SCAM and boot devices
sgaskill at QNTM.COM
Mon Feb 12 18:34:06 PST 1996
* From the SCSI Reflector, posted by:
* sgaskill at qntm.com (Steve Gaskill)
HCurley at aol.com writes:
> How does SCAM avoid the problems of the intended boot device
> being assigned to the wrong address, either on initial
> configuration or later when adding a new device?
It doesn't. SCAM is but a simple (ha ha) protocol to allow an initiator to
assign a SCSI ID to a target. Strictly speaking, it is outside the scope of
SCAM to address this.
There is a higher level document, the Plug and Play SCSI Specification (or maybe
it's just called the PnP Specification, I'm on a business trip in Japan right
now and don't have the actual document handy). I haven't looked at it in a
while, but I think it contains information that SCSI BIOS designers should
follow to avoid the kind of troublesome situation you described.
I would wager that most, if not all, of the first generation of SCAM initiators
just dole out IDs the brute force way--the SCAM target with the numerically
highest SCAM identification string gets the first available ID, and so on. This
method, as you know, will get you into trouble sooner or later.
An easy solution is for the initiator to keep track of the "known" SCAM devices
in non-volatile memory, so that it can intelligently (i.e. consistently) assign
SCSI IDs. It would be convenient for the user if the SCSI BIOS maintained a
list of ALL the SCSI devices and allowed the user to modify the "assigned ID" or
simply "boot device" status for SCAM devices.
There are certainly better long-term solutions than this, but many of those
require the collaborative efforts of the host adapter vendors and operating
I would be interested in hearing comments on this subject from host adapter
sgaskill at qntm.com
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