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From: tla@kaiser.UUCP (T Anderson)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Reading Hard disk with different DOS versions
Message-ID: <648@kaiser.UUCP>
Date: Fri, 21-Aug-87 17:28:22 EDT
Article-I.D.: kaiser.648
Posted: Fri Aug 21 17:28:22 1987
Date-Received: Sun, 23-Aug-87 06:18:20 EDT
References: <680@cblpf.ATT.COM> <683@cblpf.ATT.COM>
Distribution: na
Organization: AT&T Bell Laboratories, Liberty Corner
Lines: 44


>> 	If a hard disk has been formatted with DOS version "X",
>> 	what determines whether or not you will be able to read
>> 	it when you boot DOS version "Y" from floppy.
>> 
>> 	We just loaded 3.2 on our system (PC6300, 10 Meg).  If
>> 	you boot version 2.11 or 3.1 from floppy, and try to
>> 	access the C: drive, you get an "Invalid drive specification".
>> 	I think that it may have to do with cluster size, but the strange
>> 	thing is that another machine (also running 3.2) CAN be
>> 	booted with a different version and read the hard drive just
>> 	fine.  Any insight would be appreciated.
>> 
>> 						Mike Vehonsky
>> 						ihnp4!cblpf!jpw

> To draw an analogy to the UNIX world, the DOS format really doesn't
> "format" but rather makes a file system. (a PC "low-level" format is
> equivalent to a UNIX "format").  The DOS "file-system" changed with
> version 3, and is downward compatible with DOS 2.xx but not upward
> compatible with DOS 2.xx.  Thus a machine booted with 3.2 can read
> 2.xx disks, but a 2.xx DOS can't read 3.2 formatted disks.  

> Note this is the 12-bit versus 16-bit FAT (file-allocation-table) change.

Actually as the original author observed it is not as bad as the
above.  The only problem is DOS x with x<3.0 (or in AT&Ts case
x<3.2)  ONLY 12 bit FAT was supported so if using a later DOS the
disk was formatted with 16 bit FATs then they can no longer be read
by the earlier DOSs.  But even the newer DOSs (by default) only uses 
the 16-bit FAT for disks larger than 10M.  So even with the newer
DOS 10M and smaller disks use 12 bit FATs and CAN be read by earlier
DOSs.  Disks larger than 10 M formated by earlier DOS use 12-bit
FATs and it is even possible with the more recent DOSs to force the
use of the 12-bit FATs, which can then be read by all.  Of course
the disadvantage (and one of the reasons for introducing the 16-bit
FAT) was that on a 20M disk with 12-bit FATs the cluster size is 8k.
 The 16-bit FAT reduces that to 2k.

			     --	Terry L Anderson
				AT&T Bell Laboratories -- Liberty Corners
				UUCP:     ...!ihnp4!kaiser!tla
				TeleMail: Terry.Anderson
				(201) 580-4428