Re: [stella] Atari 7800

Subject: Re: [stella] Atari 7800
From: "Flipper - Personal" <flipper@xxxxxxxx>
Date: Tue, 17 Nov 1998 09:17:46 -0500
-----Original Message-----
From: Bob Colbert <retroware@xxxxxxxxxxxxx>
To: stella@xxxxxxxxxxx <stella@xxxxxxxxxxx>
Date: Tuesday, November 17, 1998 8:07 AM
Subject: Re: [stella] Atari 7800


>Well, last night I looked at the code a bit and didn't really get anywhere.
> I'm fairly suspicious of the decryption algorithm because the programer
>went through great pains to make the code difficult to trace.  I would


That may look hard to trace, but that's very standard 6502 coding.  The
processor has no 16 bit index registers like a Z80, so self-modifying code
is about the only way to accomplish the massive memory moves that $ff80 and
$fff8.  So I would have to assume that the only place this
>information could be is $fff8/$fff9 or $fffa/$fffb?
>
The start of code pointer is at $FFF8/$FFF9.  $FFFA-$FFFF are reserved for
use by the 6502 for it's interrupts/resets, and are used by the BIOS for the
color scrolling.


>While getting permission to use the monitor cart would be great, I still
>think it would be fun to pursue figuring out the encryption!

If you're watching while the $2536 routine does it's work, a $77 byte key
from BIOS ROM is copied to $1901 at one point( shortly after it reads the
$77 byte signature out of the cart).  Now, something tells me that no matter
how down-pat we got the algorithm, we'd need atari's version of this $77
byte key.  And to be truthful, it's computationally impossible.  Even a
128bit ($10 byte) key would be sufficient to be computationally impossible.
We would need to find an error in the algorithm that allows it to be
predicted and some of these billions of possibilities discarded as not
possible.



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