RE: [stella] Are 7800 demos legal???

Subject: RE: [stella] Are 7800 demos legal???
From: Eckhard Stolberg <Eckhard_Stolberg@xxxxxxxxxxxxxxxxxxxxx>
Date: Wed, 24 Nov 1999 17:20:43 +0100
At 09:58 24.11.1999 -0500, you wrote:

>that 4k).  You've just used the kit to design a game whose first 4k is
>_exactly_ like the template you were given.  Alternately, we could
>dissemble most any game, steal the first 4k, and jmp out, right?

It's not a template. It's the operating system inside the dev cart,
that you normally shouldn't have access to. Almost all other games
have the encryption check run over the complete code and not just the
top 4kb and they never jump out of that area, so their ROM images can't
be used for homebrew games.

>While I'm on the subject, (and out of curiousity) how _do_ you change a
>line (the jmp out) and maintain the encryption?

Not at all. That's the point of having the encryption check in the first
place. The startup vector for the ROM in the Atari dev cart points directly
to a jmp out of the area that the encryption check runs over. This is
why this ROM could be used for homebrew games.

But all that would be needed of this ROM image is that jmp instruction,
the encryption key value and the system vectors. IIRC the encryption
routine first calculates a hash value over the ROM image before it does
the encryption stuff. Is that true? If it is, would there be an easy way
to change the rest of the ROM image while still getting the same hash
value, or would that be just as complicated as guessing the new
encryption key value for a modyfied ROM?

Thanks, Eckhard Stolberg

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