On Tue, 16 Mar 1999 22:28:25 -0500 
 Thor Lancelot Simon <tls@rek.tjls.com> wrote:

 > 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...

Um... the TCDS certainly CAN DMA to host memory.  It's the PMAZ-A
which cannot (TURBOchannel single scsi).  Those the CPU must bounce
the data to/from the SRAM buffer on the card.

 > 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.

That' the TCDS (TURBOchannel Dual SCSI), and Tohru has made the driver
work on the DECstation.  He's sent some mail about this before, as I
recall...

So, anyhow, it seems perfectly reasonable that the TCDS is eing
limited by the 12.5MHz TURBOchannel.

        -- Jason R. Thorpe <thorpej@nas.nasa.gov>