SCAM and boot devices
JLOHMEYE at cosmpdaero.ftcollinsco.ncr.com
Mon Feb 12 10:25:00 PST 1996
* From the SCSI Reflector, posted by:
* "Lohmeyer, John" <JLOHMEYE at cosmpdaero.ftcollinsco.ncr.com>
HCurley at aol.com and Steve Gaskill have exchanged a couple SCSI reflector
messages regarding SCAM and boot devices. I agree with both of their
The PnP SCSI Specification is available in the /pub/standards/io/pnpscsi
directory on ftp.symbios.com, on the SCSI BBS at 719-574-0424 (file name:
pnps100.doz, a ZIP'd Word document), and on Compuserve in the PLUGPLAY
SCAM is a tool for resolving device ID conflicts. It does not solve the
whole problem and it creates new 'opportunities' (shifting device IDs).
The problem of boot devices getting shifted around is why Microsoft backed
away from requiring SCAM on SCSI devices for Windows '95. This problem can
be partially dealt with, as Steve suggested, by host adapters intelligently
assigning SCSI IDs. This partial solution does add cost to the SCSI host
adapter by requiring NVRAM. And it will still fail if a legacy device
forces a SCAM device to be moved.
The "right" solution is for the BIOS and OS vendors to solve the problem.
The BIOS guys need to examine the volume labels on all of the mounted media
and pick the boot device (or allow the user to pick the boot device as with
OS/2's boot manager). This is more work than just always booting from ID 0
or ID 6, but it is necessary to support the Plug-and-Play philosophy.
The OS vendors need to adopt a more flexible solution to media management
than merely assigning drive letters in the physical media order. Jumbling
drive letters is almost as bad as not booting.
Apple Computer has solved this problem years ago by not using drive letters
and instead naming the disk volumes. It is time for the other OS vendors to
borrow this enlightened idea. I've expressed this thought to several
Microsoft people and have always been pushed back with statements like,
'Drive letters are really engrained -- maybe in the product after the one
we've working on now'. Unfortunately, this is one of the reasons PCs remain
more difficult to use than Macs.
As an interim solution, knowledgeable users can steer the SCAM ID assignment
by setting the jumpers on their devices to the preferred IDs -- a concept
foreign to both SCAM and PnP SCSI. That is, make sure the boot drive has a
default ID of 0 (or 6, depending on the host adapter) and make sure no other
devices share this default ID.
John Lohmeyer E-Mail: john.lohmeyer at symbios.com
Symbios Logic Inc. Voice: 719-573-3362
1635 Aeroplaza Dr. Fax: 719-573-3037
Colo Spgs, CO 80916 SCSI BBS: 719-574-0424 300--14400 baud
More information about the T10