Re: Apple2 disk controller questions

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

John H. Guillory <sales@mainlinebbs.com> wrote:

> On 12 May 1999 08:49:38 GMT, supertimer@aol.com (Supertimer) wrote:
> 
> >Come to think about it, you are right.  It must be the variable
> >spin drives that give Apple 3.5" disks the extra 80k, but GCR
> >is probably a little less efficient.

I did a huge analysis of all this a few weeks ago, posted here.  GCR is
a little less efficient than MFM.

> >What I don't understand is the high density 3.5" Apple II
> >format.  Like the Mac, high density 3.5" Apple II disks are
> >1.44MB MFM.  However, there is a special 1.6MB GCR
> >format that is used by AE's driver for the IIGS.  I'm not
> >sure how the AE driver squishes those extra few kilobytes
> >into the disk.
>
>   Question is: Why did Mac limit their disk to 1.44mb in the first
> place?

Because they saw that compatibility with IBM PC disk formats and being
able to use standard drive mechanisms (eventually) was more important
than squeezing an extra 160KB on each floppy.

> Seems like if they just used the same technology that made the
> 800k disk on the high density disk, they'd naturally get 1.6megs...
> Twice the Density, 800k x 2 = 1.6megs ... Don't it seem logical?

Applied Engineering made a 3.5" high density drive which does precisely
this.  It even worked on the IIgs without needing a special controller
card.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand