Date   : Tue, 28 Jun 1983 04:51:00 EDT
From   : Allan D. Plehn <PLEHN@mit-mc>
Subject: How CP/M can determine disk format.

Steve, read the FORMATDOC file in AR47:CPM;  I think that will
answer your question.

The neat part of the format byte scheme is that the byte can be
read without any head stepping since it is on the outer track.
This makes it possible, for instance, to read a 48-TPI
five inch diskette in a 96-TPI drive.  Having read the format
byte the software can determine that double-stepping is necessary
is necessary.

My Northstar Horizon is equipped with three TEAC half-height, 960-TPI
,double sided drives plus a Tandon "Quad" (48-TPI, double-sided). I can
read the quad-written disks perfectly in any of the 96-TPI drives, including
correct calculation and display of disk capacity, remaining space, etc.
by STAT and other programs.  The "Quad" drive is retained only so that
I can WRITE Quad diskettes, to be read in standard Horizon or
Advantage configurations (which use Quad drives).

Hope this info is useful.  The BIOS source that does all this
(for N* disk controller) is also in AR47:CPM;
                       
                               Al Plehn