Clock Bits On Disk Question

["Bryan Parkoff" <BParkoff@satx.rr.com>]

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.

    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.

    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.
    Thanks...

Bryan Parkoff
BParkoff@satx.rr.com