Re: Stupid Questions 102

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

In article <4cq5l5$49m@rock101.genie.net>,  <a2pro.gelamp@genie.com> wrote:
> bpvh@primenet.com (Bradley P. Von Haden) writes:
> 
> 
> >dturner@brando.ece.utexas.edu wrote:
> 
> >Normal Apple IIgs disk drives can not read MS-DOS disks - something
> >to do with the way information is written onto the disk that is
> >hardware (?) incompatible.
> 
> yes, it is hardware incompatible.  The difference is that Apple 3.5" disk
> drives (double density) use a variable-speed motor to keep the linear
> velocity of the surface under the head constant, rather than faster on outer
> tracks than inner tracks.

This is ONE reason why the Apple 3.5 Drive cannot read MS-DOS disks,
but it is not insurmountable: the PC Transporter can read and write
720K disks with an Apple 3.5 Drive.  (Its formatting doesn't work too
well - a real PC will usually complain about the disk.)

The primary reason why you cannot read an MS-DOS disk in an Apple 3.5
Drive connected to a IIgs is that a different bit-level recording
method is used: MS-DOS disks use MFM (Modified Frequency Modulation),
while Apple disks use GCR (Group Code Recording).  The IWM chip in the
IIgs can only handle GCR.

(The same reason, the other way around, is why PCs cannot read Apple
400K/800K 3.5" disks, or Apple II 5.25" disks - the PC controller can
only handle MFM.)

The Apple 3.5 Drive has no hardware restrictions preventing the use of
MFM (apart from complications due to the rotation speed), it is just
the controller.

The SuperDrive has the abilty to disable the variable rotation speed.
It also supports high density disks.  In conjunction with a "SWIM"
chip (as provided by the Mac IIx and later machines, or the Apple II
3.5 Disk Controller Card, a.k.a. "SuperDrive Card") it can access MFM
and high density disks (720K and 1.44MB).

Note that Apple high-density 3.5" disks (1.44MB) use GCR, so they can
be physically read in a PC - you just need software to deal with the
file system.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand