SCAM and boot devices

Lohmeyer, John JLOHMEYE at
Mon Feb 12 10:25:00 PST 1996

* From the SCSI Reflector, posted by:
* "Lohmeyer, John" <JLOHMEYE at>

HCurley at 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, 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.

[soapbox on]
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.
[soapbox off]

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 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

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