Overcominb the Nes’ses mmc3 512KB limit

[johannesmutlu]

While the mmc5 chip could handle 1024KB of data, the mmc3 chip could only handle 512KB of data, but what if we could replace that 512KB rom with 512KB flash rom or ram, then we could stick a 4MB of rom with it,with data devided into sections of 512KB (or less in cases we still need some parts of previous data) then load one after the other 512 KB section of it into that 512KB ram or flash rom once needed, this way we could in theory make mmc3 nes games much bigger then 512KB rather then using the more expensive mmc5 chip?

[emerson]

I don't see this working with a stock mmc3. Maybe one of the non prg related registers could be repurposed. Seeing how most modern mmc3 homebrew pcbs use a cpld you could easily program one to function similar to the mmc3 but with expanded memory capabilities. Because mmc3 only looks at the lower 6 bits when selecting a prg bank, you will need to add additional registers to control the expanded memory.

 

Another problem you will run into is emulator support. More specifically, convincing emulator devs to support your new mapper since it is no longer mapper 004.

 

What do you need 4MB of memory for?

[johannesmutlu]

On 6/2/2021 at 7:15 PM, emerson said:

I don't see this working with a stock mmc3. Maybe one of the non prg related registers could be repurposed. Seeing how most modern mmc3 homebrew pcbs use a cpld you could easily program one to function similar to the mmc3 but with expanded memory capabilities. Because mmc3 only looks at the lower 6 bits when selecting a prg bank, you will need to add additional registers to control the expanded memory.

 

Another problem you will run into is emulator support. More specifically, convincing emulator devs to support your new mapper since it is no longer mapper 004.

 

What do you need 4MB of memory for?

Maybe for enhanced versions of power demanding games.

[Tanooki]

Another option would be a FPGA style setup an Altera chip that basically clones the memory footprint and capabilities of the MMC5.

 

I have a custom famicom cart here that had a full potential use MMC5 insane hack of Super Mario Bros 3 which turned it into Mario Allstars+mario bros arcade too.  It uses from what I've got off a few people here and there, a cheap chinese knockoff (everdrive likely) flash kit PCB with new parts added, the SD card slot removed, and it works.  Super Mario Allstars on FC/NES has SMB1-3 +2j +MB arcade -- everything has 4x saves per stage(not world like on SNES.)  The game rom size is a beast, uses the full space the chip can account for.

 

You can look into what I'm talking about on romhacking.net under Super Mario Allstars NES from Infidelity.