Linux-kernel mailing list archive 2000-07,: Re: Of removable devices

# Re: Of removable devices

David Balazic (david.balazic@uni-mb.si)
Wed, 16 Feb 2000 20:04:07 +0100

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Khimenko Victor wrote :

> > Just to mention - my old Amiga was doing this 15 years ago....
> > What a progress :)
> >
> Hmm. Plane is faster then auto and can carry more peoples but there exist
> LOTS of tricks doable with auto and not doable with plane ... Plus Amiga
> had hardware to support such operations AFAIK...

No, it had no special hardware, normal floppy drives.
There is a line called DISKCHANGE on the Shugart bus ( the one that is
used to
connect standard 3,5 floppies ). It is HIGH if there is a disk inside
the drive
and is LOW if the drive is empty ( or the opposite , I'm not sure ).
AmigaOS just polled the line to detect insertions and removals.

When a floppy with open files ( or otherwise in-use ) was removed, it
was not
ummounted, but only marked as absent. When the user inserted another
floppy,
it would mount as another logical volume. Now when there was a need to
access
the first floppy, the OS popped up a dialog ( a requester, in amiga
terminology :-)
saying "Insert volume Linux_help_disk into any drive". When the user
inserted the
floppy into one of avaliable drives, it would mark the volume as normaly
mounted again
and complete the operation.

There was/is one strange thing with the DISKCHANGE line. It goes from
INSERTED to
EMPTY when the medium is ejected, but doesn't return to INSERTED when a
medium is
inserted, _unless_ the r/w head is moved after the insertion. So when
the drive is
empty the head most be moved every now and then, otherwise the insertion
is not
detected. The reasons for this beahviour are beyond me and I would be
grateful for
any explanation.

That is why empty drives on amigas do a click every second or so.

Some newer drives support a little trick. You position the head at track
0 and
try to move it to track -1; the drive notices that that is an illegal
position
and doesn't move the head , but it still updates the DISKCHANGE line, so
there
is no annoying clicking any more.

As this feature is ( or should I say was ? ) only used on amigas it may
not be
implemented by every floppy drive unit. ( some have jumpers to activate
it or use
the line for some other purpose ).

    --
    David Balazic

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