Re: Clock Bits On Disk Question

[Eric Smith <eric-no-spam-for-me@brouhaha.com>]

"Bryan Parkoff" <BParkoff@satx.rr.com> writes:
>     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.

Beneath Apple DOS was completely wrong about this.  Apple's GCR format
does NOT use clock bits.  The whole *purpose* of GCR is to avoid explicit
clock information in order to get higher recording density as compared
to FM encoding (which uses clock bits).

However, the track, sector, and volume number as stored in the address
field are effectively stored as FM data rather than GCR nibbles.  Or,
to put it another way, those fields only use 16 of the possible GCR
nibbles, all of the form 1n1n1n1n, in order to store four bits in one
disk byte.  In the data field, more of the possible nibble values are
used, in order to store five data bits per nibble in 13-sector format,
or six data bits per nibble in 16-sector format.

>     Do MFM disk have address field and data field like GCR disk?

Yes.

> Where can I find more information how MFM disk is encoded.

Find a data sheet on the old Western Digital 1791 floppy controller
chip.  It is explained fairly well there.