Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!apple!sun-barr!texsun!pollux!attctc!chasm
From: chasm@attctc.Dallas.TX.US (Charles Marslett)
Newsgroups: comp.os.minix
Subject: Re: minix on the macintosh
Summary: What if we write them right the first time?
Message-ID: <9338@attctc.Dallas.TX.US>
Date: 12 Sep 89 18:35:52 GMT
References: <23658@louie.udel.EDU>
Organization: The Unix(R) Connection, Dallas, Texas
Lines: 35

In article <23658@louie.udel.EDU>, Leisner.Henr@xerox.com (marty) writes:
> I don't understand what the presence or absence of DMA has to do with the
> viability of Minix on a platform.
> 
> You have to rewrite the device drivers in a way that's appropriate for the
> platform.

On most systems (non-VAXen, that is), devices do not have infinite bus width
access (and VAXen just pretend).  So a well written driver will not turn on
the DMA until the data is available, and if possible wil use a burst mode
to minimize wasted bus bandwith.  Similar arguments apply (for completely
different reasons) to channel/coprocessor based mainframes (IBM 360/370/etc).

As a result, a relatively efficient portable driver (that does not need to
support unbuffered synchronous I/O devices like the IBM PC floppy controller)
will wait to transfer a block of data until it is ready.  Further, if the
machine is doing lots of I/O intensive activity, and has more than one channel
(or DMA controller -- not DMA channel), it may want to wait for multiple
blocks to be queued in the buffer.  The difference between programmed I/O
and this tightly controlled DMA or channel transfer is not very significant
(usually the only important issue is interrupt latency, and that is a
characteristic of the hardware+OS, not just the hardware).

> marty
> ARPA:	leisner.henr@xerox.com
> GV:  leisner.henr
> NS:  leisner:wbst139:xerox
> UUCP:  hplabs!arisia!leisner


===========================================================================
Charles Marslett
STB Systems, Inc.  <== Apply all standard disclaimers
Wordmark Systems   <== No disclaimers required -- that's just me
chasm@attctc.dallas.tx.us