Re: Clock Bits On Disk Question

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

Bryan Parkoff asked:

>Clock Bits On Disk Question
>
>    I read both Beneath Apple DOS and Beneath Apple ProDOS.  Chapter 3 of
>Beneath Apple DOS says that the head arm reads clock bit before it begins to
>read data.
>    Now, it looks conflict that Beneath Apple ProDOS says not to use clock
>bits.  It shows the first page of Appendix D -- The Logic State Sequencer.

Early editions of Beneath Apple DOS had this explanation wrong.  It was
fixed in later editions and in Beneath Apple ProDOS.  The authors were
more software-oriented, and did not originally understand the nature of the
Apple hardware recording scheme.

>    The Disk II family of drives uses a unique method of storing data on a
>disk.  They use a method named GCR (Group Code Recording), unlike most
>current disk drives that use FM (Frequency Modulation) or MFM (Modified
>Frequency Modulation).  This enables writing data bytes without the use of
>clock bits and thereby greatly increases the amount of data that can be
>stored on a given track.
>
>    It says that Disk II Drivers can write data bytes without using clock
>bits.  Please explain what it means.

It means that the data is encoded in a way that permits the bit timing to
be unambiguously recovered without recording separate clock bits.
Another way to look at it is that timing (clock) info and data bits are
combined in GCR.  Since some data patterns can be read without
additional clock bits, GCR permits a greater percentage of flux
transitions to carry data.

>    Do MFM disk have address field and data field like GCR disk?  Where can
>I find more information how MFM disk is encoded.  I want to know where each
>bit is located on the disk.  I am interested to study.  It does not matter
>that I spend much time while I enjoy my hobby.
>    Most web sites do not provide the information which it states bits
>located on disk.  It can be difficult to find and can be difficult to
>research.  Both Beneath Apple manuals do provide a lot of information, but
>MFM disk does not.

Information on low-level MFM encoding is not common, but I expect that
it can be found on the web.  MFM formats also encode sector addresses
in a header, since the problem of locating sectors is universal.  Older
"hard-sectored" floppies used holes punched into the inside "track" of the
disk to locate sector starts, and an extra "index" hole to locate the start
of the track (sector 0).  Hard sectoring has been abandoned as technology
has moved forward.

-michael

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