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.
