The Family Computer Disk System (FDS, FCD) was released on February 21, 1986 by Nintendo as a peripheral to their overwhelmingly popular Family Computer ("Famicom") console in Japan. It was a unit that sat underneath the Famicom and used proprietary floppy disks for data storage. It was announced, but never released, for the North American Nintendo Entertainment System. In Japan, Sharp released Twin Famicom (ツインファミコン), a composite console of both Famicom and disk system under license.
The device was connected to the Famicom deck by plugging a modified cartridge known as the RAM adapter into the system's cartridge port, which attached via a supplied cable to the disk drive. The RAM adapter contained 32 kilobytes of RAM for temporary program storage, 8 kilobytes of RAM for tile and sprite data storage, and an ASIC known as the 2C33. The ASIC acted as a disk controller for the floppy drive, and also included additional sound hardware featuring primitive FM synthesis capabilities. The floppy disks used were double-sided, with a capability of 64 kilobytes per side. Many games spanned both sides of a disk, requiring the user to switch sides at some point during gameplay. A few games used two full disks (four sides). The Famicom Disk System was capable of running on six C-cell batteries or the supplied AC adapter, even though the Famicom itself was not portable.