On-cartridge RAM

For anything related to cart (SRAM, SF2 mapper, audio, CD mode 1, ...)

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Zontar
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On-cartridge RAM

Post by Zontar » Mon Sep 30, 2013 12:07 am

I'm toying with the idea of making a 2D Minecraft-type game for the MD. As the console stands now, I can make a possible largest map size of 252^2=63.5K of RAM. If I were to go with my original idea of making switchable 3D perspectives (overhead, sideways), it would be further restricted to about 39 or 40 blocks every dimension.

Which is why I ask this question: would it even technically be possible, in theory, to design a cartridge PCB that permitted additional RAM that would be addressable (or at least bank switched somehow)? Mind you, I'm not seriously planning to sell a minecraft-type game for real hardware, but the idea's worth toying with.

Charles MacDonald
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Re: On-cartridge RAM

Post by Charles MacDonald » Mon Sep 30, 2013 1:28 am

Which is why I ask this question: would it even technically be possible, in theory, to design a cartridge PCB that permitted additional RAM that would be addressable (or at least bank switched somehow)? Mind you, I'm not seriously planning to sell a minecraft-type game for real hardware, but the idea's worth toying with.
Absolutely! You have 4MB of space for the cartridge ROM, and if you used say half of it, the remaining 2MB could be RAM. If you needed more RAM it could be bankswitched as you said.

If you wanted to put RAM into other areas, for a system without a Sega CD attached there's 4MB above the ROM range at 400000-7FFFFF you could use, but the cart wouldn't function if the SCD was present. Furthermore you can get 2MB more at 800000-9FFFFF, but it requires a little more circuitry and conflicts with the 32X.

So putting RAM into an unused part of the ROM space, and optionally banking it, is probably the most compatible option.

Mask of Destiny
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Post by Mask of Destiny » Tue Oct 01, 2013 9:32 pm

For what it's worth, the Mega Everdrive is primarily RAM based as are some of the old cartridge copiers. I'm not sure whether there is some kind of write-protect setting on those (I believe some commercial games try writing to ROM to see if they're running on a copier so write-protect can be desirable), but figuring out how disable the write-protect setting (if present) is probably a lot less work than developing a custom board.

Zontar
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Post by Zontar » Fri Oct 11, 2013 2:23 am

Mask of Destiny wrote:For what it's worth, the Mega Everdrive is primarily RAM based as are some of the old cartridge copiers. I'm not sure whether there is some kind of write-protect setting on those (I believe some commercial games try writing to ROM to see if they're running on a copier so write-protect can be desirable), but figuring out how disable the write-protect setting (if present) is probably a lot less work than developing a custom board.
This is the option I'm now exploring, as it's a lot cheaper to simply deploy a ROM online freely for use with a popular flashcart than it would be to build and sell custom carts to a rather limited audience.

Everdrive has 16MB of SDRAM. I think this is where the game is loaded. For that reason you mentioned (write-protect compatibility in old games), do you know how to make the SDRAM read/write? Is there a hardware manual released for the Everdrive?

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Stef
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Post by Stef » Fri Oct 11, 2013 8:07 am

Not sure that is what you are looking for but there is everdrive support in SGDK which allow you to program the everdrive flash cart :
https://code.google.com/p/sgdk/source/b ... verdrive.c

But as the mega everdrive is totally different i guess it does work in a complete different way.

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