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From: santiago@lerad.pa.dec.com (Ed Santiago)
Newsgroups: comp.sys.dec
Subject: Re: Caching Turbo Channel access ??
Message-ID: <1991Jun26.180424.27695@pa.dec.com>
Date: 26 Jun 91 18:04:24 GMT
References: <1991Jun26.010347.6638@mel.dit.csiro.au> <1991Jun26.024702.169@pa.dec.com>
Sender: news@pa.dec.com (News)
Organization: DEC Workstation Systems Engineering, Palo Alto
Lines: 28

santiago@lerad.pa.dec.com (Ed Santiago) writes:
>mxm@manta.mel.dit.CSIRO.AU (Mark Makies) writes:
>>Is it possible to cache I/O access from the Turbo channel
>>in a DEC Station 5000/200?
>Sorry, the hardware does not allow it.
I've been asked to clarify this a little bit.  

My interpretation of this question was "is it possible to access device
registers on a TURBOchannel option via cache space?", eg, something like:

	volatile unsigned long *CSR = <something-in-cached-space>;

The answer to this is no -- the DS5000 hardware does not support it (for
those, like me, that always ignore "unsupported" -- it doesn't work either).

However, another interpretation of the question might have been "does
TURBOchannel allow DMA into cached pages in RAM?".  The answer to this
is "well, sort of".  TURBOchannel options may perform DMA anywhere in
system memory.  However, DMA transfers do *not* go through the cache,
so if a transfer occurs to a page that is currently cached, the cache
will not know about it and keep presenting stale data to the CPU.

Hope this helps,

^E -- ex-DS5000 firmware type
-- 
Ed Santiago                                      santiago@decwrl.dec.com
DEC Workstations Systems Engineering             ..!decwrl!santiago