booting DOS on ISC+SCO, and DOS filesys handling (long-ish)

[colin manning]

Until recently, I have been using a SCO Unix 3.2.0 system on which I also 
use Windows 3 for which I have a DOS partition. When the system is reset, 
I am presented with a menu that tells me to use F3 to boot DOS, F5 
to boot SCO Unix, and F7 to boot from floppy. This is fine. I have not
wished to move this system to DOS 4, because I'm tight on memory under
DOS, and so the limitations in SCO Unix for handling larger DOS partitions
has not been a problem. This will be a different matter once DOS 5 is here.

I now have an ISC Unix 2.2.1 system as well, and need to have a similar 
functionality with this system. Although it would seem possible to have 
DOS and Unix to reside on the drive in different partitions, after reading 
the manuals it would appear that switching from one OS to the other 
necessitates using fdisk every time to change the active partition. This is
a bit long winded, and besides, fdisk is a somewhat dangerous program that
I dont want to have to use regularly.

This new system has a 1.2Gb drive, and I dont want to stick to the 32Mb 
partitions that you are restricted to with DOS 3.3; neither do I really 
want to use the 'extended partitions' of DOS 3.3 which allow you to 
sub-divide an fdisk partition into several 'secondary' DOS partitions which 
still have a 32Mb upper limit, because I will be switching to DOS 5 as soon 
as its out so I can have >32Mb DOS 4 style partitions without the extra OS 
memory overhead.  Naturally I also wish to be able to mount DOS partitions 
under both SCO and ISC, although neither SCO nor ISC supports the >32Mb DOS 
filesystems at present.

Ideally, there would not be all these seemingly unnecessary restrictions
concerning support for different partition types, sizes etc. In my opinion,
whoever designed the fdisk partition table structure left a lot to be
desired - why only allow at most 4 partitions on a drive ? and why restrict
the cylinder numbers to 10 bits ? It seems to me that to have allowed a 
reasonable number of partitions (say 32) and to use 32 bit disk addressing
would not have been too hard. The cost would have been, say, an additional
256 bytes or so to save a lot of hassle.

Also, why is it that support for DOS 4 partitions is non-existant in ISC and
SCO ? DOS 4 seems to have been around for quite long enough for support to
have been added.

Anyway, I'd be interested in information on any of the following. I'll 
post a summary in due course if there's sufficient interest.

1. Is it possible to have an easy way of selecting between booting DOS
   or Unix on an ISC system, without having to remember to use fdisk 
   every time to change the active partition ? Ideally the source to 
   a boot program that could be put on a primary DOS partition would
   be nice (like the one I use with SCO).

2. Although ISC and SCO do not seem to have any support for >32Mb DOS 4+
   filesystems:
   i.  When is support for these expected from ISC and SCO ?
   ii. When you use SCO, and you interrupt the auto-boot to boot DOS (by
       typing 'dos' at the : prompt), will it boot correctly off a >32Mb
       DOS 4 partition ?
   iii. If you try to have >32Mb DOS 4 partitions on a SCO or ISC system,
       does the Unix safely ignore them even though they cant use them ?

3. Is it possible to have both SCO and ISC Unix on the same machine, in
   different partitions, and have some way of selecting between the 2
   at boot time ?

4. Does ISC recognise SCO partitions and vice-versa ?

Many thanks in advance,

-- 
- Colin Manning, cm%jet.uucp at ukc.ac.uk (world) OR cm at jet.uucp (UK only)
- Disclaimer: Please note that the above is a personal view and should not 
  be construed as an official comment from the JET project.