> SCSI Bugs
> http://developer.apple.com/technotes/dv/dv_14.html
If ever you see a Scsi-over-Whatever device that
treats unit attentions and resets bizarrely ... maybe
here below is part of why.
Informally, among certain circles of storage device
folk, the creative approach to Scsi host
implementation cited here is Famous.
Today is the first day myself I ever saw this link,
though I can easily imagine this link has worked for
some years, maybe decades.
Pat LaVarre email@hidden
http://members.aol.com/plscsi/
> Hard Disk Hacking
> http://developer.apple.com/technotes/dv/dv_08.html
...
> SCSI Bugs
> http://developer.apple.com/technotes/dv/dv_14.html
...
ROM boot code problems
In the process of looking for a bootable SCSI device,
the boot code issues a SCSI bus reset before each
attempt to read block 0 from a device. If the read
fails for any reason, the boot code goes on to the
next device. SCSI devices which implement the Unit
Attention condition as defined by the Revision 17B
SCSI standard will fail to boot in this case. The read
will fail because the drive is attempting to report
the Unit Attention condition for the first command it
receives after the SCSI bus reset. The boot code does
not read the sense bytes and does not retry the failed
command; it simply resets the SCSI bus and goes on to
the next device.
If no other device is bootable, the boot code will
eventually cycle back to the same SCSI device ID,
reset the bus (causing Unit Attention in the drive
again), and try to read block 0 (which fails for the
same reason).
The `new' Macintosh Plus ROMs that are included in the
platinum Macintosh Plus have only one change. The
change was to simply do a single SCSI Bus Reset after
power up instead of a Reset each time through the SCSI
boot loop. This was done to allow Unit Attention
drives to be bootable. It was an object code patch
(affecting approximately 30 bytes) and no other bugs
were fixed. For details on the three versions of
Macintosh Plus ROMs, see Technical Note #154.
...
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