Path: utzoo!utgpu!attcan!uunet!lll-winken!lll-tis!ames!nrl-cmf!cmcl2!rutgers!mit-eddie!killer!chasm
From: chasm@killer.DALLAS.TX.US (Charles Marslett)
Newsgroups: comp.sys.ibm.pc
Subject: Re: screwed up directories on floppies
Summary: Never knew you were here . . .
Message-ID: <5084@killer.DALLAS.TX.US>
Date: 5 Aug 88 03:57:21 GMT
References: <1060@ndsuvax.UUCP>
Organization: The Unix(R) Connection, Dallas, Texas
Lines: 25

In article <1060@ndsuvax.UUCP>, ncperson@ndsuvax.UUCP (Missing Person) writes:
> Can anyone tell me why DOS will sometimes overwrite the directory of 
> obe floppy with that of another? I've had this happen several times in the 
> last week. 
>  it seems to simply be replacing the directory information with a copy of 
> what was on the previously inserted disk. I've heard of this happening before, but have never heard why it happens. Can somwone enlighten me?
> thaks .
> -- 
> Brett G. Person

The copying of the FAT and/or directory from one disk to another is a result of
DOS failing to realize that the diskette that was in there is removed and has
been replaced.  Some PCs (like the true blues) have sense information that
let them know if the diskette has stopped spinning -- so no real extra effort
is needed to be fairly sure of a "media change".  Other hardware does not have
this extra bit of data available, so the BIOS usually decides a "media change"
may have taken place if not I/O to the drive has been active in perhaps 1 or
2 seconds.  Then it makes sure of the fact by comparing some data on the
diskette with internal copies of that same area of the disk (assuming no two
diskettes will match) -- I think this is a chunk of the FAT, but I may be
wrong).  A bit of a kludge, but it often works !?!? (:^}].

Charles Marslett
chasm@killer.dallas.tx.us
> North Dakota State University
> uunet!ndsuvax!ncperson | ncperson@ndsuvax.bitnet