nesdev.org

A chip supports 4-screen if it has additional VRAM. But such info(whether a chip has VRAM or not) is not included in the NESDev wiki.

I'm currently using Mapper 85(Konami VRC7) in a hacking project. The 2 original games which used VRC7 didn't use 4-screen at all, and the nametable mirroring control bits only support H-, V-, and 1-screen.
So does VRC7 actually support 4-screen mirroring? Emulation for VRC7 4-screen mirroring on Nestopia works fine, but I'm still curious.

In hardware? All games can have an extra memory IC added to the cartridge to provide for four-screen mirroring in any authentic NES or Famicom.

Some clones don't support this, however: they require that the nametables only ever come from the 2 KiB in the famiclone instead.


In software, it varies by emulator. Theoretically all emulators should honor the 4-screen bit, but not all do. Sometimes you get hilarity where you get 4-screen mirroring until the first time the program writes to the mirroring control register, and then it's gone forever.

Since 4-screen does not require any mapper interaction at all (i.e. the name tables are always in the same places the whole time), any mapper can use it. If there's extra VRAM on the cart and the console's internal VRAM is permanently disabled (bypassing whatever control the mapper has over it - this requires a bit of rewiring), 4-screen will work regardless of the mapper.

You have to be careful with CHR bankswitching though, because some mappers respond to $2000-$2FFF accesses like they were $0000-$0FFF accesses (mapper 118, an MMC3 variant, uses this to have finer control over the name table mirroring than normally possible with an MMC3), so if your extra VRAM is in the same chip as the CHR-RAM, the 4-screen might get screwed up if you switch CHR banks (not that you'd have much reason to with CHR-RAM). To completely avoid this problem, you'd have to keep the extra VRAM in a separate chip (something you'd have to do anyway when using CHR-ROM) and connect the raw address lines to it (as opposed to the address lines that the mapper outputs).

Now, whether emulators support these configurations is a different issue... It's possible that some emulator authors have chosen to give full control to the mapper regardless of what the header says about mirroring. You might have to test and see which one you can use for development, or modify an open source one to fit your needs.

Technically, most mappers mappers should support 4-screen mapping. The only exception is mapper who already deals with nametable mappring in a standard way : MMC5, and TLSROM/TKSROM (I think it's mapper 118 or something like this).

Napoleon Senki does it in an interesting way: $0000-$07FF is CHR ROM, $0800-$27FF is an 8K RAM on the cart, and $2800-$2FFF is the NES's internal RAM. The program sees it as $0000-$07FF CHR ROM, $0800-$1FFF CHR RAM, and $2000-$3FFF a four-screen nametable.

Yes, this mapping is absolutely awesome !!
Too bad it was used on such a bad game.

Also it doesn't matter where internal and external RAM is mapped, as long as $0800 - $2fff is RAM. ($3000-$3fff is just unused mirror of nametables)

(Pedanticism) On Napoleon Senki, PPU $3xxx is actually open bus. (A completely unimportant detail.)

Bregalad wrote:Yes, this mapping is absolutely awesome !!
Too bad it was used on such a bad game.

Off topic, but I don't see why you think such a well made game is a bad game.

Pokun wrote:Off topic, but I don't see why you think such a well made game is a bad game.

From what I can tell, Bregalad thinks all games suck...

Would a game about managing Pedro's class president campaign suck any less?

Probably not. lol