Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!usc!apple!oliveb!amiga!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga
Subject: Re: Commodore New Products/Price Reductions COMDEX Announcements
Message-ID: <8830@cbmvax.UUCP>
Date: 5 Dec 89 22:42:27 GMT
References: <22891@gryphon.COM>
Organization: Commodore Technology, West Chester, PA
Lines: 26

in article <22891@gryphon.COM>, jdm@gryphon.COM (John Mesiavech) says:

>  Regarding the extra MMU on the CBM A2630 board......

There ain't one...

> Had read on a technical board that the reason for the extra MMU
> is that the one that's built into the 68030 is a SUBSET of the
> 68851 MMU.  So, CBM put the full MMU on the board for better UNIX
> performance.

The performance under UNIX isn't noticably lessened by the difference
in MMUs; perhaps a little, due to the smaller ATC in the '030, but
then again, the '030 has some advantages the '851 doesn't.  In any
case, there's not enough of an advantage to the '851 over the '030
MMU to justify the $100 or more those '851s cost.  Even if the '851
could be used as an external MMU to the 68030, which it can't.  The
MMU coprocessor address isn't passed by the 68030, so a 68851 just
sits there eating power and adding a wait state or so if it's put
into a 68030 system.

> John
-- 
Dave Haynie Commodore-Amiga (Systems Engineering) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: hazy     BIX: hazy
                    Too much of everything is just enough