- The circuitry that effects the connections between the Game Pak's PRG- and CHR-ROMs/RAMs and the NES Control Deck, whether it holds additional ICs to do so or not.
- An ASIC that handles the primary load of this task.
- The description that emulators use for a particular combination of circuitry and hardware on the game pak doing this and other tasks.
The hardware and circuitry on a cartridge's printed circuit board perform several other functions:
- (always) Mirroring: controlling which way the nametables are arranged, as the NES only has two nametables' worth of CIRAM but four spaces.
- (rarely) Providing cartridge-side CHR-RAM or ROM to supplement or replace the nametable CIRAM (4-Screen mirroring).
- (often) Bank switching: dynamically changing the mapping of cartridge ROM and RAM chips into CPU/PPU address spaces. By doing so, the cartridge may have more data than the address space would allow.
- (often) Using a battery-backed volatile RAM chip, or rarely an EEPROM, to maintain data between play sessions
- (commonly) Generating interrupts, esp. by scanline
- (rarely) Providing additional sound-generation. The Famicom sent its audio through the cartridge port to allow the cartridge to mix in additional channels; the NES does not without modification.
- (rarely) Can use the special features of the cartridge-side, such as extended attribute.
- and occasionally other sundry functions like fast multiplication or graphics enhancement.
Most mappers fall into one of two categories: discrete logic, and ASIC based. Some discrete logic mappers are susceptible to bus conflicts. Nintendo uses the term Memory Management Controller (or MMC for short) for its ASIC mappers ("Why Game Paks Never Forget" article in Nintendo Power) (note: it may have originally stood for "Multi Memory Controller", at least according to Japanese).
Discrete logic mappers are often referred to by the name of a board that they are commonly used in (e.g. "UNROM"). ASIC mappers are named after the ASIC (e.g. "MMC1" or "FME-7"), except in boards that use an ASIC in an unusual way (such as "TQROM", "TLSROM", or "NES-EVENT"). The emulation community generally refers to mappers by a numbering scheme that originated with the iNES emulator (e.g. "mapper 002").
The notation used in Disch's docs describing mappers is explained here.
The suggested method to assign a mapper number is not to assign one unless you have at least either A. a hardware implementation or B. an emulator implementation and a sketch of hardware. (You should probably also have written at least part of the game too)
iNES 1.0 mapper grid
This is the plane 0 table. These mappers do not require a NES 2.0 header.
Most icons next to mapper numbers refer to publishers. Nintendo-made boards with numerous publishers get the Nintendo icon; Nintendo-made boards dominated by one publisher get that publisher's icon. Other icons refer to status:
- This mapper is "bad": it was used for something other than an actual cartridge. Some are mapper hacks designed for early disk-based copiers. Others are duplicate mappers that were assigned by mistake.
- "Bad" mappers reserved for use by tools, such as emulator internal use or private use during development of a new mapper.
- Mappers developed by members of the NES homebrew scene for cart releases.
- "Pirate MMC3" mappers. Many are used for unauthorized demakes of 16-bit fighting games.
- Mappers used primarily by illegally copied games, usually multicarts, sometimes single-game mapper hacks.
- Mappers for which we have some information but no identified manufacturer.
- Mappers that are not yet documented on this wiki. Do not assume undocumented mappers are currently unassigned; consult the Nestopia and Nintendulator source code.
Plane 1 (NES 2.0 mappers 256-511) is reserved for mappers used by games released outside the East Asian market. By the end of 1996, commercial development of NES games had ceased. The vast majority of mappers used in commercial NES games prior to the end of 1996 had been discovered prior to 2013, when the "planes" system for NES 2.0 mapper number allocation was described. A few ultra-rare prototypes of commercial games may still be discovered, but most of these mappers will be new mappers devised for homebrew games.
Plane 2 (NES 2.0 mappers 512-767) is reserved for new East Asian mappers. Famiclones were still popular in China in the 2000s decade, and companies were still producing new games and new pirate multicarts for the Chinese market. Unlike games for the North American, South American, and European markets, games for the Chinese market need to provide for writing systems with hundreds of characters, and this needs either CHR RAM, ExGrafix, or precomposed phrases. (See CHR ROM vs. CHR RAM.) In 2015, UNIF was declared dead, yet may of CaH4e3's dumps still had only a UNIF board type.
- Nintendo: MMC1, MMC2, MMC3, MMC4, MMC5 with audio, MMC6
- Konami: VRC1, VRC2, VRC3, VRC4, VRC6 with audio, VRC7 with audio, VRC IRQs
- Others: Color Dreams, Namco 163 with audio, NINA-001, NINA-03/06, SUBOR, Sunsoft FME-7 with audio (Sunsoft 5B)
- List of mappers with board names
- List of discrete logic mappers
- List of iNES mapper numbers
- Comparison of Nintendo mappers
- List of mapper names and corresponding iNES mapper numbers
- Hardware pinout, including mapper pinouts
- Nintendulator source code
- Nestopia source code
- FCEUX source code
- Planes proposal