Re: UniDisk and SE/30 Driver

[nparker@cie-2.uoregon.edu (Neil Parker)]

In article <50e2nt$706@darla.visi.com> nathan@visi.com (Nathan Mates) writes:
>In article <AE4F3C94-C9508@198.161.156.43>,
>Octavian Micro Development <octavian@agt.net> wrote:
>>I am so shocked that there is an alive Apple // user community...
>
>   [Don't be so shocked; we hate dealing with the paperwork when
>deaths are involved]
>
>>That said, has anyone written a driver for the 5 1/4" UniDisk on a Mac?
>>I know it is possible.
>
>    The only drivers (and the only supported way of connecting an
>Apple 5.25" drive (the unidisk is the old 3.5" drive)) are for 5.25"
>drives connected to a //e card in certain (LC, LC II, LC III, some
>performas) Macs. As the Apple II 5.25" format requires _exact_ timing
>and logic, only with the special hardware on the card can they be
>successfully connected. You'd have to lock out all interrupts when
>accessing the disk, and then sit in a precisely coded and speed
>calibrated loop.
>
>   It is possible to physically connect such a drive to the mac. I'm
>pretty sure that extra hardware is needed before you can get around
>to even thinking of writing a driver.

Actually, given that the Mac's IWM chip is compatible with the IIGS IWM chip,
this might not be quite as impossible as it sounds.

The IWM chip contains some internal control bits that the original Disk II
interface doesn't have...one of them turns on a feature whereby the chip can
tell the software when it's time to write the next byte, so it's no longer
necessary for the software to count out exact 32-microsecond intervals.
This feature is used by the GS's 3.5-inch disk driver.  I've never seen
it used for 5.25-inch disk access, but I don't know of any reason why it
couldn't be.

Locking out interrupts isn't as much of a problem as it seems--remember,
the Mac's 3.5-inch disk driver already has to do so.

Of course someone would still have to write the driver for it.  Personally,
I'm planning to let somebody else tackle that task.  I'll also let
somebody else figure out how to deal with the fact that Macs are designed
around the assumption that the user can't remove a disk without the
computer's permission (5.25-inch drives have no means of enforcing that
prohibition, or even of informing the computer that the user has switched
disks).

          - Neil Parker

P.S.  Speaking of IWM features not normally used for 5.25-inch disk access,
I've occasionally wondered if the IWM's 2-usec-bit-cell feature could be
used to make 280K 5.25-inch disks on a IIGS.  Can the average DSDD disk
handle that recording density?
-- 
Neil Parker, nparker@{cie-2,cie}.uoregon.edu, http://cie-2.uoregon.edu/~nparker

     "Evolution is vastly overrated."  -- Ambassador Delenn, _Babylon_5_