Re: [stella] RIOT RAM test

Subject: Re: [stella] RIOT RAM test
From: "Eckhard Stolberg" <Eckhard_Stolberg@xxxxxx>
Date: Wed, 27 Nov 2002 20:25:18 +0100
> Is this what cause problems with certain 2600 carts not working?
> Are they relying on the contents of RAM at powerup?

I'm not sure about that. I don't know any games that fail because of
the RAM. The early 7800s had problems with some superchip games,
because the timing for this is a bit critical. Therefore Atari added
a little circuit to the later models of the 7800 that changes the
signal timing on the bus. This made the superchip games more reliable,
but broke the Activision games with FE bankswitching for example.

Another problem is that the handling of the undefined bits for the
TIA read registers was changed. On the 2600 the 6 lower bits will
always read whatever data was on the data bus last. Usually this is
the address of the register. On the 7800 these bits will always be
zero. Mountain King for example starts to (accidentally ?) use the
TIA read registers as level data pointers when you go too high.
This is how the secret kingdom comes to be. On the 7800 you can't
get there, since the accidental level data will be different.

Also the 7800 starts at 1.79 MHz during the signature test, which
might cause some bankswitching methods to lock up. Chad had a problem
with this during the development of the Cuttle Cart, but was able to
fix it.

Ciao, Eckhard Stolberg

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