Multi-Memory Controller

Multi-memory controllers or memory management controllers (MMC) are different kinds of special chips designed by various video game developers for use in Nintendo Entertainment System (NES) cartridges. These chips extend the capabilities of the original console and make it possible to create NES games with features the original console cannot offer alone.

With the advent of NES emulators, the code written to emulate these chips became known as mappers.

Early NES MMCs are composed of 7400 series discrete logic chips. The UNROM, implemented with two such chips, divides the program space into two 16 KB banks. The MMC allows a program to switch one bank while keeping one bank always available. Instead of a dedicated ROM chip to hold graphics data (called CHR by Nintendo), games using UNROM store graphics data on the program ROM and copy it to a RAM on the cartridge at run time.

The MMC1 is Nintendo's first custom MMC integrated circuit to incorporate support for saved games and multi-directional scrolling configurations. The chip comes in at least five different versions: MMC1A, MMC1B1, MMC1B2, MMC1B3 and MMC1C. The differences between the different versions are slight, mostly owing to savegame memory protection behavior. The MMC1 chip allows for switching of different memory banks. Program ROM can be selected in 16KB or 32KB chunks, and character ROM can be selected in 4KB or 8KB chunks. An unusual feature of this memory controller is that its input is serial, rather than parallel, so 5 sequential writes (with bit shifting) are needed to send a command to the circuit.

The MMC2 is only used in Mike Tyson's Punch-Out!! and the later rerelease which replaced Mike Tyson. A single 8KB bank of program ROM can be selected (with the remaining 24KB locked) and character ROM can be selected in two pairs of 4KB banks, which would be automatically switched when the video hardware attempts to load particular graphic tiles from memory, thus allowing a larger amount of graphics to be used on the screen without the need for the game itself to manually switch them.

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