This is a piece of hardware that was designed with hardware garbage collection, and evaluating properly lexically scoped closures as a computation model. The average environment circa 1979 would have been a machine language monitor on a 6502, or an assembler on a Z80, or a primitive BASIC implementation. Compared to that sort of code-writing-experience, SIMPLE blows me away. Hell, I had to write a singly-linked list #define “class” in plain C the other day (…which is of course just plain nuts, but that’s another story).

Of course, there are some limitations to SIMPLE. Looking at it now, limiting all memory words to homogeneous two-word cons pairs (with no vectors/arrays) seems very constrained. This was hand-waved away by saying that this processor could be paired with an APL processor to handle array-based math. That it would today just be a similar NV GPU instead doesn’t say much for the advancement of our discipline.

The non-realtime GC also sticks out, though there was a paper the next year on how to make it realtime, for some definition of realtime. Unfortunately, that paper is only available via HellSpawn, sorry, ACM Digital HELLSPAWN Library, so I haven’t read it yet. Having a process or thread block for a GC is one thing, having the whole processor block seems very foreign.

There’s also no ALU! It’s not really needed for basic evaluation, but I think its lack is more due to wanting to demonstrate that it wasn’t necessary and saving on die space, rather than believing it didn’t belong there.

All the way

So, SIMPLE is the basis of my plan for a new (but very old) processor. Lisp all the way down. If all goes well, phase 2 will be trying to lift some of those limitations if they actually turn out to be limiting.