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SubjectRe: Floppy problem
Date

In article <Pine.SOL.3.91.961221231221.8954A-100000@fan1.fan.nb.ca>,
William Burrow <aa126@fan.nb.ca> wrote:

>Can't. The floppy drive hardware is backward and stupid. It does not
>notify when a disk is removed, it notifies (if you check!!) if a disk
>change has occurred.

Throwing back a few years, Atari ST machines used to poll the write
protect switch several times per second to detect a disk change. Write
protect would always toggle back and forth a few times as a disk went in
or out from simple mechanical causes, so it was a fairly good indicator
that something had been done.

A change of state meant the disk had been moved and the drive would
immediately be rescanned to check if the filesystem had changed by 1st
checking the dosfs serial number and if it matched the previous one, the
filemap compared with the machine's cached filemap.

This latter check was because quite a few dain bramaged formatting
programs would churn out disks with the same serial number and many
commercially available preformatted disks suffer the same problem.

It worked extremely well and was almost totally immune to being fiddled.
The only time things tended to get out of sync was when a program
disabled the polling to get a bit more speed - and there was a simple
sequence to refresh things if that happened.

One only needs to implement the polling when there's a disk mounted.
Atari ran at 60 checks per second which is a bit extreme as it takes
at least 0.5 second to change a disk :-)

Of course, Atari never had to deal with write cached filesystems (though
one 3rd party programs I ran on my 4Mb STe switched on write caching and
would put up impolite demands to "reinsert the disk immediately" if one so
much as touched the eject button.)

If the disk is mounted readonly, all thats needed if a change occurs is
automatic unmounting...

AB
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