Risc vs Symbolics (was: Poor Sun timings, as competition...)

[RWK@fuji.ila.com (Robert W. Kerns)]

    Date: Mon, 13 Aug 90 16:50 EDT
    From: Reti@riverside.scrc.symbolics.com
	I know that real users compare whole systems (i.e. hw+so+compiler+env.+...)
	but i was wandering if someone has tried to answer to the following
	(silly) question:
    I don't think it is a silly question.
I agree!

    My gut feel is that you'd need at least a factor of 3 and 4 to break
    even, though you'd have to do the architectural design and then code up sample
    frequent instruction sequences and sum their timings to be sure.

I'd rate this another way:  I'd say it would take about a 50%-100% larger
factor than the usual CISC:RISC break-even ratio.  That's my feel for
how often the Symbolics architectures' parallel paths save cycles for
Lisp.

    Also, the storage of tags is a problem; if you want 32-bit integers and floating
    point numbers, but also want tags, you must have either a word size bigger than
    32 bits (Ivory has 40) or do some kludging around to simulate it which will no
    doubt have a negative impact on the performance of most if not all memory
    references.

It should be pointed out that Genera depends on being able to do
fixnum arithmetic on addresses in a few places.