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From: time@tbomb.ice.com (Tim Endres)
Newsgroups: comp.sys.mac.programmer
Subject: Re: async write to disk (MPW C)
Message-ID: <1CE00001.a1e4o2@tbomb.ice.com>
Date: 30 Jan 91 02:54:36 GMT
Reply-To: time@ice.com
Organization: ICE Engineering, Inc.
Lines: 36
X-Mailer: uAccess - Mac Release: 1.0.5+


In article <9341@dog.ee.lbl.gov>, beard@ux5.lbl.gov (Patrick C Beard) writes:
> In article <1016@ra.MsState.Edu> krn1@ra.msstate.edu (Ken Newton) writes:
> #I am working on a real-time application to run on a Mac fx that 
> #requires very fast data collection to disk.
> 
> The Macintosh OS is an unsuitable OS if you need asynchronous
> access to the hard disk.  The SCSI manager can't work asynchronously in
> its current design.  Thus you can't take advantage of DMA on the IIfx until
> a later system (later even than system 7).  A new disk driver will be required,
> I believe, or a new SCSI manager.

This is true with one exception. Several manufacturers now sell NuBus
cards that implement SCSI. These are designed in such a way as to
allow asynchronous I/O and dual I/O. I helped write a driver for just
such a card that allowed disk writes to occur while data was being
read from an image card. This worked very well.

I know MicroNet has such a card, I programmed the Golden Triangle
ASP card, and I think DayStar has a card.

> However, under A/UX, DMA can and is being used for disk access speedup on
> the IIfx.  Ironically Unix is likely to give you better disk performance,
> even though you will probably be spending a lot of time swapping programs
> to and from disk.  A/UX has no SCSI manager, just different device drivers.

Oh so true, however it is straight forward to design A/UX applications
such that they do not swap and are very efficient. Plus I believe
you can access more memory under A/UX than under MacOS currently.

tim.

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