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NES programming: Going beyond 24KB.

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Why are you making your game 24KB in the first place?
You can get 32KB PRG and the full 8KB CHR with a regular NROM cartridge (like SMB does), without needing to bank switch.

If you want more than that, you'll need a cartridge with any mapper chip. If you aren't limited by production means/costs, MMC3 is a really simple easy one to work with, that gives you a ton of power. How you switch banks depends on which PCB you're using, and if you are creating your game to be played on emulator, you need to tell it which PCB by assigning a mapper number and rom sizes in the header.

Here's information about how to interface with some of the more useful mappers:

MMC3 (very powerful, but can be a little weird to set up, use a good emulator like Mesen to test)
MMC1 (not as powerful as MMC3, but much simpler. The way you change banks is oddly cumbersome, but once you've managed to do it, it should work fine - the page has code examples)
BNROM (very easy to interface with, but since it uses 32KB banks, writing your code to bankswitch can be a little confusing)

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