Path: utzoo!attcan!uunet!portal!cup.portal.com!ts
From: ts@cup.portal.com (Tim W Smith)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Apple HD SC 80 does not support Asynch i/o.
Message-ID: <13369@cup.portal.com>
Date: 10 Jan 89 07:54:15 GMT
References: <11605@dartvax.Dartmouth.EDU> <271@berlin.acss.umn.edu> <27318@ucbvax.BERKELEY.EDU> <13271@cup.portal.com> <5958@polya.Stanford.EDU>
Organization: The Portal System (TM)
Lines: 27

Apple means not to poll from interrupt level ( such as vertical retrace
tasks ).  These can interrupt while another SCSI operation is in
progress.

Network devices that hook up via SCSI have to poll from interrupt
level, however, because accRun events do not get generated when
you are stuck in a synchronous file operation over a file server.

Also, removable media devices have to poll at interrupt time if
they are waiting for a disk to be inserted.  This is because you
don't get accRun events when the system is displaying the "Please
Insert Disk So-and-so" dialog.  At least these don't really have to
worry about wiping out someone else's use of SCSI, since the system
is hung waiting for the disk anyway.

The hard part is that there are people out there that assume that
they are the only ones going against Apple recommendations.  If all
the people that had to poll at interrupt time got together, we could
agree on some convention to avoid wiping each other out.

						Tim Smith

ps: note that it is not sufficient for network devices that connect
via SCSI to only be careful when they poll via vertical retrace tasks.
They must also be careful when called to send a packet, because many
users of AppleTalk send packets during interrupt time.  Removable
media drivers at least don't have to worry about this one.