Good description of Apple ][ disk controller

Vintage Computer Festival vcf at
Tue Mar 22 15:57:51 CST 2005

On Tue, 22 Mar 2005, Eric Smith wrote:

> I wrote:
> >> You want the book "The Apple II Circuit Description", or "The Apple IIe
> >> Circuit Description", by Winston Gayler.
> Sellam wrote:
> > While that's a good book, it has nothing on the disk controller.  See
> > my previous message for a better selection.
> Oh.  I suppose I must be hallucinating when I look at this copy in
> front of me.  The table of contents lists chapter 9 as being "The
> Disk Controller".  And pages 9-1 through 9-45 seem to give a detailed
> explanation of how it works.  But I guess this must just be a
> particularly strange side effect of the drugs that I'm not taking.

Yes, the drugs you are taking are so good that you're reading the title of
your book, which is "Understanding the Apple II" by Jim Sather, as, "The
Apple II Circuit Description".  Pass the mushrooms!

> > 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.

Ok, this is what I thought.  And I haven't read far enough into
UNDERSTANDING THE APPLE II to have dispelled that rumor.

> Fortunately they understood the higher-level stuff, so the rest of
> the book is much more accurate.


> > According to my reading (in Beneath Apple DOS), you can only have *one
> > pair* of consecutive zeros in any one byte, but it was not clear if that
> > was a hardware constraint or an artificial constraint to pare down the
> > number of "legal" values.
> Artificial constraint.  The hardware (w/ the 16 sector state machine)
> can deal with any nybble that has the MSB set, and no more than two
> consecutive zeros.

Ok, cool.


Sellam Ismail                                        Vintage Computer Festival
International Man of Intrigue and Danger      

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