Re: DOS 3.3 Nibble Encoding

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

In article <mark-171193100945@128.157.9.145> mark@trillian.jsc.nasa.gov (Mark Manning) writes:
> In article <Nov17.020819.75996@acs.ucalgary.ca>, sbdocker@acs.ucalgary.ca
> (Sean Brendan Dockery) wrote:
> 
> [Stuff Deleted]
> 
> > First, the boot sector obviously conforms to the format of the rest of
> > the disk.
> 
> This is not necessarily true.  If this were true, then the Apple could not
> boot both a ProDos disk and a DOS 3.3 disk from the same disk drive (ie: a
> 5.25" Apple drive).  This is because one is formatted one way and the other
> another.

No they aren't.  A DOS 3.3 and ProDOS disk use the same physical disk
formatting scheme (35 tracks, 16 sectors per track).  The only
difference is the ordering of the sectors on the track (which doesn't
affect the ability to read or write, just the speed).

For both operating systems, track 0 sector 0 is loaded when you boot
the disk.  The code in the sector is different from DOS 3.3 and ProDOS,
and the boot proceeds in totally different ways after the first sector
has been read.

> Correct me if I am wrong, but I thought the initial boot track was more
> or less a byte stream set-up.  Where there were a set of sync bytes at
> the beginning of the byte stream and then there were just a set of bytes
> with no sync information to them (sort of like reading a tape).  These
> were loaded into memory and then the first byte jumpped to (which in turn
> read in the next section and so on and so on...).

You're completely wrong.  There is a set of self-sync bytes at the
start of every sector on the disk, which are used to synchronise the
disk reading hardware with the address marks.  There is a larger gap
before sector 0, but this is only because there is a little space left
over on the track after 16 sectors have been written.

There is absolutely nothing special about track 0, sector 0.
-- 
David Empson                                                               
dempson@swell.actrix.gen.nz                                                
Snail mail: P.O. Box 27-103, Wellington, New Zealand