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TheFamily Computer Disk System (orFamicom Disk System) is an add-on for the Famicom that utilized distinctive-looking "Disk Card" floppy disks (primarily yellow, though sometimes blue) as its media. A number of its games were later ported to the Famicom and NES cartridge format, and a number of early Famicom and NES cartridges were later released (or were planned to be released) in the FDS disk format.

The Twin Famicom, meanwhile, combined the Famicom and FDS into a single unit that supported both formats and the Famicom's other peripherals.

One notable aspect of the BIOS itself, which later made cameos inPaper Mario: The Thousand-Year Door andTetris DS, is the "PLEASE SET DISK CARD" screen: While waiting for the player to insert a disk, Mario and Luigi (using slightly-altered versions of theirMario Bros. sprites) change the color scheme and chase each other around. The FDS' mascot, Diskun, has similarly made cameos in various games, as well as appearing on a sign inThe Super Mario Bros. Movie.

Three versions (01, 01A, 02) of the FDS BIOS exist, as indicated by the markings on the RP2C33 chip containing it. 01 and 01A correspond to standalone units, while 02 corresponds to the Twin Famicom.

RAM Check

HoldStart + Select during the boot screen and pressReset. If no problems are detected, the screen will display "PRAM CRAM OK"; if an error occurs, the address of the first memory cell where the error was detected will be displayed.

Programmer Credit

01	01A/02

Use the RAM Check code above, but before the text appears on-screen releaseStart + Select then holdRight + A to see the programmer credit of Takao Sawano. This works on both the standalone unit and the Twin Famicom, the latter of which otherwise makes no reference to Nintendo.

The 01 revision only includes the programmer credit. The 01A revision onwards features some space-saving code optimizations (including the removal of the code responsible for adding a drop shadow to the font, seen below) that allowed Sawano to add both the name of the ASIC and the team responsible for FDS development,Nintendo R&D2.

Revisional Differences

01, 01A, and 02

01	01A	02

The 01 and 01A revisions display the Nintendo logo, whereas the 02 revision uses the "FAMICOM" logo present on the Twin Famicom.

The 01A revision removed the heavy shading on the bottom edges of the Nintendo logo and the drop shadow behind the alphanumeric character set.

Animal Crossing and Wii Virtual Console

To do: See which BIOS is used in the Wii U/3DS Virtual Console and the Nintendo Switch Online emulators. Verify that the emulators hook into the modified instructions.

The 01A revision BIOS was modified for use by emulators inAnimal Crossing^[6] and theWii Virtual Console^[7]. The table below lists the modifications:

The unofficial STP/KIL instruction normally causes the CPU to crash/halt execution until the system is reset. It was likely used by the emulators to hook into disk access routines and provide high-level replacements (i.e. to bypass disk loading/saving). The JMP instruction simply skips over a section of timed code originally meant to wait for the CRC checksum to be written to disk.

References

• Games developed by Nintendo R&D2
• Games published by Nintendo
• Games published by Sharp
• Games published by Simon & Toys
• Games released in 1986
• Games released in February
• Games released on February 21
• Games released in July
• Games released on July 1
• Games released in 1991
• Games with hidden developer credits
• Games with debugging functions
• Games with revisional differences
• To do
• Famicom Disk System games
• BIOS/Firmware ROMs