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A mapper is a piece of hardware soldered to a cartridge's printed circuit board that performs address decoding, bank switching, and possibly other tasks. Some mappers can generate interrupts for timing; some Famicom games' mappers even have extra audio channels.

NES cartridges can include extra hardware which allows the use of large program and graphics ROMs and extra features. This extra hardware is usually referred to as a "mapper", as it is used primarily to map the relatively small CPU and PPU address spaces to a portion of the larger address space of the ROM(s) on the cartridge.

Mappers vary in how they translate addresses. The various CPU memory mapping schemes expand the maximum program size above the standard 32 KiB to larger powers of 2, while PPU memory mapping schemes can add new graphic features.

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).

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").

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