Re: Apple ][ plus 13 sector! Problems...

[Eric Smith <eric-no-spam-for-me@brouhaha.com>]

mjmahon@aol.com (Michael J. Mahon) writes:
> Actually, Bryan is right, the state machine PROM is different
> to allow reliable reading of disk nibbles with two consecutive
> zero bits.
> 
> I don't follow his "9-bits...10-bits" points, since both controllers
> have always used 8-bit nibbles on disk.

The controller doesn't care if there are a few extra zero bits between
the end of one nibble and the beginning of the next one.  This is used
for the self-sync method.

In 13-sector format, the self-sync pattern consists of a series of
self-sync nibbles that each have eight 1 bits followed by a single 0 bit
(total of nine bits each).  If the controller starts attempting to read
at an arbitrary point in the sequence, the first byte it reads will
probably contain at least one 0 bit, rather than being the desired all
1s pattern (hex FF).  Because the zero bits can NEVER be interpreted as
the high bit of a nibble, as it reads each succeeding byte from the
controller, it will "catch up" to the point where it reads synchronized
FF bytes.

In 16-sector format, the self-sync uses eight 1 bits followed by two 0
bits (total of ten bits each).  This allows the controller and software
to get in sync faster.