Messages in this thread |  | | From | (Alan Brown) | Subject | Re: Floppy problem | Date | 27 Dec 1996 05:25:37 +1300 |
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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...
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