Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!caen!kuhub.cc.ukans.edu!markv
From: markv@kuhub.cc.ukans.edu
Newsgroups: comp.sys.amiga.hardware
Subject: Re: 68030 options.
Message-ID: <1991Mar26.095729.29291@kuhub.cc.ukans.edu>
Date: 26 Mar 91 15:57:29 GMT
References: <47678@nigel.ee.udel.edu> <19944@cbmvax.commodore.com>
Organization: University of Kansas Academic Computing Services
Lines: 43

>>> stale data in either cache after a DMA transfer.  The A2091 software will
>>> actually flush the cache after an I/O operation completes, but in general,
> 
>>Perfect timing, Dave! Someone just asked me how to flush the
>>cache(s), and I don't have the foggiest. 
> 
> ...This will clear the caches properly for 68020 and 68030, but not
> for 68040 or any external cache that might need...Under 2.0, there's
> a system call to be used for cache clearing, which can be called for
> any system, and would even work with external caches...

A query and a request?  One thing I havn't seen mentioned is the
68040's MMU's ability to monitor the bus, and update its caches on
accesses to cached RAM, and to inhibit RAM and supply a valued for a
cached address that hasn't been "written thru".  This would eliminate
the cache coherency problem and avoid the severe performance hit your
going to take if you have to flush 4K of cache on *every* DMA I/O
(unless there's some more intellegence in the process).

My questions are, does the 3000's CPU slot have the signals available
that would let it do this for motherboard RAM, or would it be
restricted do any "on-board" RAM.  Second, I don't think anyone doubts
that Commodore is working on some kind of 040 board/machine and it
sure would be nice if this feature is supported...

Otherwise, what about a few more OS extensions that expliot the PMMU
to inhibit cacheing of regions used for DMA buffers (this of course is
in the direction of my dream of OS support for the MMU in general,
maybe some flags like MEMF_NOCACHE and MEMF_NOPAGE, locking blocks, etc).

> -- 
> Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
>    {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: hazy     BIX: hazy
> 	"What works for me might work for you"	-Jimmy Buffett
-- 
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