Path: utzoo!utgpu!attcan!uunet!husc6!cfa!ward
From: ward@cfa.harvard.EDU (Steve Ward)
Newsgroups: comp.sys.nsc.32k
Subject: do it yourself project?
Message-ID: <1268@cfa.cfa.harvard.EDU>
Date: 10 Nov 88 18:39:14 GMT
Organization: Harvard-Smithsonian Ctr. for Astrophysics
Lines: 44


Subject: Re: 32K "do-it-yourself" kits...
Newsgroups: comp.sys.nsc.32k
Keywords: The Ultimate Scrooge...
Summary: why no MMU for SOME situations
	 homogenous software environs, passive backplane
References: <2613@sultra.UUCP> <2629@sultra.UUCP>

...
MMU:  The point about MMU's being great to protect the system from the
errant process is true, true, true.  Of course, one also gets virtual
memory out of it, too.  This can be bad for some things.

I do realtime work and predictable, fast context switches are a must.
I measure these in low microseconds.  Virtual memory inflicts
non-deterministic latencies and if process parts are swapping, quite
often many milliseconds of overhead.  Sometimes one can just turn
off the MMU.  I am not familiar with the 32K family to know what is
possible and with what effect.  No MMU IN EFFECT would be just as
good as no MMU physically.  Maybe one design will work for both user
camps.

I prefer a single homogenous software environment using only the
selected 32K cpu.  Still, the IBM PC bus and I/O harware can be
utilized by designing the CPU board to function with an IBM bus
passive backplane (you get them for $75 - $99), acting as the IBM
PC bus master.  This gives you one software environment, but uses
all that PC hardware.  Just pop out your PC mom board and pop in
a passive backplane.

The first place to start is with a real design and I have only
heard D. Rand offer one, so far.  I guess we'll see what pops
up in the next few days.  For the record, nobody responded to
my posted message offering some CAE/CAD service toward a board
project - I called for direct email to me on it.  Two messages
have been posted here that touch on my message.  I am not sure
there is much interest in making a board, though there may be
interest in using a board, providing SOMEBODY ELSE makes it,
of course.  Also, the real doers may already have made something
as there have been cryptic references to some secret project.
And, of course, a few days isn't enough time for all interest
to be expressed.  Wait and see, I guess.

Steve W.    ward@cfa.harvard.edu