Good description of Apple ][ disk controller

[Eric Smith]

Sellam asked about details of Apple Disk II GCR encoding.

I wrote:
> By adding the somewhat arbitrary
> prohibition of two double-zeros in a nybble, the number of valid
> nybbles is reduced to 72, which is still 6 more than necessary.  A
> few more are also eliminated; I don't recall whether there was an
> easily stated criterion that was used to eliminate the remaining 6.

Now I remember.  They added the requirement that there *must* be a
pair of consecutive one bits in the nybble, not counting the MSB.
They don't require that for the address/data field marker bytes (D5
and AA).  But that requirement leaves exactly 64 valid nybble values,
which can encode six bits, and it does it in such a way that the
boot PROM can easily generate the decode table.

There's a GPL'd C program on my web site that shows how a table of
valid nybbles can be programmatically generated:
    http://www.brouhaha.com/~eric/software/diskii_nybble/

The output from the program is:

13 sector nybbles:
   0:  ab ad ae af b5 b6 b7 ba bb bd
  10:  be bf d6 d7 da db dd de df ea
  20:  eb ed ee ef f5 f6 f7 fa fb fd
  30:  fe ff

16 sector nybbles:
   0:  96 97 9a 9b 9d 9e 9f a6 a7 ab
  10:  ac ad ae af b2 b3 b4 b5 b6 b7
  20:  b9 ba bb bc bd be bf cb cd ce
  30:  cf d3 d6 d7 d9 da db dc dd de
  40:  df e5 e6 e7 e9 ea eb ec ed ee
  50:  ef f2 f3 f4 f5 f6 f7 f9 fa fb
  60:  fc fd fe ff

Eric