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

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

Dan Foster wrote:

<snip>

>The *DEFINITIVE* work that explains how DOS works, including its RWTS
>(read/write track/sector) routine and the tight timing loop involved was
>in, I think, an article that Dr. Sandy Mossberg wrote for Nibble in the
>'80s?

Although Mossberg does a nice job of disassembling DOS and
explaining its logic, the definitive work on the low-level encoding
and the state machine that decodes it is undoubtedly Sather's
"Understanding the Apple II" and "Understanding the Apple //e".

There were earlier reverse-engineered explanations of the state
machine, but Sather puts it all together, providing a clear and
thorough explanation of both the earlier and the later controllers.

<snip>

>Well, one chapter is devoted to DOS 3.3. I'm going to presume that Mr.
>Irwin will forgive me for significant quotations from his excellent book
>since it's been 20 years (and if you can locate a copy -- do so!).

Unfortunately, Mr. Irwin also misunderstands both the actual
magnetic encoding on the disk and the way that DOS converts
data bytes to "disk nibbles" for writing.

Making the same error as the early editions of "Beneath Apple
DOS", Irwin says that all bits are written to disk interleaved
with clock pulses.  This is false.

Apple's (Wozniak's) GCR scheme was designed precisely to
_avoid_ having to use clock bits, thereby increasing the data
capacity of a track by 50%.

Also, the method he describes for converting a stream of
8-bit data bytes to 6-bit chunks for encoding is incorrect.
DOS 3.3 actually takes the upper 2 bits from three data
bytes to create another 6-bit chunk, rather than taking
a "moving" 6 bits from the concatenated stream.

It is unfortunate that this many years after the fact, so
many misconceptions about the Apple disk technology
still persist.  I have long regarded the Disk ][ controller
as Wozniak's engineering masterpiece, surpassing even
the Apple ][+ in the creativity and simplicity of its design.

-michael

Check out parallel computing for 8-bit Apples on my
Home page:  http://members.aol.com/MJMahon/