The Expansion Pak is a RAM expansion for the Nintendo 64 which adds four additional megabytes of RAM to the console while in use, making the total amount of RAM accessible to eight megabytes. The Pak was released on its own in late 1998 in Japan, The Expansion Pak is added to the console by removing the cover marked "Memory Expansion" in front of the cartridge slot and inserting it into the slot underneath, replacing the system's pre-installed Jumper Pak. It was originally meant to be used with the Nintendo 64DD, its games requiring the Expansion Pak to play, but after the failure of the disk drive it was instead used to enhance select Nintendo 64 cartridge games.
Donkey Kong 64 and The Legend of Zelda: Majora's Mask are the only cartridge games to require the Expansion Pak in order to play, although other games like Perfect Dark and Starcraft 64 only require the Expansion Pak to play specific modes (Perfect Dark requires the Expansion Pak for single-player, while Starcraft 64 requires it for multiplayer). As such, when Donkey Kong 64 was first released, a free Expansion Pak was included with each copy for a limited time since the Expansion Pak fixes a game-breaking bug during development; later shipments of the game did not include it.