Disk Protection (was: Re: How to create .dsk images)

[mjmahon@aol.com (Michael J. Mahon)]

Andy McFadden wrote:

>Paul Schlyter <pausch@saaf.se> wrote:
>>> Some of them might be copy protected but I'm not sure if that's a problem.
>>  
>> For ADT its a fatal problem: it works only with unprotected floppies.
>> OTOH as long as the floppy is unprotected, it doesn't matter whether
>> it's a Pascal, CP/M, DOS 3.3 or ProDos floppy -- ADT handles them all.
>
>You can transfer many copy protected floppies by using SST to create a
>NIB image, but it's a bit of an ordeal.  You use SST to create two
>unprotected disks from a single protected disk, then use ADT to transfer
>the two images to the PC.  There you either run SST in an emulator to
>combine them, or use the "SST Merge" feature of CiderPress.
>
>It's a tedious process (especially if you only have one 5.25" drive), but
>disks that don't use track synchronization or nibble counting will usually
>work.

This raises the question of what fraction of protected disks do
not employ track synchronization or nibble counting.

By my recollection, quite a number of disks were protected
using one or the other (occasionally both) of these methods.

It would be very interesting to catalog the copy protection means
for a large sample of protected disks.  (The study is made a bit
more difficult by the scarcity of protected disks.  ;-)

Some methods (in rough order of complexity):

  Bad checksum; on particular sector
  Changed epilog byte(s)
  Changed prolog byte(s) (address or data)
  Dynamic prolog bytes (e.g.: odd/even track)
  Bit-stuffed prolog(s) (and epilogs?)
  Nibble-counted track(s)
  Signature nibble(s) on track
  Synchronized tracks
  Spiral partial tracks
  ..and more (help fill out the list).

Perhaps a good starting point would be to analyze the "copy
parameters" for various disks, or the "softkey" for deprotecting
them.

I'm sure that the methods would evolve and change over time,
as copy programs adapted to the various methods...  Darned
if it doesn't sound like there's a thesis in there somewhere.  ;-)

-michael

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