The Nintendo MMC1 Mapper

NES games come in cartridges, and inside of those cartridges are various circuits and hardware. Different games use different circuits and hardware, and the configuration and capabilities of such cartridges is commonly called their mapper. Mappers are designed to extend the system and bypass its limitations, such as by adding RAM to the cartridge or even extra sound channels. More commonly though, mappers are designed to allow games larger than 40K to be made.

The term mapper arises from the concept of memory mapping: translating memory hardware into the CPU's and PPU's address spaces. A memory map describes which addresses correspond to which physical locations of memory.

The Nintendo MMC1 is a mapper ASIC used in Nintendo's SxROM, NES-EVENT and 2ME circuit boards. Unlike almost all other mappers, the MMC1 is configured through a serial port in order to reduce its pin count. CPU $8000-$FFFF is connected to a common shift register. Writing a value with bit 7 set ($80 through $FF) to any address in $8000-$FFFF clears the shift register to its initial state. To change a register's value, the CPU writes five times with bit 7 clear and one bit of the desired value in bit 0 (starting with the low bit of the value). On the first four writes, the MMC1 shifts bit 0 into a shift register. On the fifth write, the MMC1 copies bit 0 and the shift register contents into an internal register selected by bits 14 and 13 of the address, and then it clears the shift register. Only on the fifth write does the address matter, and even then, only bits 14 and 13 of the address matter because the mapper doesn't see the lower address bits (similar to the mirroring seen with PPU registers). After the fifth write, the shift register is cleared automatically, so writing again with bit 7 set to clear the shift register is not needed.

When the serial port is written to on consecutive cycles, it ignores every write after the first. In practice, this only happens when the CPU executes read-modify-write instructions, which first write the original value before writing the modified one on the next cycle. This restriction only applies to the data being written on bit 0; the bit 7 reset is never ignored.

At least seven different versions of the MMC1 are known to exist: MMC1, MMC1A, MMC1B1, MMC1B1-H, MMC1B2, MMC1B2F, MMC1B3. The known differences relate to bit 4 of $E000. MMC1 is the most commonly used mapper by NES games.