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From: fnf@mcdsun.UUCP
Newsgroups: comp.sys.amiga
Subject: Re: Comments on future ASDG products (68020)
Message-ID: <284@mcdsun.UUCP>
Date: Mon, 6-Apr-87 11:19:25 EST
Article-I.D.: mcdsun.284
Posted: Mon Apr  6 11:19:25 1987
Date-Received: Fri, 10-Apr-87 00:24:59 EST
References: <6043@amdahl.UUCP> <276@mcdsun.UUCP> <969@crash.CTS.COM>
Reply-To: fnf@mcdsun.UUCP (Fred Fish)
Organization: Motorola Microcomputer Division
Lines: 48
Keywords: 68020 ASDG MMU cooperation future

In article <969@crash.CTS.COM> billd@crash.CTS.COM (Bill D'Camp) writes:
>Fred,
>
>Something I wonder about with all this talk of standard versus
>custom MMU's, if the standard MMU for the '020 is so great, how
>come none of the companies making '020 boxes use it?  From what I've
>gathered by talking with people from Sun, SGI, and Masscomp, to name 
>a few, they all use custom MMUs because there is a SIGNIFICANT penalty
>involved in using the standard chip.  Is this correct, or is it just
>an excuse to limit software portability?
>
>Any personal opinions on this issue, I don't expect you to speak for
>Motorola, but I'm sure that there is a lot of interest if you could
>get an official reading from.

I have an MC68851 Manual that has been sitting on my bookshelf collecting
dust for the last six months.  Maybe I should read it sometime :-)
(tells you what I know about the details of the chip itself).

I can speak from my experience at Unisoft.  Only a relatively small number
of companies that implemented MC68000/MC68010 systems a few years ago
used the standard Motorola MMU available at the time (MC68451) precisely
because of performance penalties and limitations of the chip itself.
As fate would have it though, since that *was* the standard at the time,
ports to the systems using the *451 were very straightforward (after the
first one) and Unisoft actively sought out such customers because of this.
I expect the situation re the 020/851 to be much the same.

As far as the performance considerations go, perhaps we could coerce
someone at Apple to release dhrystone figures for the new Mac II under
their implementation of Unix.  I don't have any figures for percentage
of *current* 68020 designs that use the 68851, but my impression is that
the 851 has a much higher acceptance in the hardware development 
community than the 451 ever had.  I believe the main impediment to its
use is still low availability.

I hope this helps to clarify things.  Again, remember that these are
my personal opinions only.  The last time I did any hardware design
was around the time the first Mac came out and Unix was something
most people outside of the university environment had never heard of.
It would be interesting to get some opinions from current system
designers.

-Fred
-- 
= Drug tests; just say *NO*!  (Moto just announced new drug testing program)  =
= Fred Fish  Motorola Computer Division, 3013 S 52nd St, Tempe, Az 85282  USA =
= seismo!noao!mcdsun!fnf    (602) 438-5976                                    =