Reading about LISP machines has given me the sad realisation that Unix is RISC.
LISP machines were as CISC as you could get, closing the "semantic gap" by all the way up to LISP primitives.
RISC was "don't do that, don't make assumptions about the high-level workload, just provide fast machine-aligned primitives".
LISP OSes aligned with LISP. Unix may align with C, but in a fairly generic way. Really it's an OS trying to be dumb and hardware aligned. Push the clever up the stack.
At some level, I have to ask myself "Is this true?". In the same way that fish might not question water, is the C model really a "neutral" model on which to build flexibly?
Well, C was, at one point, pretty "hardware independent assembly", so I think it's fair to treat it as quite hardware-aligned.
Probably the weirdest bit of hardware/OS co-evolution is the MMU. It feels a little contrived.
The rest, though? Reasonably good hardware sympathy for a load-store machine.
@sgf Certainly the way the MMU works, with TLBs and page descriptors, shapes a lot of Unix (and Windows) internals, and those in turn constrain how hardware can implement those features.
The most infamously different variant of this is the i432, which would have been... interesting... to port Unix/Windows to.
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