Disk Manager 6.x and above is a piece of software that performs the
translation necessary to access harddisks of more than 1024 cylinders with
DOS/Windows. This is achieved by installing a Dynamic Drive Overlay (DDO)
to translate drive parameters. The driver provides only the basic
translation functions, without EDPT or extended int13 calls. Of course,
this is less of an issue in a software driver than in a system BIOS.
If Disk Manager (DM) is used to format only the slave drive, the DDO can be
installed in the config.sys as an ordinary device driver
(device=dmdrvr.bin). On the other hand, using DM on the master drive
isn't quite that easy from a technical point of view. Since you must boot
DOS from the master drive, and you must load the DDO before you can access
the drive, the only option is to load it very early during the boot
process, even before the operating system itself. Changes are made to the
Master Boot Record (MBR) to accomplish this 'pre-boot loading'.
This scheme works fine, but has a few drawbacks.
- First, DDO as a pre-boot loader has implications for floppy boots. If
you boot directly from a floppy, DDO does not have a chance to boot and the
partition does not make sense. This can be remedied by having a line in the
config.sys on the floppy which reads "
device=dmdrvr.bin". You can also
watch for the press spacebar to boot from floppy message when booting
from the hard drive.
- The second drawback is that operating system installations routinely
overwrite the MBR with their own boot code. The DDO is no longer loaded and
your partition will be inaccessible until you let Disk Manager write a new
MBR.
- Third, a nonstandard partitioning scheme is used whereby the data is
offset by one track. This is completely transparent as long as the drive is
accessed through the DDO; however, many operating systems want to replace
the DDO by their own disk routines and will have to be aware of this
scheme.
Windows 95 will support Disk Manager 6.x and above 'out of the box', as
will new versions of OS/2 Warp, Windows NT 3.5.1 and Linux 1.3.x. IBM and
Microsoft have created fixes that allow older versions of OS/2, Warp and
Windows NT to work with Disk Manager.
- Fourth, corruption of the DDO sector will result in a DDO Integrity
Error. While it is fairly easy to re-write the DDO sector (use the
dmcfig.exe utility on your DM diskette), this is is a sign of a bigger
problem (eg. virus) rather than a problem in itself---contact Ontrack tech
support (
tech@ontrack.com
)
for assistance.
An advantage of formatting the master drive with DM instead of loading the
DDO from config.sys is that you can use Windows for Workgroups' 32-bit file
access on both drives---if you use dmdrvr.bin, the slave drive is
restricted to 16-bit file access.
The 6.x versions of Disk Manager have some additional disadvantages which
are corrected in version 7:
- They are not fully compatible with the device drivers of most VLB
ATA(-2) interfaces; also, ATAPI CD-ROM and tape devices on the chain are
not supported.
- A final concern is disk utilities. If the utility in question goes
directly to the hardware, without going through the DD overlay, it can
potentially be destructive. Ontrack's policy on this is to refer
compatibility questions to the manufacturer of the utility as they cannot
possibly maintain compatibility charts for all versions of all utilities.
You can find more information on the various versions of Disk Manager on
OnTrack's www site
.
32-bit disk access (32BDA), also known as FastDisk, is a set of protected-mode
drivers that direct int13 calls to the hard disk controller through a protected
mode interface. For the latter the hard disk controller has to supply an
appropriate virtual device driver (VxD).
Windows ships with one such driver built in: *wdctrl. Unfortunately, this
device only supports controllers that are strictly compatible with the WD1003
standard; this excludes SCSI, ATA-2, LBA or CHS translation, disks with more
than 1024 cylinders and even some commonplace features of ATA such as block
mode. If it detects one of these during the initialization phase it will refuse
to load. In today's computers, this means that *wdctrl will rarely do the
job and an external VxD must be used.
32BDA has two advantages over disk access through the BIOS. First, since the
FastDisk VxD is re-entrant, it enables Windows to use virtual memory for DOS
sessions. Using virtual memory without 32BDA could create a deadlock situation
if a page fault is generated during the execution of BIOS routines. Since the
BIOS is not re-entrant, it is not possible to use a BIOS call to read the page
from disk until the first BIOS call has terminated; on the other hand, this BIOS
thread must remain suspended until the swapped out page has been read.
So 32BDA enables Windows to manage memory much more efficiently with one or more
DOS sessions open.
The second advantage of 32-bit disk access is that it saves two (relatively
slow) switches between virtual and protected mode per disk I/O call. Take, for
instance, a disk read performed by a DOS application. In the absence of 32BDA,
each such call causes the following sequence of events:
|
1 | Application | calls INT21 to read from disk | |
2 | Windows | traps the call, switches to protected mode |
|
3 | Windows | switches to real mode, returns to DOS |
|
4 | DOS | makes int13 call to BIOS disk routines |
|
5 | Windows | traps the call, switches to protected mode |
|
6 | Windows | switches to real mode, returns to BIOS |
|
7 | BIOS | acts upon int13 call and does the read |
|
8 | Windows | traps the return from int13, switches to PM |
|
9 | Windows | switches to RM, returns the result to DOS |
|
10 | DOS | receives the result, passes on to application |
|
11 | Windows | traps the return from DOS, switches to PM |
|
12 | Windows | switches to RM, returns result to application |
|
13 | Application | receives the result from the INT21 call |
|
|
Using 32-bit disk access replaces steps 6 to 8 by a single call to the FastDisk
VxD. This removes two mode switches, resulting in a usually small disk
performance improvement. (Steps 3-11 also apply to native Windows applications).
