Re: DOS 3.3 Nibble Encoding

[dempson@swell.actrix.gen.nz (David Empson)]

In article <mark-181193104935@128.157.9.145> mark@trillian.jsc.nasa.gov (Mark Manning) writes:
> In article <CGos7L.GFD@actrix.gen.nz>, dempson@swell.actrix.gen.nz (David
> Empson) wrote:
> > 
> > There is absolutely nothing special about track 0, sector 0.
> 
> I can not agree with the last statement.  Logic alone tells me that
> the boot track on a ProDos disk is the same as the boot track on a
> DOS 3.3, Apple Pascal, CP/M, or any other formatted disk for the Apple
> computer.  The rest of the disk may be formatted differently, but the
> boot track has to be standardized so the system will boot properly.

As I said to you in E-Mail: an unprotected Apple II 5.25" disk uses
the same physical format for every sector on every track.

DOS 3.3, Pascal, CP/M and ProDOS use the same physical disk formatting
method (with differing sector interleave factors) but have different
directory structures and file storage mechanisms.  The CODE in track
0, sector 0 is different for each operating system because of the way
in which the operating system is stored on the disk.

DOS 3.3's boot sector reads RWTS from track 0, sectors 1 through 9,
then transfers control to a second-stage loader which loads the rest
of DOS from tracks 0, 1 and 2.  It then tries to run the HELLO program.

ProDOS's boot sector reads the second sector belonging to block 0,
then the pair of them attempt to read the file called PRODOS into
memory and transfer control to it.

Pascal's boot sector is similar in concept - it loads SYSTEM.APPLE and
SYSTEM.PASCAL into memory, then transfers control to the Pascal
operating system to do the rest.

CP/M?  I haven't looked at it in detail, but from memory, CP/M version
2.2 will load the BDOS from the first three tracks, then transfer
control to the 'cold entry' point of the BDOS, which will load the
command processor (also in track 2, I think).


Programs exist to transfer files between the various operating
systems, and they use the standard "read a sector" or "read a block"
call provided by the operating system to read data from another
operating system's disks.


Only protected disks use special formatting techniques, and even they
must have standard formatting for track 0, sector 0 (with some rules
relaxed since the boot ROM doesn't check everything about the boot
sector).
-- 
David Empson                                                               
dempson@swell.actrix.gen.nz                                                
Snail mail: P.O. Box 27-103, Wellington, New Zealand