RE: [stella] 7800

Subject: RE: [stella] 7800
From: Matt Pritchard <MPritchard@xxxxxxxxxxxxxxxxxxx>
Date: Fri, 13 Nov 1998 14:11:03 -0600
Harry's cart does not bypass the validation check.. it passes it with flying
colors.  

The important point is that the entire ROM does not need to be used for
validation (good, especially if you have RAM on your cart).
The Monitor cart uses the minimum amount of ROM the validation code will
allow; which is the top 4K;

I believe, but have not confirmed, that the vector the 7800 jumps to hand
control off to the cartridge has to be in the range of addressed used for
the validation check.  That is, the code the cart first starts running has
to be used for validation.  What harry did, was to make that code
immediately jump through a vector that is not stored in validation area;
i.e., the code in the 4K block that's validated is just a stub that
transfers execution to the portion of the ROM that's not in the validation
area.  We should check that ROM out to see if he filled it up with library
routines which may be useful by others.

    If I recall, Harry (and everyone else) had to send their code to Atari
to get the 126 Byte validation code generated for them.  To avoid having to
do this each time the code changed, developed used machines that did not
have the validation ROM.  This is why 7800 prototypes are so far and few...
they wouldn't work on a regular 7800 unless that particular ROM revision had
been sent to Atari and returned to them.  I suspect that 7800 games shown at
CES shows that have never surfaced, were shown on machines with development
ROMs.  When found later, they'd just show a black screen, and off into the
trash they go.

BTW, I may be the only person to come by an original 7800 Montior cart
(complete) by thrifting in the wild... ;-)

-Matt Pritchard
Graphics Engine and Optimization Specialist
MS Age of Empires & Age of Empires ][

> -----Original Message-----
> From:	danboris@xxxxxxxxxxxxxxxx [SMTP:danboris@xxxxxxxxxxxxxxxx]
> Sent:	Friday, November 13, 1998 12:51 PM
> To:	stella@xxxxxxxxxxx
> Subject:	Re: [stella] 7800
> 
> > 
> > How similar is this "backdoor" to the trick used to make Lynx carts
> without
> > Atari encryption? 
> 
> The backdoor he was talking about in the 7800 was just in Harry's
> cartridge. It still doesn't explain how Harry's cart passes the valdation
> test in the first place. I have looked over the bios and don't see
> anywhere where you could steel control away from the bios before the
> validation check happens, short of putting some special circuitry on the
> cartridge.
> 
> Dan Boris
> 
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