Re: Disk II 6-2 Encoding Question

[russotto@grace.speakeasy.net (Matthew T. Russotto)]

In article <4730f90e$0$20622$4c368faf@roadrunner.com>,
Bryan Parkoff <nospam@nospam.com> wrote:
>Hello,
>
>    I review page C-4 from Beneath Apple ProDOS book.  The Disk Byte is 
>total possible 72, but Disk II chooses only possible 66.  Sixth-four Disk 
>bytes are used for data and two Disk bytes as AA and D5 are used for Address 
>Field and Data Field.  I am curious why six Disk bytes as 95, A5, A9, CA, 
>D2, and D4 are not chosen.  Could it be maximum accurate to read and write 
>6-2 encoding?  They have a pair of consecutive zero bits and they do not 
>have a pair of consecutive one bits.  Disk II prefers to have a pair of 
>consecutive zero bits "AND" a pair of consecutive one bits.  Do you have 
>explanation?

Yes, according to Beneath Apple DOS, the reason those bytes were
excluded was because they don't have consecutive 1 bits in the lower 7
bits.  I believe this was somewhat arbitrary, to make the mapping
exact.
-- 
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  result in a fully-depreciated one.