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Number   :  7925
Date     :  2004-05-21
Author   :  Kan Yabumoto
Subject  :  Re: Using /CLONE to restore the bootdrive in FAT16 ?
Size(KB) :  4



Bob wrote:

>Garry wrote:
>
>
> > However this isn't a problem as far as your drive assignments
> > go as you can re-assign the drive letters in windows
> > to be whatever you want.
>
>Kan responded
>
>This is not true in any of Win9X/ME systems. The ability
>to assign a drive letter to a disk by the user is only
>with the NT-family Windows (which is done through entries
>in the system registry). It is my understanding that
>Win9X/ME simply takes over the file system from the
>underlying DOS (which initiates the disk I/O operation
>before it enters the Windows environment) which in turn
>relies its drive-letter assignment on what the BIOS detects
>at the beginning.
>
>

> In W9x/ME it is possible to fix drive letters using
> Device Manager - Disk Settings by pre-setting the start
> and end drive letters to be the same. I set W for writer
> and V for DVD using this technique (with FAT32) and they
> are accessible from the DOS box prompt. It does not appear
> to affect the BIOS start up sequence with four IDE devices
> and (in ignorance) I have assumed that it was some form
> of Windows "re-allocation" at a later stage during boot.
>
> Bob

I wrote in my original message about exceptions in the
drive-letter assignment issue.

----------------------------------------------------------
The only exception to this rule is via the installable
file system (e.g., MSCDEX.EXE, DATMAN.EXE, etc. which
usually mimic the network drive creation like NET.EXE).
There are other techniques that allows a program to
inject a new drive letter by emulating a hard disk
in which cases, the driveletter-assignment must follow
the usual BIOS rule. There might be other non-Microsoft
techniques to manipulate driveletters which is outside
the scope of my discussion here.
----------------------------------------------------------

The CD and DVD drive letters are created by an installable
file system either MSCDEX.EXE or a software module that
is equivalent to MSCDEX. It can assign a drive letter
that is not assigned to something else. But the standard
hard disk (MFM, IDE, floppy) is handled directly by BIOS
and typically the user has no control on the drive letter.
The same thing is also true when you create a "RamDisk"
(boy, it was only a few years ago that we are all so familiar
with the technique which is mostly forgotten). Most RamDisk
is implemented by a low-level device driver which mimics
the presence of sector-based storage (hard/floppy) disk.
the built-in standard file system (part of IO.SYS ---- the
"DOS Kernel") assign such a device on a first-come-first-
served basis. This is why most RAMDISK's driveletter
assignment does not have as much user-choice as the
drive letter of CD or DVD (MSCDEX allows you to select
any free drive letter).

As to the start and end drive letters, DOS old timers
may remember the "LASTDRIVE=Z" setting in the CONFIG.SYS
file. This is one of the efforts in DOS where you try
to squeeze every byte on your system. DOS allocates a
table of drives in its internal memory. I used to know
all this by heart but my memory is getting very fuzzy.
I don't think you can define what's the first drive letter.
The LASTDRIVE= parameter simply sets the last drive letter
that is accepted as legal. Without the LASTDRIVE= statement,
DOS will set E: as the default value. Once you commit your
system (at the boot up time via the CONFIG.SYS statement),
you will not be able to run Ramdisk to add more drive letter.
So, by declaring LASTDRIVE=Z, you can reserve the memory
for the extra drive letter. We were talking about something
like 32 bytes per extra drive letter. It amounts to less
than 1KB even if you set it to LASTDRIVE=Z but in those
days with lots of TSRs, memory in the 640KB area was at
premium.

Anyway, in Win9X, the freedom of choosing the drive letter
is limited to network volume mapping and devices that
are supported via installable file system such as CD and
DVD. The hard disks and floppy disks are not in that
category and that is why you could not find similar
drive letter setting option in the property sheet for the
hard disk.

Kan Yabumoto



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