Re: High and Double density floppies

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

In article <2kbvdu$nd2@news.tamu.edu> akitomom@cs.tamu.edu
(Akitomo Morita) writes:
> The main problem is that the Apple Disk ][ uses a different method to write
> data to the disk than IBM style drives.  The Disk ][ uses a method which is
> call GCR (Group Code Recording) and IBM style drives use MFM (Modified
> Frequency Modulation) which are incompatible, i.e. a GCR drive cannot read
> disks written using MFM and vice versa.  I believe that there are drives
> which are capable of reading both type of disks, though.

Actually, it is the disk controller which determines the format, not
the drive.

The PC's 5.25" drive is able to read/write Apple II disks, provided you
have a disk controller which can handle the GCR data.  I believe the
"IIe on a card" for the PC (I forget its name) can do this.

The opposite isn't true - most Disk ][ drives cannot access 40 tracks,
let alone both sides of the disk.

The 3.5" drive is a different matter.  Apple's 400k and 800k drives
squeeze more data onto the disk by varying the rotation speed of the
disk according to the track (slower on outer tracks, if I remember
rightly).  The drive also uses GCR.

The PC's 3.5" drive has a fixed rotation speed and uses MFM.
-- 
David Empson                                                               
dempson@swell.actrix.gen.nz                                                
Snail mail: P.O. Box 27-103, Wellington, New Zealand