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Re: [Spock@SAMSON.cadr.dialnet.symbolics.com: IFU]

    Date: Fri, 2 Dec 88 12:32 CST
    From: gadbois@sygmund.cgs.utexas.edu (David Gadbois)

	Date: Fri, 2 Dec 88 08:46 PST
	From: DE@PHOENIX.SCH.Symbolics.COM (Doug Evans)

	[ stuff deleted ]

	Also, it's pretty difficult to squeeze performance out of a 40bit
	computer with a 32bit address space when it has to use a bus structure
	with 36bit datapaths and 24bit addresspaths.

    I thought the address path was 28 bits, since the word organization is
    (I'm told) 28-bit address + 6-bit tag + 2-bit cdr code or 32-bit
    immediate data + 2-bit tag + 2-bit cdr code.  If the address path is
    24-bit, how do you deal with the extra 4 bits?

It's a 28-bit address path.

    While I'm asking questions:

    How does the MacIvory do 40-bit (or 48 -- see next question) words with
    the 32 bit NuBus?

48 bits -- 32 bits of address or data, 6 bits of tag, 2 bits of
cdr-code, and 8 bits of parity.  Three 32-bit NuBus words combine to
form 2 Ivory words + ECC.

    I may be confused by this, but the literature I've read implies about
    the XL400 says it's memory organization includes 8 bits of error
    correction/detection, and that its Ivory board handles the ECD itself.
    Is the MacIvory set up that way too?  Since this is a substantial amount
    of memory overhead (and maybe silicon, too), I was wondering what the
    design argument was to add it in.  Is the memory potentially bad enough
    to require a lot of error-fixing overhead?

Lisp machines are expected to stay up for long periods of times (as far
as "workstations" go, anyway), typically several weeks.  The memory is
not low quality.  The way I like to put the question is "well, the
Titanic has been shown to float, so why not just throw out all these
heavy lifeboats and life-preservers?"  ECC (and tags) buy you a large
measure of robustness.  We certainly think that the memory overhead is
worth it.

    I think that the 3600 architecture allowed for up to 34 tags.  The
    Ivory, with a full 6-bit tag field (assuming you're still using 2 bits
    out of the 40 for a cdr code), lets you have up to 64.  Aren't there
    some big incompatibility potentials here?

What kinds of incompatibility do you have in mind?

    --David Gadbois