SCAM Default ID Control
jlohmeye at ncr-mpd.FtCollinsCO.NCR.COM
Thu Mar 24 16:55:50 PST 1994
Last week at the Plug and Play SCSI meeting, Larry Lamers brought up a
deviant behavior (he is good at that) that a SCAM device could have
which would cause difficulties for early users of SCAM devices. The
problem is that today's operating systems and today's PC BIOS routines
do not handle non-fixed resources very well. Until the software
learns to cope with devices moving around (or disappearing and
reappearing at other addresses), users of larger PC systems will
continue to have to control SCSI address assignments.
In particular, if a user adds a second hard disk to his PC system,
he/she may wish to control which one gets assigned a higher or lower
SCSI ID. This will affect which one becomes the boot device and
which drive letters get assigned to which device.
Most of the group assumed that such users could use the default ID
setting mechanism (switches or jumpers) to steer the SCAM assignment
of SCSI IDs. Larry pointed out that an implementation of SCAM could
ignore the default ID if it detected SCAM protocol and just report the
preferred PnP SCSI default ID for the device type. In this case, such
disk drives would always report a default ID of 6 no matter what the
The people present want to make sure that such behavior is not
permitted. It would prevent users from overcoming the BIOS and OS
limitations in today's machines. We are proposing that this point be
clarified in SPI as SCAM is incorporated. Particularly, the default
ID reported in the type bytes in a SCAM system must be the same as the
default ID that would be used in a non-SCAM system.
A somewhat related topic discussed at the meeting is whether it makes
sense to define a SCAM Mode Page. Such a page could be useful for
software that wants to get a device's SCAM information without going
through SCAM protocol (could be difficult due to software layering).
Such a mode page could also be useful for changing default IDs without
using jumpers or switches.
John Lohmeyer E-Mail: John.Lohmeyer at FtCollinsCO.NCR.COM
NCR Microelectronics Voice: 719-573-3362
1635 Aeroplaza Dr. Fax: 719-597-8225
Colo Spgs, CO 80916 SCSI BBS: 719-574-0424 300--14400 baud
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