RE: [stella] 7800 programs -- potential problems?

Subject: RE: [stella] 7800 programs -- potential problems?
From: Eckhard Stolberg <Eckhard_Stolberg@xxxxxxxxxxxxxxxxxxxxx>
Date: Wed, 19 Jan 2000 21:02:56 +0100
>How hard is it to build larger carts like what the 7800 had in its final
>days?

John was talking about building RAM carts. Building a RAM cart that is
compatible with Atari's Super Game bankswitching is a bit complicated,
as the bankswitching is triggered by writing to the game's ROM space.

Modifying existing Super Games to take EPROMs shouldn't be much more
complicated than modifying a 4kb 2600 game, since the Super Games have
the BS logic separated from the ROM chip. But I'll leave the hardware
details to John, as I always do. ;-)

>I know the 6502 runs at VCS speed when in VCS mode on the 7800, but when 
>the 2600 game is on the 7800 cart board, can you really access the extra 
>address and read/write lines too?  If so, that would really open up all 

Yes you can. While experimenting with our RAM cart, we tried running
VCS Decathlon on it, and it worked. Decathlon uses two 4kb banks, one
at $D000 and one at $F000 and switches between them with JSRs.(The
JSRs is what the bankswitching logic seems to detect.)

>sorts of cool things, basically giving you the ability to write 
>Supercharger-like games without all the hardware and software 
>hoop-jumping.  Also, the 6502C has the halt line and hardware 
>interrupts.  Could these be used for anything productive when in 2600 
>mode?  

I don't think so. Generating the the display line interrupt is handeled
by the DMA in 7800 mode. You can't even mess around with DMA in 2600 mode,
since the registers for it aren't available.

>And wasn't it determined that you can't access the extra internal 
>7800 RAM while in 2600 mode?

The 7800 OS turns off the extra internal RAM when switching into 2600
mode, since the address space for normal 2600 games partly overlaps
with the extra RAM. If you had a game with a proper encryption key
that would have all address lines connected to the cart and that in
it's first instructions turns on the TIA but doesn't turn off the
extra RAM, you could write 2600 games that use the RAM. But if you
are going to crate a game that works only on the 7800 console and
that has access to the MARIA, why would you want to limit yourself
to 2600 mode graphics?


Thanks, Eckhard Stolberg



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