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

[mjmahon@aol.com (Michael J. Mahon)]

Eric Smith replied:

>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.

I understand self-sync bytes, Eric, I just don't see the relevance to
data nibbles or to the controller state machine.  Whether self-sync
bytes are 9- or 10-bit nibbles doesn't affect their operation with
either controller state machine.  Of course, 10-bit self-sync nibbles
sync faster in the worst case...

-michael

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