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From: markz@microsoft.UUCP (Mark Zbikowski)
Newsgroups: net.micro.pc
Subject: re: unerase
Message-ID: <8700@microsoft.UUCP>
Date: Wed, 18-Jul-84 16:37:34 EDT
Article-I.D.: microsoft.8700
Posted: Wed Jul 18 16:37:34 1984
Date-Received: Fri, 20-Jul-84 03:48:34 EDT
Organization: Microsoft Corporation
Lines: 25


	"...Is there a public-domain program that will "unerase" an accidentally
	deleted DOS file?..."

It purely by accident that you can recover erased files.  When DOS erases a
file, it zeros the relevant clusters in the FAT and then marks the directory
entry as being free.  If (as some non-public domain programs attempt) you
go and un-mark the directory entry, you *may* recover the first allocation
unit only.  Here are the list of difficult problems such a program would
have to solve:

o   Reuse of the directory entry.  Consider using wordstar and deleting a
	file.  Wordstar may have done another create which *could* have reused
	the directory entry.  Tough luck.

o   File is > 1 allocation unit.  On single-sided floppies, this is 1 sector
	(512 bytes). On the XT hard disk, this is 8 sectors (4K).  If you have
	done much work on the disk, the files will tend to be scattered.  Also,
	there is NO guarantee that MSDOS makes as to the contiguity of files.  Any
	reliable sort of recovery would need to scan the free allocation units
	and 'guess' at which ones belonged where in the recovered file.  Tough
	luck again.

o   Reuse of freed allocation units.  These now belong to another file.
	Tough luck finally.