Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!samsung!usc!apple!portal!portal!cup.portal.com!ts
From: ts@cup.portal.com (Tim W Smith)
Newsgroups: comp.sys.mac
Subject: Re: Mac Plus Radius Upgrade Info Plus a Favor
Message-ID: <26585@cup.portal.com>
Date: 4 Feb 90 10:48:33 GMT
References: <1000@manta.NOSC.MIL>
Organization: The Portal System (TM)
Lines: 27

The Mac ROMs have trouble with SCSI.  Asking them to work with
accelerators is asking for a miracle.

If the author of the disk driver supplied with the hard disk never
encountered an accelerated Mac, the code is unlikely to perform the
bizarre rituals needed to work correctly.

For example, after the driver does SCSIGet and SCSISelect, it will
do SCSICmd.  Under an accelerated Mac, SCSICmd will fail sometimes.

Any normal person would treat this as an error and pass an error code
back to the Device Manager.  However, someone who has been warped by
the unholy combination of the ROM SCSI Manager and an accelerator will
either loop issuing SCSICmd calls until it works, or will check the
SCSI bus to see just what phase it is in and issue the appropriate
SCSI Manager call.

I prefer the "loop until it works" approach, because looking at the
bus requires either trusting SCSIStat in the ROM, or looking at the
hardware directly.

When I was experimenting with an SE and a couple different accelerators,
I noticed that how the SCSI Manager gets screwed up depends on the
accelerator.  For example, with the Radius accelerator, it was SCSICmd
that tended to screw up.  With the Dove accelerator, it was SCSIRead.

						Tim Smith