Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!pasteur!ucbvax!agate!labrea!polya!kaufman
From: kaufman@polya.Stanford.EDU (Marc T. Kaufman)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Apple HD SC 80 does not support Asynch i/o.
Keywords: Just what is the problem here?
Message-ID: <5879@polya.Stanford.EDU>
Date: 4 Jan 89 04:22:04 GMT
References: <11605@dartvax.Dartmouth.EDU> <271@berlin.acss.umn.edu> <11614@dartvax.Dartmouth.EDU>
Reply-To: kaufman@polya.Stanford.EDU (Marc T. Kaufman)
Organization: Stanford University
Lines: 32

In article <11614@dartvax.Dartmouth.EDU> earleh@eleazar.dartmouth.edu (Earle R. Horton) writes:

.The Mac OS supports asynchronous I/O for all drivers installed in the
.system.  Inside Macintosh gives quite precise instructions for an
.application developer to use it.  There can only be one of two reasons
.why this feature is not implemented for SCSI hard disks, even the
.Apple HD SC 80:

.	a)  Disk driver writers were too lazy to put it in, and
.	    figured most people wouldn't miss it.

.	b)  The present SCSI interface is not capable of it.  I don't
.	    know how this could be true, but I am extremely unwilling
.	    to believe that the Apple drive cannot do asynch I/O because
.	    of the first reason.

.Anybody have any idea which of the above is true?  I've started to
.wonder, since the Apple HD SC 80 failed Oster's Asynch test program
.last night.

The problem with SCSI is that, if you give the CPU back to a user (the same
or another user .. it doesn't matter), the user may try to start another SCSI
operation .. perhaps on a different device.  Now SCSI supports such things, via
the Disconnect-Reselect messages, but the MacOS does not (BTW: A/UX does
support disconnect-reselect).  The reselect comes from the peripheral, so the
OS has to do some work to determine which driver is the correct one to restart.
There are words in the SCSI sections of Inside Mac that hint that SOMEDAY (real
soon?) the MacOS will support disconnect-reselect, but until that time comes,
the OS cannot take the chance that two operations will be active on the bus
at the same time.

Marc Kaufman (kaufman@polya.stanford.edu)