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From: zink@panix.uucp (David Zink)
Subject: Re: Dos and unix on same Disk
Sender: zink@panix.uucp (David Zink)
Message-ID: <1991Mar29.055801.15505@panix.uucp>
Date: Fri, 29 Mar 91 05:58:01 GMT
Organization: PANIX - Public Access Unix Systems of NY

People keep mentioning the problem with DOS 4 and SCO etc., but so
far nobody has mentioned the base problem here.

I saw the problem solved once, if you can't figure it out from my vague
descriptions mail me (david@jyacc.com) and I will hunt up the guy who actually
does this stuff.

The problem is that SCO only works with DOS 3.3. Or tries to.  You can spoof
it, however.  Apparently 3.3 added (or didn't add) a boot sector or some
such before each DOS partition, and 4.0 does the reverse.  Therefor SCO looks
a track or so off when trying to find the dos info, and problems arrive.

However, DOS stores partition information by head and track, while SCO stores
it by absolute sector number.  All you have to do is jiggle the numbers by
the right values and everything finds the DOS partition where they expect it.
[ Note: this means the partition table has conflicting information about
partition info. ]

There was also a problem with granularity; I believe it was that DOS expects
its partitions to be a whole number of tracks and will format to the end of
the last track containing the partition.  Or some such.

Anyway I remember we always had to format the dos partition before installing
Xenix (way back when).  Dos would otherwise overwrite the start of the
Xenix partition. Probably really a granularity problem.

Also, since SCO expects 3.3, you can't access big dos partitions.  And you 
can't see more than one from SCO.  So we end up formatting the disk:
32M dos partition; 150M BigDos partition; UNIX partitions

We have a system in the office formatted that way last summer that is still
humming happily away.

-- David Zink  david@jyacc.com (Work) zink@panix.uucp (Play)
Actually this post is so scatter-brained that I'll probably look up the info
and post it no matter what.  I only post this because a lot of people who
could probably figure it all out need the pointers, and I won't be near
the office for a while.