Subject: Re: PMAZ-A option card on 3MIN and MAXINE
To: Toru Nishimura <nisimura@is.aist-nara.ac.jp>
From: Michael L. Hitch <mhitch@lightning.oscs.montana.edu>
List: port-pmax
Date: 06/04/1998 20:08:18
On Jun  5, 10:41am, Toru Nishimura wrote:
> That was my original suspection.  Gee... the PMAZ-A figure I posted is
> worse than AHA-1542B with 486DX2/66.  IOASIC SCSI DMA outperforms
> 12.5MHz PMAZ-A completely.

  It should also outperform a 25Mhz PMAZ-A as well.

  I was trying to test the transfer rates under Ultrix, and it seemed
like Ultrix was able to transfer just as fast with the PMAZ-A as
with the IOASIC SCSI.

> BTW, I'm interested in IOASIC DMA "chaining".  How can we program "DMA
> pointer update"?  (I can not believe commented-out asc_dma_intr() has
> ever worked so far, although it seems to be there for-loooog-time).

  I think that commented out code was a leftover from a much earlier
driver.  The DMA stuff has been reworked quite a bit since then.  I
would guess it's been left in as an example of what needs to be done,
and as a reminder.

  All (!) it takes is to figure out the physical address of the next
page for the DMA and stuff it into the specified register on an
IOASIC SCSI interrupt.  The tricky part is that the register needs
to be loaded quite quickly - the amount of time it takes to transfer
the previous page of data.  To keep up with a sustained rate of
5MB/sec, this would be around 800 micro-seconds.  This may sound
like a lot of time, but there are too many things in the kernel which
can keep the interrupts disabled longer than that.  Just outputing
to the screen on a Maxine display would kill it completely.

  I doubt the SCSI pointer reload interrupt can be used until the
spl*() and interrupt handling is significantly revised.  I suspect
that the SCSI pointer reload interrupt would need to be kept
enabled all the time, even at splhigh() to ensure that the interrupt
can be processed quickly.

Michael

-- 
Michael L. Hitch			mhitch@montana.edu
Computer Consultant
Information Technology Center
Montana State University	Bozeman, MT	USA