Date   : Thu, 29 Sep 2011 01:12:51 +0200
From   : conca@... (Carlo)
Subject: Superior Software ADFS Disc Protection

Hello everybody,

fearing that 25 years old floppy discs could soon start failing, I've been
recently making disc images of my BBC Master Compact floppies.
The only ones I haven't been able to copy are the Superior Software original 
games, that employ a rather sophisticated copy protection scheme. I remember
that a friend who owned an Amiga was the only one who was able to create
fully working copies of those discs; don't know what copier software he used
though.

Finding no info on the Internet I dug out the old disassemblers and debuggers
I wrote circa 20 years ago and set out to investigate the copy protection scheme.
To make a long story short it turns out that Superior discs are split into
three parts:

* Several regular ADFS MFM tracks that contain boot files and loaders;
* One special track which is checked by the copy protection scheme;
* Many FM tracks carrying the games themselves; these tracks are read by
the loaders once the special track is validated.

The special track is a really tricky beast! The core of the protection scheme
is that it is readable in both FM and MFM mode, and in each mode it sports
four 512-byte sectors with logical IDs $F0 to $F3.
Now, if you do the math, you will find that 4*512 bytes in MFM mode plus
4*512 bytes in FM modes take up more than a full track!

Using the mighty Advanced Disc Investigator ROM I found out that the odd 
numbered sectors start halfway through the preceding even ones, thus saving
considerable space. Incidentally, this wreaks havoc with the odd sectors
which assume a bad CRC, which is required _to be bad_ by the loaders anyway!

Unfortunately, work and real life eat away a lot of time from fun, but what
I'd like to do is write a piece of software to create such a track by direct
access of the WD1770 FDC.
I'm thinking about writing the four MFM sectors, then suddenly switching
the controller to FM mode and writing the four FM sectors while the disc
is spinning. Is it feasible? Is there any floppy guru who can give me some
advice?

I hope this rant of mine sparked the interest of at least some of you :)

Ciao,
Carlo.