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

Subject: RE: [stella] Are 7800 demos legal???
From: Eckhard Stolberg <Eckhard_Stolberg@xxxxxxxxxxxxxxxxxxxxx>
Date: Thu, 25 Nov 1999 16:10:18 +0100
At 11:44 24.11.1999 -0500, you wrote:

>Okay, if you're willing to stomach a few more naive questions...

Sure, no problem. ;-)

>I assume Atari did the encrypting for game publishers, and that's why the
>encryption check is "run over the complete code"?  How does the check work
>that the check's run over only the first 4k in the homebrew scenario and
>over 16k (or what have you) of another game?  Can you tell the encryption
>check to just check out the first, say, 4 bytes of a game which includes a
>jmp (that is, what part of the cart tells the check to stop after 4k in the
>homebrew example)?

There are two informational bytes right after the signature key.
$fff8 was meant to be a regional code, but since the PAL version of the
7800 came out in late 1989 when Atari had already given up on the
encryption issue, this byte is always $ff. (It might not nessesary
have been export restrictions that made Atari change the BIOS code for
the PAL version, since the build in Asteroids game first checks if
it is running on a PAL or a NTSC console to set up the display properly.
So Atari might have planned to include this ROM in NTSC consoles as well.)

The lower nibble of the byte at $fff9 has to be $7 for a 7800 game.
Since most 2600 games don't have a $7 there, this is one of the
checks before the encryption check is executed. The high nibble of
that byte has to be the high nibble of the ROM start. So if you have
a 4kb game, this nibble will be $f. If you have a 16kb game for example
this nibble has to be $c. So you can tell the encrytion check to
test the ROM in steps of 4kb. This is usefull, if the lower part of
the address space holds some extra RAM or is bankswitched. The startup
vector has to point into the signed area of the ROM and since the
addresses where the system vectors reside are always part of the signed
area, you can't change them after the encryption key was added to the ROM.

>Sorry for the totally uniformed questions.

If you apologize for asking questions, then my last reply must have
sounded kind of harsh which wasn't intended. Sorry about that. ;-)

Ciao, Eckhard Stolberg

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