Introduction
============

There are several notes discussing coexistence of DOS, NT and OS/2 on
one system.  I did it on a system with two SCSI disk drives.

There must be many ways to accomplish this, but there are some that I
tried which can lead to disastrous results.  This happened apparently
because MSDOS 6.0 and OS/2 appear to treat partitioning on a second
physical hard disk differently vis-a-vis logical DRIVE designations.
I'll describe this situation.

Some claim that a successful installation can be made in which the OS/2
boot manager can select DOS or OS/2.  If you then select DOS, then the
NT boot manager will allow you to select NT or DOS.  I tried to follow
this scenario, but there appears to be a bug in the OS/2 Boot Manager
that prevents NT from starting.  The bug may only be if NT is installed
on a separate NTFS partition.  I've isolated this problem and propose a
work-around.

The work-around involves a C program that can save and restore partion
records.  There are 4 partion records stored in the master boot record
on each physical disk.  This 512 byte sector can be saved, modified and
restored by a simple program.  Even if the NT installation puts zeros in
the OS/2 Boot Manager partition record (it did), it can be restored.
Even if the the OS/2 Boot Manager modifies a byte in the NT partion
record (it does), it can be restored.  The technique is sketched at the
end of this note.

The usual recommendation is to install NT then OS/2.  I reversed that
order, and discovered that NT deleted the OS/2 Boot Manager partition
data in the master boot record.  I restored it with the partition record
program.

In all cases OS/2 is version 2.1 on CD-ROM and NT is the March Beta.

Machine Base
============

My machine is a 486/33 with May '91 AMI Bios, 16M RAM and a 632M primary
and a 495M second SCSI disk.

The first 2 partitions of the first disk are Primary Dos and Extended
DOS respectively.

The 3rd partition is "NT ONLY" and is formatted as NTFS.

The 4th partition was 1M reserved for the OS/2 Boot Manager at the end
of the phyiscal drive.

Initial Trials
==============

FDISK and FORMAT from DOS 6.0 was used to format the second disk
followed by several attempts to load OS/2 on the second drive.  Some
failed and others produced side effects I didn't like.

If the second disk is partitioned with a primary DOS partition, then
that becomes logical drive D:.  The logical drive assignments on the
first disk move up a letter.  D: becomes E:, E: becomes F:.  I couldn't
live with this reassignment so I tried other solutions.

One try took advantage of an MSDOS 6.0 "feature" which allows you to
create an extended DOS partition without having a primary DOS partition
on the same drive.  I thought that this might solve the drive letter
re-designation by eliminating the primary drive on disk 2 and it did --
sort of!

After reinstalling OS/2 and the OS/2 Boot Manager things seemed OK, so I
tried NT. It did not boot, and reported the following:

  HardError c000021a {fatal System Error}
  Exception Processing Message c000021a Parameters e10c2088 0 0 0

Next I moved the data from one logical drive (E:) on the first disk to
one on the second disk.  Then I installed OS/2 on E:.  NT didn't boot.
I reinstalled NT, but the result was the same -- NT wouldn't boot.

Some detective work showed the System Indicator byte in the partition
record for NT for the NT partition had been modified (to 0x17) by the
OS/2 Boot Manager.  Restoring it to the original value (0x07) and making
DOS the active partition allowed NT to boot successfully.

I have not seen this result reported before.  IBM's tech support did not
recognize the problem either.

Disaster Installation
=====================

NOTE: In the discussion that follows the "normal" behavior of the DOS
and OS/2 operating systems as concerns LOGICAL DRIVE TO PHYSICAL DRIVE
MAPPING is as follows:

Logical C:  Primary Dos partition on first physical drive (0x80).
Logical D:  Primary Dos partition on SECOND physical drive (0x81).
Logical E:, F:, etc.  are assigned in order first exhausting the logical
drives in the extended DOS partition on the first physical drive (0x80)
followed by the logical drives in the extended DOS partition on the
second physical drive (0x81).

UNLESS the Primary DOS Partition on drive 0x81 HAS BEEN DELETED!  In
that case all logical drives on 0x80 are assigned from in order starting
at C: then all logical drives in the extended DOS partition on 0x81 are
assigned.  The 'HOPSCOTCHING" back and forth from drive to drive in the
"NORMAL" case is avoided.

NT was happy in its own partition, why not put OS/2 in its own
partition?  This would free E: so it could be used as original and the
use of the separate partition might solve the changing of NT's partition
System Indicator byte.  I could put an HPFS in an unused logical drive
in the extended DOS partition on the second drive.

The OS/2 FDISK reported the free space in unassigned partitions.  I
designated the size and exited FDISK.  The install process now correctly
indicated the partition was not formatted, I agreed, and FORMAT
completed.  Next it automatically tried to reboot, but with this
message:

  "Selected Partition is not formatted. Hit any key"

I booted DOS and discovered drive D: was not found.  The DOS command:

  UNFORMAT /PARTN /L

Showed:

  Disk 1	632
	C:	128
	E:	128	!!!
	E:	128
  Disk 2	495
	D:	120	Not accessible by DOS (HPFS?)
	F:	128
	G:	128

All data on the original D: was lost.  I believe that MSDOS 6.0, OS/2
FDISK, OS/2 Boot Manager do not collectively agree on the logical drive
designations for a 2 disk installation with no primary DOS partition
designated on the 2nd disk.  My use of a new feature of MSDOS 6.0
probably contributed to this problem.

Good Result
===========

So I started over nearly from scratch (after restoring many 100s
megabytes from tape)...

I replaced MSDOS 6.0 with IBM PCDOS 5.0.  Then deleted all logical
drives and partitions except the PRIMARY DOS (with C: in it.)

The new configuration:

  Disk 1
    :0	128M DOS Primary
	C:	     (IBM PCDOS 5.0)
    :1	336M Extended DOS
	D: (128)
	E: (128)
	F: (80)      (OS/2 2.1)
    :2	167M Non-DOS (NT on NTFS)
    :3	1M   Non-DOS (Boot Manager)

  Disk 2
    :0	1M   DOS Primary
	(Set System Indicator byte to 0)
    :1	494M Extended DOS
	G: (128)
	H: (128)
	I: (128)
	J: (110)

I changed the System Indicator to 0 on the primary partition of disk 2
so it would be not be recognized.  Then I followed this sequence:

  1.  Format drives D:-J:.

  2.  Made sure there were no remnants of the NT or MSDOS 6.0
      installation on C: (both put hidden files in C:\.)

  3.  Install OS/2 (After slowing machine down by turning off all cache
      and shadow ROM switches -- see the OS/2 installation notes for
      non-IBM hardware).  Installation went smooth this time albeit
      slowly.  Play with OS/2. It seems stable.

  4.  Capture the OS/2 partition record (see below).

  5.  Install NT.  The NT installation destroyed the OS/2 Boot Manager
      information in the partition record.  Shame on NT.

  6.  Capture the NT partition record.


OS/2 Boot Manager changes the partition record
==============================================

Create a new partition record that combines the OS/2 and NT data.  I did
this by creating a copy of the NT partition record and hex edited the
OS/2 boot manager partition data to it.

With this configuration the OS/2 Boot Manager is now active, but still
modifies the NT partition record, so I keep two boot partition records
and load the appropriate one from DOS when I need to change from NT to
OS/2.


Impressions
===========

MSDOS 6.00 and the OS/2 installation process are incompatible in the
area of partition management for a second disk (perhaps only if you have
not specified a primary partition on that disk.) This is what I believe
led to loss of data in my cited in the discussion above.  I believe the
OS/2 and MSDOS 6.0 behave differently with respect to LOGICAL DRIVE
LETTER ASSIGNMENT when there is no PRIMARY DOS partition on a second
disk (Physical 0x81.)

Certainly, OS/2 FDISK and FORMAT and the same named programs in DOS 6.0
view the MSDOS 6.0 partition record on disk 2 (0x81) differently from
each other.  Otherwise, I would not have had the confused drive letter
assignment which led having lost data.  I solved the incompatibility by
returning to an earlier DOS version which in turn forced a change of
procedure.  I'll let the reader complete other squares on this test
matrix.

The NT installation zeros SYSTEM INDICATOR of the OS/2 Boot Manager part
of the partition record.  This can, of course, be reversed (e.g.  by
saving the partition record before installing NT and restoring it when
you wish.)

The OS/2 Boot Manager changes a byte in the NT boot partition record if
NT is installed in its own NTFS partition on disk 1.  As a result, the
OS/2 Boot Manager can not be active when I want to boot NT.

I solved the partition record problems by the use of a special program
that can save and load partition records.  To boot NT I load a partition
record that makes DOS the active partition and changes the System
Indicator byte on the NT partition to the correct value.  To boot OS/2,
you can just make the OS/2 Boot Manager partition active.  The details
of this program are discussed below.

Appendix: Partition record management
=====================================

The partition information is saved in sector 1 of track 0 and has the
following structure:

  typedef unsigned char Byte;
  typedef unsigned long Ulong;
  typedef struct
  {
    Byte boot_ind;	// boot Indicator.  0 or 0x80 if active
    Byte beg_cchr[3];	// Beginning of partition Track sector & head
    Byte system_ind;	// 0x06 is BIG DOS primary
			// 0x05 is Extended DOS
			// 0x07 is NT - NTFS
			// 0x0a is OS/2 Boot manager
    Byte end_cchr[3];	// Ending of partition Track, sector and head
    Ulong rel_sector;	// Starting sector number
    Ulong num_sector;	// Length of partition in sectors
  } PartionRecord;

  typedef struct
  {
    Byte bootStuff[ 0x1be ];
    PartionRecord par[ 4 ];
    Byte endStuff[2];
  } BootRec;

To read a boot partition from disk, you can use the INT 0x13 function 2
call.  To write use INT 0x13 function 3.

  INT 13 parameters:
    AH = 02h - read, 03h to write
    AL = number of sectors to read
    CH = track (plus bits 8,9 of CL)
    CL = sector (01 to number of sectors per track for the drive)
    DH = head
    DL = drive
    ES:BX = buffer pointer

We don't have to worry about translating some large track number into
appropriate bits of CL but can do something like the following:

  union REGS in;
  struct SREGS seg;

  #define AX in.x.ax
  #define BX in.x.bx
  #define ES seg.es
  #define CX in.x.cx
  #define DX in.x.dx

  char buf[512];
  char far * p = ( char far * )buf;

  AX = 0x0201;		// read 1 sector
  BX = FP_OFF( p );	// into buf
  ES = FP_SEG( p );
  CX = 0x0001;		// from track 0, sector 1
  DX = 0x0080;		// in drive 80 (81 is the second)
  int86x( 0x13, &in, &in, &seg );

Now by casting the buffer to a boot record:

  BootRec *rec = (BootRec *)buf;
  PartitionRecord *par = rec->par;

We can read or change data in the buffer.

  for (i = 0; i < 4; i++)
    printf( " 0:%d %02x %02x\n", i, par[i].boot_ind, par[i].system_ind );

Here is the results of the above printf() statement with an active OS/2
boot manager in the 4th partition and the NT System Indicator set to 07.

  0:0 00 06
  0:1 00 05
  0:2 00 07
  0:3 80 0a

Reboot and the OS/2 Boot Manager comes up.  Select DOS and run the
program again.

  0:0 00 06
  0:1 00 05
  0:2 00 17	 !! Boot manager changed NT's System Indicator
  0:3 80 0a

Just 1 little bit has changed.	But NT will not boot.  Change record
again to allow NT to boot:

  0:0 80 06	 Make 0 active
  0:1 00 05
  0:2 00 07	 Good NT System Indicator
  0:3 00 0a

Now NT boots (and by the way does not change the OS/2 Boot Manager
record).

Given that we can read the partition data from the boot record, it is
easy to change the data and write the modified boot record back to the
disk.

Thanks to Steve Behman for suggestions, code and encouragement.

Dave Hillman