Future Domain SCSI controller for A

[neese at cpe.UUCP]

>The problem is not Xenix, but the way IBM designed the AT.  What happens
>is that most controllers use the same interrupt.  The AT cannot support
>multiple devices with the same interrupt.  Also, they both might be using
>the same address space.  Either or both of these situations will cause 
>problems.  Perhaps some of the hardware types will be able to shed some
>more light on the matter.

Actually, the real problem is:
1)  In order for SCO xenix to support DOS partitions they had to stick with
the DOS kinda things.  AT BIOS's only support 2 hard drives,...so they had
to live with that restriction.
2)  The BIOS is used by Xenix to do the boot and load the kernel.  This implies
that you must have a hard drive scheme that is compatible with the BIOS
architecture.
3)  MS-DOS only understands 2 hard drives, so for compatibility sakes, SCO
was forced to use the same scheme so they could support DOS and Xenix on
the same drives.  MS-DOS has to be able to understand the partition table
that Xenix creates or they couldn't co-exist on the same drive.

  Now some of this stuff can be worked around.  In SCO Xenix 2.2.4, the OS
supports multiple HD controllers.  (SCSI and ST-506).  A newer version of
2.3 called 2.3GT will be able to support ST-506, ESDI, and SCSI.  This is
all done without breaking any compatibility but it did require the engineers
at SCO to do some very creative things to the OS.
  So if you want to point fingers, IBM did the first stupid thing by only
allowing the AT BIOS to support only 2 hard drives.  Of course, Microsoft
followed suit and created a DOS that could only deal with 2 physical hard
drives, and then Western Digital added fuel to the fire by creating a line
of controllers that only supported 2 hard drives.  This is how we got here
today.

					Roy Neese
					Tandy Computer Product Engineering
					UUCP@ killer!{ninja, merch}!cpe!neese