On Wed, Mar 17, 1999 at 11:25:21AM +0900, Toru Nishimura wrote:
> >>   The 5000/240 uses the IOASIC SCSI normally, and the problem appears to
> >> only be in the TurboChannel-option SCSI that the 5000/200 has.  It's
> >> possibl that the problem may show up on the TC-option card on the other
> >> systems, but not too many people are using the TC-option card when they
> >> have the IOASIC SCSI.  [The IOASIC SCSI will use DMA directly to memory,
> >> while all the TC-option SCSI cards have to copy data to/from the on-card
> >> memory buffer.]
> >
> > This machine has 2 additional TC SCSI cards.  So you're saying I should be
> > worried?
> 
> Single channel SCSI TURBOchannel option card PMAZ-A, the same design
> as 5000/200 builtin, runs slower than IOASIC SCSI. 
> 
> To me dissappointment, dual channel SCSI TURBOchannel option card
> PMAZB-A (TCDS) could not achieve better performance than IOASIC SCSI
> of my Personal DECstation builtin.  Probably, the limiting factor is
> its artificially lowerd 12.5MHz TC clock rate.  I'm day-dreaming to
> have a memory-rich DECstation 5000/260 with the regretful loss of
> DECsystem 5900s two years ago... 

It seems to me the issue is quite probably not just that the TCDS card is
on a 12.5MHz TurboChannel bus, it's also that, as mentioned above, it
can't DMA directly to/from host memory -- everything has to be copied to
a buffer on the card first.  That's burning a lot of memory bandwidth and
CPU cycles on a machine that's not exactly got a whole lot of either to
spare...

I think Jonathan and I discussed a couple of times whether it was feasible
to put one of the TurboChannel SCSI cards that shipped with the Alphas --
which *could* DMA directly to/from host memory -- into a TC DECstation.
AFAIK nobody has mashed the appropriate device drivers into shape and tried
it yet.

This is all a pretty fuzzy memory to me, I hope not *too* much of the above
is wrong...

-- 
Thor Lancelot Simon	                                      tls@rek.tjls.com
	"And where do all these highways go, now that we are free?"