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

["Gary M. Palter" <Palter@alderaan.scrc.symbolics.com>]

    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?

    While I'm asking questions:

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

By doing 2 bus writes per Ivory word.  One full word write for the 32
bit data field; one half word write for the 8 bit tag plus 8 bit 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?  

Yes.  (It's actually the Ivory chip that does ECC.)

					  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.  

It means you need 6 bytes per Ivory word instead of 5.  I wouldn't call
that substantial.  It actually simplifies the MacIvory's Nubus interface
board as you don't have to do a read to get the halfword with the tag in
order to preserve the other tag therein.

				       Is the memory potentially bad enough
    to require a lot of error-fixing overhead?

We think so.

    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?

Ivory is source level compatible with the 3600.  Most of our beta
test customers who have converted their applications to the Ivory were
able to do so by recompiling without any source changes.

    --David Gadbois