Size limits

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metalforever
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Size limits

Post by metalforever » Sun Jul 15, 2018 3:44 pm

Is it possible using any technique (such as perhaps implementing an SD card inside of a cart) to run a game over 8MB (incl bank switching)? For example, perhaps if a group of people were very clever, we could create a larger game and put it on a cart? Maybe an elaborate bank switching scheme could be used?

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cero
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Re: Size limits

Post by cero » Sun Jul 15, 2018 4:42 pm

Yes, the SSF2 mapper lets you go over the 4mb limit, and you could design a custom mapper too. Everdrives and emulators support SSF2, so you can test your game easily.

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Re: Size limits

Post by Chilly Willy » Sun Jul 15, 2018 7:26 pm

The standard SEGA mapper (often incorrectly called the SSF2 mapper as it was the only game to use it) allows up to 32MBytes of rom via bank switching. Most available flash carts don't go that high. For example, the current Mega Everdrive only allows up to 15MB. If you make the cart yourself and use the standard mapper, you could go to 32MB. If you don't care about standards, you could easily make your own flash cart that allowed GBytes of storage. It would be silly, but quite easy to do.

metalforever
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Re: Size limits

Post by metalforever » Sun Jul 15, 2018 9:10 pm

Thank you for the answers. It wasn't clear which kind of restrictions I had to impose on the development of a homebrew game.

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Sik
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Re: Size limits

Post by Sik » Mon Jul 16, 2018 5:43 am

For the record, I'm not sure where the 32MB limit comes from (as we don't have any actual official docs from the mapper - what we know is from reverse engineering I think?). But yeah, you can use the mapper to go over 8MB easily if you have a cartridge that actually provides that much memory.

In theory you could also use your own custom mapper too if you're willing to build the cartridge (and cope with the lack of support in emulators).
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Stef
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Re: Size limits

Post by Stef » Mon Jul 16, 2018 7:59 am

As far i remember the Sega mapper allow to remap bank of 512 KB with a register of 6 or 7 bit wide (it was in an official doc) so it means 64 or 128 banks of 512KB (so 32 MB or 64 MB at max).

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Re: Size limits

Post by Miquel » Mon Jul 16, 2018 11:44 am

If you develop your own bank switching there is no limit of size other than what technology provides.

And you can use any type of storage as long as the worst case scenario in read response is smaller than a read cycle on the 68K. In the case of SD+FAT you will need a quite big fpga-like chip, not the best option.

Or another option is to have 2/4MB of RAM and reading consciously from the SD.
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Re: Size limits

Post by Chilly Willy » Mon Jul 16, 2018 2:40 pm

The mapper is fully documented in the 32X docs in the 32X Development Equipment section. It uses registers from $A130F1 to $A130FF as controls for eight banks of 512KB each. Bank 0 cannot be changed and it's register is a control register for save ram. The other seven use six bits to set the bank selected from the rom (or ram if using the dev cart). Six bits gives 64 banks of 512KB, or 32MB of space. While the mapper handles seven banks, SSF2 only uses banks 6 and 7, so some early flash carts don't support the full mapper spec.

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Stef
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Re: Size limits

Post by Stef » Mon Jul 16, 2018 2:49 pm

Ok so it was indeed 6 bit :p

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Re: Size limits

Post by Chilly Willy » Mon Jul 16, 2018 5:35 pm

Yes, but they show always setting the two upper bits to 0, so they could have thought about expanding it later. That never happened, of course, and roms on the Genesis/32X never reached 32MB, much less bigger. They did reach 64MB on the N64, so if the 32X had been a run-away success ( :lol: ), it might have gotten expanded. Since there's little chance of SEGA going back and releasing something new, you could make your own mapper using all 8 bits for backwards compatibility, but allowing 128MB. What I think would be more interesting would be make the banks 7 bits, and make b7 a ram/rom select line, allowing carts to have both ram and rom that can be bank selected.

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Re: Size limits

Post by Sik » Mon Jul 16, 2018 8:45 pm

It's possible that particular mapper only had enough pins to map 32MB. And yeah, if I recall correctly there's work to make one that can address up to 128MB (we should call it the GIGA POWER cartridge since it's 1 gigabit :v). And technically nothing prevents you from using the remaining 8 bits from the word (Mega Everdrive uses the upper bits for tagging, so it's doable).
Chilly Willy wrote:
Mon Jul 16, 2018 5:35 pm
What I think would be more interesting would be make the banks 7 bits, and make b7 a ram/rom select line, allowing carts to have both ram and rom that can be bank selected.
Or use bit 15 :​P
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Re: Size limits

Post by Chilly Willy » Tue Jul 17, 2018 1:53 pm

Yes, I think a full word per bank would be better. They clearly wouldn't have had enough pins or gates for that at the time, but these days, why not? :D

At that point, amount of memory you can address gets silly.

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