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From: wheels@mks.UUCP (Gerry Wheeler)
Newsgroups: comp.os.minix
Subject: Re: bad blocks on hard disk
Message-ID: <297@mks.UUCP>
Date: Fri, 4-Sep-87 09:38:42 EDT
Article-I.D.: mks.297
Posted: Fri Sep  4 09:38:42 1987
Date-Received: Sat, 5-Sep-87 22:41:03 EDT
References: <196@turbo.RAY.COM> <489@ast.cs.vu.nl>
Organization: Mortice Kern Systems, Waterloo, Ont.
Lines: 31
Summary: MS-DOS, as I recall it...

In article <489@ast.cs.vu.nl>, ast@cs.vu.nl (Andy Tanenbaum) writes:
> I believe that the DOS approach to bad tracks is to keep the last couple
> of tracks in reserve, and to have a map that tells which of the regular
> tracks are bad.  It then subsitutes the reserve tracks (or maybe just
> sectors, I am not sure) for the bad ones.
> 
> Andy Tanenbaum (ast@cs.vu.nl)

I am NOT a DOS expert, but I do seem to recall reading this somewhere.
I think DOS simply writes a magic number in the FAT entry corresponding
to the bad sector so it will never be used. I don't think any replacement
sectors are allocated anywhere. This would explain how chkdsk can report
the number of bad sectors.

Note, however, that sometimes one can map out bad sectors or tracks at
a lower level. Some disk controllers allow bad sectors to be remapped
at the hardware level during the low-level format process. Once this is
done, the disk appears to be perfect to the software, albeit a little
smaller. The Adaptec 4000 series controllers, for example, can do this.
Nevertheless, a scheme for mapping out bad blocks at the OS level is nice
for those occasions when the disk develops new trouble spots. Saves going
through the low-level format again.

Why not simply have a utility which will append blocks to a file called
BAD_BLOCKS? Just don't try reading the file. I guess all the permissions
could be removed from it.
-- 
     ll  // // ,'/~~\' Gerry Wheeler {decvax,ihnp4,seismo}!watmath!mks!wheels
    /ll/// //l' `\\\   Mortice Kern Systems Inc.         (519) 884-2251
   / l //_// ll\___/   43 Bridgeport Rd. E., Waterloo, ON, Can. N2J 2J4
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