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From: binder@fizbin.DEC.COM (Sold - but we have others)
Newsgroups: comp.sys.apple
Subject: Re: IBM/Apple disk interchange?
Message-ID: <8701022246.AA16363@decwrl.dec.com>
Date: Fri, 2-Jan-87 16:51:00 EST
Article-I.D.: decwrl.8701022246.AA16363
Posted: Fri Jan  2 16:51:00 1987
Date-Received: Fri, 2-Jan-87 21:36:43 EST
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 28

Query from TMPLee@dockmaster.arpa:

> Does anyone know of any software that would run on an Apple II and allow it
> to read and/or write IBM PC format (5.25") disks (with a regular Apple
> drive) or which would run on an IBM PC and allow it to read and/or write
> Apple-formatted disks?  (I'm talking about software, not a hardware card,
> for instance.) (either Apple DOS or ProDOS format) Seems like it ought to be
> possible on the Apple since the Apple drive is driven at a very low-level by
> the 6502. 

Nope.  Can't be done.  IBM's disks are written and read by an industry-
standard disk control chip that will do either FM (double frequency) or MFM
(Miller Code) and nothing else.  The chip requires an Index signal from the
drive once per revolution; recording is synchronized to that Index pulse
during the formatting operation.  The Apple's controller is equally restricted
- its custom state machine can't do anything except the special GCR (group-
coded recording) that Woz invented to make it so cheap.  And the Apple drive,
also cheapified, does not provide an Index signal, because it's not needed -
Woz used self-synchronizing preamble formats.  The low-level control by the
Apple's 6502 consists of handling spinup/down, stepper motor sequencing, and
so on - there's no direct access between the read/write path and the 6502. 

Cheers,
Dick Binder   (The Stainless Steel Rat)

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