Write precomp for Compaq/Miniscribe ESDI Xenix?

[Chip Salzenberg]

[This is a weird one.  Put on your wizard hats, everyone.]

We at TCT have had intermittent hard disk errors on the machine called
"tct".  Tct is a Compaq Deskpro 386/20 with a 300M ESDI drive, running
SCO Xenix/386 2.3.1.

The hard drive is a Miniscribe 9380E with a Compaq faceplace.

The controller is a Compaq-created oddball.  It presents the same
interface as a WD1003 (standard AT controller), so we use normal
Xenix.  However, the controller does one bit of trickery: it pretends
that the drive has half the cylinders and twice the heads.

(The controller's deception makes the entire drive accessible through
the standard BIOS interface, which can only handle 1024 cylinders.
Thus there is no problem with booting off a kernel that's way out at
the end of the disk.)

I became suspicious that write precompensation may be involved in the
disk errors when I realized that the errors were all coming from the
last 25% of the disk.  Furthermore, when I checked the Xenix
configuration, I found that the write precompensation had been set to
track 65535 -- i.e. it had been turned off.

Also, some of the errors just evaporate when I turn the computer off
and on again -- but some errors persist.  Sigh.

So here, at last, is the question: Does the Miniscrive 9380E drive,
when used with the Compaq ESDI controller, require write
precompensation?  And if so, should I program it with the "real"
precomp track, or with the track divided by two to match the pretend
geometry that the Compaq controller simulates?

I'm really confused, and our drive is acting weird.  Any help,
especially from people with Compaqs, Xenix, and/or 300M ESDI drives
would be greatly appreciated.  (I do mean GREATLY.)

By the way, for perspiring minds that want to know: You can discover
and/or change your Xenix low-level disk shape and write precomp by
logging in as root and typing "dkinit 0" (for the first drive) or
"dkinit 1" (for the second drive).  Beware--here there be dragons...
-- 
Chip Salzenberg at ComDev/TCT   <chip%tct at ateng.com>, <uunet!ateng!tct!chip>