Re: PowerPC Mac's Dont like 800k disks!!

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

In article <Pine.SUN.3.91.951213125448.2102A-100000@sunspot.tiac.net>,
Programer  <progrmer@tiac.net> wrote:
> On Tue, 12 Dec 1995, Mark Munz wrote:
> 
> > I've got a PowerMac 7100/80 (system 7.5) and I haven't had any problems
> > with 800k Apple II floppies. I do know it's very slow to write to them,
> > especially if you are using subdirectories.
> 
> well its probably slower cuz apple took out the SWIM (sp?) chip, this 
> changed the speed of the disk drive spin depending on where the head's 
> where so that they could get more data into the larger outer rings (this 
> is something that IBM clones never did thats why they are 720k vs our 800k)

The SWIM chip has nothing to do with the drive rotation speed.

On early Macs (128 and 512) the drive speed was controlled by the Mac
using a pulse train going out to the drive.  This pulse train was
generated by one of the VIA chips, I think.  The software varied the
frequency according to the track that was being accessed, causing the
disk rotation speed to vary accordingly.

The Apple 3.5 Drive (and SuperDrive when accessing 400K/800K disks)
has built-in hardware to vary its own rotation speed according to the
track being accessed (five speed bands, 16 tracks per band).

The SuperDrive disables this mechanism when accessing MFM disks
(720K/1.44M).  I'm not sure if this is done by the controller writing
to a specific register in the drive, or automatically because it is
switched into MFM mode (also under control of the computer).

Apple's more recent (cheaper) drives have simplified hardware to
reduce their production costs.  The most obvious example is the
removal of the automatic "inject" and "eject" mechanism.

I haven't had a close look or listen to one of these drives, but it is
quite possible that they also removed the speed variation.  I also
have not looked inside recent Macs to see whether they still use the
SWIM chip (I expect they do, or a more recent derivative of it).

To access 800K disks reliably in a drive without variable rotation
speed, the computer would have to vary the WIDTH of the bits being
sent to the drive (or cope with narrower/wider bits when reading).
This is probably difficult, without doing something like adjusting the
clock speed of the disk controller.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand