Good description of Apple ][ disk controller

Eric J Korpela korpela at
Wed Mar 23 11:01:09 CST 2005

On Tue, 22 Mar 2005 10:43:26 -0800 (PST), Eric Smith <eric at> wrote:
> > I see.  And this is where I'm confused.  Beneath Apple DOS shows diagrams
> > that indicate there's a clock pulse between every data bit.  So either
> > that diagram is wrong, and there are no clock pulses,
> The authors of Beneath Apple DOS had no clue whatsoever as to what was
> going on at that level, so they apparently just assumed that it worked
> the same as other disk controllers in that regard.
> There are no clock bits.  That's the whole point of GCR.

Either that or they read the patent application....   In the patent,
Woz describes the (13 sector) nybble format as being interleaved data
and clock bits, with the clock bits all set to 1.  While this has a
different meaning than clock bits in an [M]FM controller, you can
still think of them as clock bits.  I would assume the patent text is
an accurate description of what Woz said he designed.


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