Good description of Apple ][ disk controller

Eric Smith eric at
Thu Mar 24 13:37:08 CST 2005

Eric wrote:
> 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.

The patent does not describe the 13-sector format at all!!!  It only
describes an 11-sector non-GCR format that was never used by any software
Apple actually publicly released.

The patent is describing FM encoding, not GCR.  The Disk II controller
is capable of FM encoding, by explicity using only nibble patterns that
have bits 7, 5, 3, and 1 set.

In released versions of DOS, FM encoding is only used for the address
field.  There's no point in using GCR for the address field, as it only
stores four bytes.

So if the authors of Beneath Apple DOS based their description on the,
patent, they clearly didn't actually understand it.  For one thing,
the hardware does not EVER automatically generate those clock bit, as
Beneath Apple DOS claims.  It doesn't do it if you write FM, and it
doesn't do it if you write GCR.  (The controller has no idea which
you're using.)

Nothing in the patent suggests that the hardware inserts clock bits.
In fact, the patent pretty clearly explains that those clock bits are
generated by software using the appropriate bit patterns.

By the way, the reason that the 13-sector and 16-sector GCR data
patterns, in which 8 channel bits are used to encode 5 or 6 data
bits, are called nibbles (or nybbles) is a carryover from the FM
format in which the 8 channel bits encode 4 data bits, a classical
4-bit nybble.

> I would assume the patent text is
> an accurate description of what Woz said he designed.

It's an accurate description of the hardware, and of one way for
software to use it.  If you're writing a book about the product that
actually shipped, you should:

1)  Not assume that it works exactly the same way as a patent disclosure.

2)  If you're going to use the patent as a source, take the time to
    actually understand what it says.


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