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Does SNES SRAM size matter?
Posted: Sun Sep 17, 2017 1:19 am
I'm just getting into the SNES Repro scene and have a question about SRAM. As there are 2 sizes (64k and 256k) I was wondering, as there is no real price difference between the 2 chips can I just use a 256k on every board or does it have to be specific to the ROM I'm using?
Re: SNES SRAM Question
Posted: Sun Sep 17, 2017 6:50 am
You can use a 256 kbit RAM everywhere except for a few obscure Japanese games like Dezaemon, which require a bigger RAM.
But the number of address lines actually connected to the RAM is specific to the ROM because some ROMs do a self-test and halt if the RAM size isn't exactly as expected. To use these ROMs, connect the SRAM's A14 and A13 inputs to ground (and disconnect them from the address lines on the cart edge) so that only the first 8192 bytes (64 kbits) are usable. Furthermore, to use ROMs that expect a 16 kbit RAM, also ground A12 and A11. (If it makes routing easier, you could connect these address pins to power or another address pin instead of ground.)
The other way is to patch the game to skip the self-test, but that's not always easy. Some ROMs, such as Earthbound, have redundant code strewn about to verify the self-test in some manner.
Re: Does SNES SRAM size matter?
Posted: Sun Sep 17, 2017 10:17 am
Excellent, thanks very much!!