Re: Why can't Gs 800k drives read 720k IBM disks?

[dempson@atlantis.actrix.gen.nz (David Empson)]

In article <LwZ32c2w165w@railnet.nshore.org>,
Adam Barr <barr@railnet.nshore.org> wrote:
> DOes anyone know why this can't be done? Whats the differance? It is a 
> disk drive right?

There is one major reason why the Apple 3.5 Drive cannot access a 720k
disk: a different method is used to record data on the disk.

On an Apple 5.25" or 3.5" disk (400k/800k), a method called "GCR"
(Group Code Recording) is used to store data on the disk.  GCR uses a
coded lookup table to convert between legal disk bytes and actual data
bytes, with data bytes split into 6-bit and 2-bit portions.

On a PC 5.25" or 3.5" disk (or on Apple 3.5" disks formatted to 720k
or 1.44MB), a method called "MFM" (Modified Frequency Modulation) is
used.  MFM is based on FM, which is a very simple technique: all data
bits are written to disk separated by clock bits.  This is effectively
the same as the "4-and-4" encoding method used on GCR disks to write
the address information for each each sector.

MFM is the same as FM, but the clock bits are eliminated except when
there are adjacent zero bits, and the timing is altered in those cases
so that no extra space is required on the disk.  The end result is
that MFM has twice the density of FM, but requires more precise timing
in the controller.

In short: the controller chip (IWM, or "Integrated Woz Machine") used
in the Apple II and early Macs can only handle GCR-encoded data.
Similarly, PC disk controllers can only handle FM and MFM-encoded data.


Apple came out with an improved version of the disk controller called
the SWIM ("Super Woz/Wendell Intregrated Machine", I think).  In
addition to GCR, this chip can handle high density disks and MFM
formats.  This chip is used in later Macs, and on the SuperDrive card.
A version of the same chip is used on the PC Transporer.

However, even this chip doesn't allow an Apple 3.5 Drive to access MFM
disks.  The main remaining problem is that Apple's 400k and 800k disk
formats use variable rotation speeds to allow more data to be packed
onto the outer tracks (the disk speed varies in five bands of sixteen
tracks).

MFM disks do not use variable speed rotation.  This is the main reason
they only hold 720k.  If Apple hadn't varied the rotation speed, they
would only have been able to fit 640k on the double sided 3.5" disk
(MFM is more efficient than GCR because its timing has a higher
resolution for the same bit cell widths).

In early Macintosh 400k and 800k drives, the variable rotation speed
was controlled by a signal from the computer.  In the Apple 3.5 Drive,
it is hardwired into the drive and cannot be disabled.

If you were to use an MFM disk in an Apple 3.5 drive, the width and
timing of the bits on the disk would change depending on which track
band was being accessed.  This is not something that the standard SWIM
chip can cope with.  At worst, the timing is distorted by about 50%.

Having said that, the PC Transporter _is_ able to access 720k MFM
disks in an Apple 3.5 Drive, but it doesn't do it very well.  It is
very slow, and disks formatted by the PCT tend not to work very well
on real MFM drives.  The PCT must cope with the variable rotation
speed in software, or using a specially modified SWIM chip.  It
doesn't quite get it right, though.


The SuperDrive is an improved version of the Apple 3.5 Drive.  It is
fully compatible with the earlier drive (you can plug it into a IIgs
disk port, for example, and the IIgs treats it just like a 400k/800k
Apple 3.5 Drive).  It also supports MFM disk formats and high-density
disks.  When operating in MFM mode, the variable rotation speed feature
is disabled.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand