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Re: is it sensible to think of an extended address Lisp for the

I didn't say that using two words per CONS cell was a kludge;
I said that DEC20 extended addressing is a kludge.  The giveaway
is the word "extended" -- if it's extended, then it doesn't fit
well into the architectural framework.  As for my long list of
reasons, perhaps I neglected to make it clear that they were
not reasons for never doing it, but reasons why MacLISP didn't
naturally "grow" into an extended-addressing implementation.
Bringing up NIL on a DEC20 wouldn't be too much worse than bringing
it up on any other machine.
An extended-addressing DEC20 is a fine machine, viewed as such.
It is not, however, to be viewed as a terribly upward-compatible
version of the PDP-10 architecture; the use of extended addressing
would pervade everything, however superficially similar the
instruction sets appppear to be.  (DEC bills the VAX as a member of
the PDP-11 family, too, but calling a tail a leg doesn't make it one.)