Re: Hardware musings

["Michael J. Mahon" <mjmahon@aol.com>]

Mark McDougall wrote:
> mdj wrote:
>
>
> > The only automatic way I can see of getting around this would involve
> > sampling the read head directly in analog, and compare the signal
> > amplitude of adjacent tracks, but this would involve a hardware hack to
> > the drive and much more sophisticated software.

Such an approach could probably determine *approximately* what info had
been written, but there would be no certainty.

Consider two tracks written "3/4 track" apart.  If similar information
is written at some angular position on both tracks, it is quite likely
that micro speed variations will cause some of that info to interfere
destructively on the quarter tracks between them, and that, a few
nibbles away, they will interfere constructively, making those nibbles
look quite strong--as if they had been written on one of the intervening
quarter tracks.

Even if you used a much narrower head to read, this case would be very
hard to distinguish.

> Speaking from a theoretical perspective, rather than someone with any
> detailed knowledge of the A2 disk controller, IMHO Michael and Matt have
> hit the nail right on the head.
>
> From what I can gather reading this thread, the design and subsequent
> flexibility of the A2 disk controller allows an almost 'analogue'
> control over the contents of the media - something which amplifies the
> problem of automatically capturing this information exponentially.

I think the analog approach is a red herring.  No copy protection
scheme that uses a Disk ][ and Disk ][ Controller can depend on any
behavior that is not observable with those devices.

> I think the best approach would be to catalogue the known protection
> schemes and modify/expand whatever disk image file format is most
> appropriate to handle each scheme. When more schemes are
> discovered/documented, then the format is necessarily extended.

Not a very satisfying approach for a "dead" computer system.  ;-)

> I don't think there's any other viable approach?

I think that I already expressed one:  save all the data that a Disk ][
Controller can read from each quarter track (including its length,
determined by scanning for repeats), together with its angular
alignment WRT track 0, sector 0, and a map of all bits that are "soft"
(read differently on different read attempts).

Can anyone think of any protection scheme that would use more
information than that?

> To go to the extremes of capturing the analogue data on the disk would
> require not only, as mjd noted, hardware hacks and more sophisticated
> software, but also a software 'emulation' right down to the analogue
> behaviour of the disk controller/drive in order to be able to read (and
> hence duplicate) the preserved images.

Successfully *writing* a protected disk does raise some issues not
involved in reading.

Reading is a passive activity, and can be done "worst case", without
knowing exactly what will be required when the image is used.

Writing is active, and must be switched on and off precisely to get
the needed data recorded to the disk in the right place without
clobbering other needed data.  The critical part of this is knowing
*exactly* what data is *needed*--and that involves human effort.

(BTW, if a program is single load, and never accesses the disk again,
then the need for human action may be eliminated.  It is "only"
necessary for an emulator to "observe" the program's interaction with
the disk to definitively determine what data is relevant.)

-michael

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