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From: raymond@pioneer.arc.nasa.gov.arpa (Eric A. Raymond)
Newsgroups: comp.arch
Subject: LISPMs not RISC? - Re: CISCy RISC? RISCy CISC?
Message-ID: <16921@ames.arc.nasa.gov>
Date: 22 Oct 88 01:26:11 GMT
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Reply-To: raymond@pioneer.arc.nasa.gov.UUCP (Eric A. Raymond)
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DIn article <19191@apple.Apple.COM> malcolm@Apple.COM (Malcolm Slaney) writes:
>>must be backward compatible.  Even so, there are no new CISC designs
>>being done, that I know of.  
>
>Ummm, I think Symbolics and TI would probably argue with this statement.
>Both companies have recently introduced Lisp machines on a chip....and nobody
>will ever call a conventional Lisp machine architecture a RISC.

Hmmm.  Have you ever looked at a Lisp Machine?  Sure its not a RISC because
its microcoded, but there are some similarities.  Things like barrel shifters
(for extracting tags) come to mind.  The instruction set is not all that
complex.  Sure it doesn't have lots of registers, but its paradigm is a
stack machine (you might consider its on-chip stack to be registers).
Anyway, nobody ever said that RISC meant lots of registers.

This is where TI ends (I believe) and Symbolics goes on (the tags that is).
At least that's where the CADR (LMI and TI didn't really vary from this
design)  left off.  The neat trick which the Symbolics introduced in the
3600 (and now in Ivory) was to allow parallel execution of instructions, 
type checking, GC related checks ,etc.  If one of these traps suceeded, then 
a different instruction would be called.  These extra checks caused no overhead
in the base case.  (i.e. Assume operands to add are integers.  If it turns out 
that they are not, then execute the floating point add.  The beauty is that if 
they were integers, then you already have your answer and the type check was
free.)  Does this sound familiar?  They also did things like go to larger word 
sizes (36 & 40) and CDR coding (I don't think CADR had this - might be wrong).

Please flame on me and set me straight if I'm wrong.  Note that both
RISC and LISPM-chip (read stack machine with tag bits) are vague terms.
Also note that in every respect other than development environments (and
their not really that far behind anymore)  RISC machine LISP >= LISPM.
By the way, Envos, a Xerox spinoff, sells the Xerox LISP environment for
SUN 4's which is 2x faster than the 1108's were.  Not that the d-machines
were nkown for speed ....   Xerox does not sell machines anymore.

Name: Eric A. Raymond
ARPA: raymond@pioneer.arc.nasa.gov
SLOW: NASA Ames Research Center, MS 244-17, Moffett Field, CA 94035

Nothing left to do but :-) :-) :-)