SCAM and boot devices

Steve Gaskill sgaskill at QNTM.COM
Mon Feb 12 18:34:06 PST 1996

* From the SCSI Reflector, posted by:
* sgaskill at (Steve Gaskill)
HCurley at 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 
system vendors.

I would be interested in hearing comments on this subject from host adapter 

  Steve Gaskill

  Quantum Corporation
  sgaskill at

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