Family Computer Disk System

The Famicom Disk System was a Japan-exclusive storage device for the Famicom, designed to reduce Nintendo's cost of making copies of games by switching from mask ROM chips to a storage medium based on Mitsumi's Quick Disk. Unfortunately for Nintendo, it also reduced the pirates' cost of making copies of games.

Overview

• PRG ROM : 8 KB BIOS at $E000-$FFFF
• PRG RAM: 32 KB at $6000-$DFFF
• CHR capacity: 8 KB RAM
• Storage capacity : ~64 KB per disk side
• Nametable mirroring: Controlled by mapper
• Subject to bus conflicts: No

Games are stored on one or multiple disk sides. The FDS BIOS is used to load data from disks to PRG RAM or VRAM, and games can execute from there.

Hardware

The Famicom disk system comes in two parts : The disk drive and the RAM adapter.

The RAM adapter is a special shaped cartridge that contains the RAM chips and an ASIC with DRAM controller, IRQ hardware, sound generation hardware, serial interface for the disk drive, and parallel port. The Disk Drive has to be powered separately and is only connected to the Famicom/NES via a serial cable to the RAM adapter.

Most disk drives contain two motors: a spindle motor that spins the disk at a specific speed, and a stepper motor which moves the read/write head between each circular data track. By comparison, the FDS only contains a single motor that does both at once, so it instead stores the data in a single spiral-shaped track. There is a mechanism that detects when the head reaches the end of the disc and makes it return to the start (making an audible click). Because of this limitation, random access to the disc is impossible, making FDS disk drive data access behave similarly to a reel of tape (but much faster). Data can only be accessed by spinning the disc, waiting for the head to reach the inner edge of the disc, then waiting again until the desired data file is reached. A complete cycle through the entire disc takes about 7 seconds.

The disc drive only contains basic electronics, there is no "intelligence" in it; therefore, the serial interface almost directly represents what is stored on the disc.

Disks

The FDS disk is a modified version of the Mitsumi Quick Disk.

See:

• FDS disk format - the disk data format and file structure
• FDS file format (.FDS) - an archival file format for storing and emulating FDS disks

Registers

$402x registers are write-only $403x registers are read-only

IRQ reload value low ($4020)

7  bit  0
---------
LLLL LLLL
|||| ||||
++++-++++- 8 LSB of IRQ reload value

IRQ reload value high ($4021)

7  bit  0
---------
LLLL LLLL
|||| ||||
++++-++++- 8 MSB of IRQ reload value

Unlike $4022, $4020 and $4021 are not affected by the $4023.0 (disk registers enabled) flag - the reload value can be altered even when disk registers are disabled.

IRQ control ($4022)

7  bit  0
---------
xxxx xxER
       ||
       |-- IRQ Repeat Flag
       +-- IRQ Enabled

When $4022 is written to with bit 1 (IRQ enabled) set, the reload value is copied into the IRQ's counter. Each CPU clock cycle the counter is decremented by one if the enable flag is set.

When the counter's value is 0 and the IRQ enable flag is on, the following happens on every CPU cycle:

• An IRQ is generated.
• The IRQ counter is reset to its reload value (contained in $4020+$4021)
• If the IRQ repeat flag is NOT set, the IRQ enabled flag is cleared and the counter stops.

Notes:

• This register is affected by the $4023.0 (Enable disk I/O registers) flag - if disk registers are disabled, it is impossible to start the IRQ counter (writing to $4022 has no effect).
• Clearing $4023.0 will immediately stop the IRQ counter and acknowledge any pending timer IRQs.
• Writing to $4022 with bit 1 (IRQ enabled) cleared will stop the IRQ counter and acknowledge any pending timer IRQs.
• Enabling IRQs when the reload value is set to 0 will cause an IRQ immediately. Doing this with the repeat flag enabled will cause an infinite loop of IRQs on every CPU cycle.
• Since the disk transfer routine also uses IRQs, it's very important to disable timer IRQs before doing any access to the disk.

There are only 3 known ways to acknowledge the timer IRQ:

• Read $4030
• Disable IRQs by writing to $4022
• Disable disk registers by writing to $4023

Master I/O enable ($4023)

7  bit  0
---------
xxxx xxSD
       ||
       |+- Enable disk I/O registers
       +-- Enable sound I/O registers

This register sounds obscure. FDS bios just writes $00 then $83 to it.

As noted above, disabling disk registers also disables timer IRQs.

Write data register ($4024)

The data that this register is programmed with will be the next 8-bit quantity to load into the shift register (next time the byte transfer flag raises), and to be shifted out and appear on pin 5 of the RAM adapter cable (2C33 pin 52).

FDS Control ($4025)

7  bit  0
---------
IS1B MRTD
|||| ||||
|||| |||+- Drive Motor Control  
|||| |||     0: Stop motor
|||| |||     1: Turn on motor
|||| ||+-- Transfer Reset
|||| ||        Set 1 to reset transfer timing to the initial state.
|||| |+--- Read / Write mode
|||| |     (0: write; 1: read)
|||| +---- Mirroring (0: vertical; 1: horizontal)
|||+------ CRC control (set during CRC calculation of transfer)
||+------- Always set to '1'
|+-------- Read/Write Start  
|            Turn on motor.  Set to 1 when the drive becomes ready for read/write
+--------- Interrupt Transfer  
             0: Transfer without using IRQ
             1: Enable IRQ when the drive becomes ready for 

A FDS game that wants to change mirroring probably don't want to touch motor related bits, so it should do a read-modify-write from the pseudo registers (see FDS BIOS).

External connector ($4026)

Output of expansion terminal where there's a shutter on the back of the ram card. The outputs of $4026 (open-collector with 4.7K ohm pull-ups (except on bit 7)), are shared with the inputs on $4033.

Disk Status Register 0 ($4030)

7  bit  0
---------
IExB xxTD
|| |   ||
|| |   |+- Timer Interrupt (1: an IRQ occurred)
|| |   +-- Byte transfer flag. Set every time 8 bits have been transfered between the RAM adaptor & disk drive (service $4024/$4031). 
|| |       Reset when $4024, $4031, or $4030 has been serviced.
|| +------ CRC control (0: CRC passed; 1: CRC error)
|+-------- End of Head (1 when disk head is on the most inner track)
+--------- Disk Data Read/Write Enable (1 when disk is readable/writable)

Read data register ($4031)

This register is loaded with the contents of an internal shift register every time the byte transfer flag raises. The shift register receives its serial data via pin 9 of the RAM adapter cable (2C33 pin 51).

Disk drive status register ($4032)

7  bit  0
---------
xxxx xPRS
      |||
      ||+- Disk flag  (0: Disk inserted; 1: Disk not inserted)
      |+-- Ready flag (0: Disk readу; 1: Disk not ready)
      +--- Protect flag (0: Not write protected; 1: Write protected or disk ejected)

External connector read ($4033)

7  bit  0
---------
BIII IIII
|||| ||||
|+++-++++- Input from expansion terminal where there's a shutter on the back of the ram card.
+--------- Battery status (0: Voltage is low; 1: Good).

When a bit is clear in $4026 port it will read back as '0' here (including battery bit) because of how open collector input works. Battery bit should be checked when the motor is on, otherwise it always will be read as 0.

Sound ($4040-$4092)

For details on sound information, see FDS audio.

BIOS

The FDS contains a fixed 8KB BIOS at $E000-FFFF. This controls the Famicom at power-on and reset, dispatches the NMI and IRQ, and offers an API for accessing the data on disk.

See: FDS BIOS

See Also

• FDS BIOS
• FDS disk format
• FDS file format (.FDS)
• FDS audio
• FDS RAM adaptor cable pinout
• RP2C33 pinout
• iNES mapper 20 - Reserved for FDS dumps, but not widely used for it.
• Github repository: Simple FDS example for ca65
• Forum post: Skipping the FDS license screen

References

• FDS technical reference.txt by Brad Taylor
• Enri's Famicom Disk System page (Japanese)
• Enri's Famicom Disk System page (Japanese) (old/outdated)
• fds-nori.txt - FDS reference in Japanese by Nori
• FDS Lister by ccovell
• Forum post: .fds format: Can checksums be heuristically detected? - Includes a CRC implementation in C.
• Forum post: FDS IRQ reload flag/value