Message-ID:	<03e601c08deb$f647b760$b608e289@mpaul>
From:	"Matthias Paul" <Matthias DOT Paul AT post DOT rwth-aachen DOT de>
To:	<opendos AT delorie DOT com>
Cc:	<fd-dev AT topica DOT com>
Subject:	How to utilize up to 8 Gb disk space for DR-DOS in multi-boot scenarios with MS-DOS 7.1
Date:	Sat, 3 Feb 2001 15:15:23 +0100
Organization:	Rechenzentrum RWTH Aachen
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< (Although *this* article is not directly related to FreeDOS, all
< users of non-LBA DOSes have similar problems than described
< below when they have harddisks greater than 8 Gb. It is nothing
< but fortune that DR-DOS inheritely offers a solution by utilizing
< its C5h partition type. This is why I am CCing this into the
< FreeDOS mailing.)

Hi all,

Given the ongoing discussion about OEM DR-DOS 7.05, hereīs a tip
how to maximize the disk storage space available for DR-DOS 7.03
(and earlier) to up to 8 Gb per harddisk even in multi-boot scenarios
with MS-DOS 7.10 without loosing the space above 8 Gb.

One annoying issue with DR-DOS 7.03 on modern systems is, that if you
have a harddisk larger than 8 Gb and donīt use special multi-boot loaders,
you will most probably be forced to use the new type 0Fh extended partition
(LBA) instead of the traditional type 05h extended partition (CHS).
This is because of design limitations in MS-DOS 7.10+, which will always
access logical drives in a type 05h extended partition via CHS, even if the
individual logical drives in there are of LBA type, or go beyond 8 Gb...
(Although this workaround is sometimes used in conjunction with OS/2,
using a 05h partition going beyond 8 Gb may cause MS-DOS 7.10 to hang
or corrupt your data...)

The problem is, that DR-DOS 7.03 does not recognize the 0Fh partition
type (DR-DOS 7.04/7.05 does, but again, itīs an OEM issue), so you can
either use only 2 Gb (instead of up to 8 Gb) on each hard disk for DR-DOS
(and only as a primary partition), or, if you would use a 05h partition, you
could use up to 8 Gb under DR-DOS, but would also be limited to these
8 Gb under MS-DOS 7.10 and thereby loosing all the harddisk storage space
above 8 Gb. (There may be exceptions if you would use multiple primary
partitions, but this would require a multi-boot facility beyond the functionality
offered by DR-DOS LOADER.)

However, there is one possible solution, when you create *two* extended
partitions (in this order):

1. The first extended partition should occupy everything up to 8 Gb (which
is not used for primary partitions) and it must be of the C5h type ("DR DOS
secured"). You can use DR-DOS FDISK to create a 05h extended partition
and all the logical drives in there, and afterwards change the extended partition
type to C5h (for example with DEBUG or a disk editor). Of course, such
partitions will not actually be secured, but using C5h instead of 05h helps to
hide the partition from MS-DOS, so you can create another extended
partition:

2. The remainder of the drive (above 8 Gb) can be allocated in a type 0Fh
partition (using MS-DOS 7.10+ FDISK).

DR-DOS will then only see the logical drives in the C5h partition, and
MS-DOS 7.10 will only see the logical drives in the 0Fh partition.
(MS-DOS/PC DOS up to 7.0 and other current non-LBA DOS implementations
will see neither of them.) Each OS will recognize the other extended
partition as non-DOS primary partition, so there is *no* actual conflict
by MS specs, and disk utilities will *not* get confused by this.
Another advantage is that the DR-DOS logical drives wonīt get polluted
by long filenames and other incompatible stuff. There is one restriction:
Due to the fact, that DR-DOS normally uses the C5h type for secured
partitions, it will not log-in any logical drives in there when booting
from floppy. So always keep a set of emergency utilities on floppy or
the C: primary partition, which (in this example) is visible under both OS.

This is an ad-hoc solution that should work with DR DOS 6.0-7.03.
I donīt recommend to use it under 7.04/7.05, because this OS recognizes
both, C5h and 0Fh extended partitions and would only log-in the
logical drives for the physically *first* extended partition entry in the
partition table of the Master Boot Record. (Of course, this can also be
a feature.)

Alternatively, you can combine type 05h and 0Fh extended partitions,
but then you must ensure, that the type 05h partition is created after
the 0Fh partition, and that the *entry* for the 0Fh partition is physically
located before the 05h entry in the partition table. Since MS-DOS
FDISK makes it difficult to control the physical location of a partition
and some disk utilities may change the order of partition *entries* in the
partition table, this can led to all kind of trouble and serious data loss
in some scenarios. Therefore, I donīt recommend it. But if you know
exactly what youīre doing, it may have some advantages compared
to the use of the C5h type (for example, this helps if you want to
access these logical drives under MS-DOS/PC DOS up to 7.0
as well).

Of course, the best solution is to introduce new partition types,
but obviously this will work only for future DOS releases.
I am currently writing a proposal for new partition types to be
introduced with FreeDOS, which would allow almost the same
under FreeDOS (and all other future DOS releases) at virtually
no memory impact for the kernel and disk tools (FDISK)
(maybe 20-30 bytes).

This model works great for me and I would like to
learn about *your* thoughts and experiences with it.

Thanks,

 Matthias

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Matthias Paul, Ubierstrasse 28, D-50321 Bruehl, Germany
<Matthias DOT Paul AT post DOT rwth-aachen DOT de> <mpaul AT drdos DOT org>
http://www.rhrz.uni-bonn.de/~uzs180/mpdokeng.html
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