SCAM Default ID Control

John Lohmeyer 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 
jumpers/switch settings.

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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