Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!ucsd!sdcsvax!ucsdhub!hp-sdd!hplabs!hpcea!hpnmd!hpsrla!brucek
From: brucek@hpsrla.HP.COM (Bruce Kleinman)
Newsgroups: comp.arch
Subject: Re: Harvard Architecure
Message-ID: <3460012@hpsrla.HP.COM>
Date: 10 Mar 88 20:09:38 GMT
References: <8803011911.AA06922@decwrl.dec.com>
Organization: HP Network Measurements Div - Santa Rosa, CA
Lines: 37

+-------
| >Ahh, those massive 256-Byte caches are really going to speed this
| > puppy up :-)
|
| Actually, it will. Remember, the CDC 6600 got a win from an "instruction
| stack" of 480 bits ! 
|
| Plus, the two caches access in parallel (versus the one cache of the 68020).
| Plus, the caches now take one clock (versus 2 on the 68020).
| Plus, the caches now have burst refill (if the board designer supports it,
| of course.)
| 
| All in all, a clear improvement. I don't hear any suggestions as to a better
| use for the silicon.
+-------

All in all a clear improvement?  Over the '020, perhaps ...
The 256-byte data cache is of questionable value, as the miss-rate will be
fairly high.  I would supply numbers for "fairly high," but I can't seem to
find any miss-rate data for caches of smaller than 1 Kbyte.  Furthermore, the
restricted implementation of the data cache makes it useless in multi-processor
systems as well as systems with DMA.  The data cache has no facilities for
coherency.  Solution - disable the data cache or flush, flush away.

Suggestions as to a better use for the silicon?  OK ...
Expand the I-cache.  I suspect that loosing the D-cache could make room for
a 1 Kbyte I-cache.  This would offer honest improvements in almost all systems.
High-perf machines can surround the '030 with an hefty sys-cache for
increased performance.  Memory bandwidth problems, you say?  Bring the
Harvard architecture out to the pins - which is exactly what Motorola did
for the 88000.


                              Bruce Kleinman
          brucek%hpnmd@hpcea.hp.com  -or-  ...hplabs!hpnmd!brucek

              Hewlett Packard - Network Measurements Division
                          Santa Rosa, California