Re: Apple2 disk controller questions

[pg@sff.net (Paul Guertin)]

dempson@actrix.gen.nz (David Empson) wrote:
 
> The Apple II uses a 6-and-2 encoding scheme, in which each data byte is
> broken up into 6-bit and 2-bit fields.  The 6-bit value is used as an
> index into the lookup table to get the corresponding valid disk byte.
> The 2-bit leftover is combined over three bytes to get another 6-bit
> number which is used similarly.  (The data corresponding to the leftover
> bits is written at the end of the sector.)

Thank you for that message, David. As usual, it was well written and
full of accurate information.

I'd just like to add that Apple (Woz?)'s decision to write the leftover
bits at the end of the sector, instead of every three bytes, means that
it takes at least two revolutions to read or write a full track: there's
no time to do the 6-and-2 encoding or decoding on the fly. But if the
letover bits were interleaved every three bytes, it could be done. This
is how Locksmith's fast disk copy can read a whole disk in 8 seconds
with full error checking.

Paul Guertin
pg@sff.net