FDS disk format: Difference between revisions

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At the end of each block, a 16-bit CRC is stored. On loading, the CRC is *not* calculated by the 6502 in the BIOS, but by the RAM adapter, which monitors disc transfers and calculates the CRC. It will automatically send an error code if both CRCs doesn't match.
At the end of each block, a 16-bit CRC is stored. On loading, the CRC is *not* calculated by the 6502 in the BIOS, but by the RAM adapter, which monitors disc transfers and calculates the CRC. It will automatically send an error code if both CRCs doesn't match.
The CRC used is the common [https://reveng.sourceforge.io/crc-catalogue/16.htm#crc.cat.crc-16-kermit CRC-16/KERMIT] algorithm. The '1' bit at the end of the gap is included in the calculation.


=== True disc capacity ===
=== True disc capacity ===

Revision as of 21:32, 24 February 2023

The Famicom Disk System uses disks that are a modified version of Mitsumi Quick Disk hardware, with a custom data format stored on the disk.

Each side can hold more than 64KB of data.

For archive and emulation, dumps of these disks are often preserved using the FDS file format (.FDS).

FDS Disk Side format

Each disk side must be structured into block as follows :

1, 2, 3, 4, 3, 4, ...., 3, 4

The 3, 4 pattern should be repeated once per file present on the disk.

Blocks type 1 and 2 represent information about the disk and how many files it has, and each block type 3 and 4 pair represents a single file. See Block format below.

Gaps

Physically on the disc, there are "gaps" of 0 recorded between blocks and before the start of the disc. Length of the gaps are as follows:

  • Before the start of the disc : At least 26150 bits, 28300 typical.
  • Gap between blocks : At least 480 bits, 976 bits typical.

Gaps are teminated by a single '1' bit. In terms of bytes, it would be $80, as the data is stored in little endian format.

CRCs

At the end of each block, a 16-bit CRC is stored. On loading, the CRC is *not* calculated by the 6502 in the BIOS, but by the RAM adapter, which monitors disc transfers and calculates the CRC. It will automatically send an error code if both CRCs doesn't match.

The CRC used is the common CRC-16/KERMIT algorithm. The '1' bit at the end of the gap is included in the calculation.

True disc capacity

In the commonly used FDS file format, its disk image size is limited to 65500 bytes, but does not contain CRCs or gaps.

The actual disk capacity is somewhat larger, and there are a few variant disk formats that may have even more capacity.

Block format

Disk info block (block 1)

Offset Length (bytes) Description Details
$00 1 Block code Raw byte: $01
$01 14 Disk verification Literal ASCII string: *NINTENDO-HVC*
Used by BIOS to verify legitimate disk image
$0F 1 Licensee code See Licensee codes
$10 3 Game name 3-letter ASCII code per game (e.g. ZEL for The Legend of Zelda)
$13 1 Game type $20 = " " — Normal disk
$45 = "E" — Event (e.g. Japanese national DiskFax tournaments)
$4A = "J" — Unknown. May indicate Family Computer Network Adapter
$52 = "R" — Reduction in price via advertising
$14 1 Game version Starts at $00, increments per revision
$15 1 Side number $00 = Side A, $01 = Side B. Single-sided disks use $00
$16 1 Disk number First disk is $00, second is $01, etc.
$17 1 Disk type (FMC) $01 for FMC blue-disk releases, and $00 otherwise [1]
$18 1 Unknown $00 in all known games
$19 1 Boot read file code Refers to the file code/file number to load upon boot/start-up
$1A 5 Unknown Raw bytes: $FF $FF $FF $FF $FF
$1F 3 Manufacturing date See Date format
$22 1 Country code $49 = Japan
$23 1 Unknown Raw byte: $61. Speculative: Region code?
$24 1 Unknown Raw byte: $00. Speculative: Location/site?
$25 2 Unknown Raw bytes: $00 $02
$27 5 Unknown Speculative: some kind of game information representation?
$2C 3 "Rewritten disk" date See Date format. It's speculated this refers to the date the disk was formatted and rewritten by something like a Disk Writer kiosk.
In the case of an original (non-copied) disk, this should be the same as Manufacturing date
$2F 1 Unknown
$30 1 Unknown Raw byte: $80
$31 2 Disk Writer serial number
$33 1 Unknown Raw byte: $07
$34 1 Disk rewrite count Value stored in BCD format. $00 = Original (no copies). Also see Price
$35 1 Actual disk side $00 = Side A, $01 = Side B
$36 1 Disk type (other) $00 = yellow disk, $FF = blue disk, $FE = prototype, sample, or internal-use (white or pink) disk
$37 1 Disk version Unknown how this differs from game version ($14). Disk version numbers indicate different software revisions. Speculation is that disk version incremented with each disk received from a licensee.
$38 2 CRC (Omitted from .FDS files)

The *NINTENDO-HVC* string, stored in ASCII, proves that this is a FDS disk. If the string doesn't match, the BIOS will refuse to read the disk further.

If the FDS is started with a disk whose side number and disk number aren't both $00, it will be prompted to insert the first disk side. However, some games make this number $00, even for the second disk to make it bootable too.

All files with IDs smaller or equals to the boot read file code will be loaded when the game is booting.

The FDS also has a trademark security system similar to what Sega used on some of its consoles:

The 224-byte text at PPU $2800-$28DF must match the data in the BIOS, starting at $ED37.
This data contains an English licensing message stored in the SMB1/Zelda character encoding. The BIOS ensures that it is present in the nametable and displays it on the screen for several seconds after the boot files are loaded.
Traditionally, the first file on a disk is a nametable type file loaded into $2800, which is named KYODAKU- (きょだく or means approval). It must be one of the boot files, or the license message test will fail (error 20) before it proceeds to run the program.
The license screen test and display can be bypassed by using a boot file to enable NMIs, which will interrupt the boot loading process early.[2]

Date format

All bytes are stored in BCD format, in order of year, month, and day. To accurately calculate they year, add 1925 to the BCD value. For example, values of $63 $11 $28 would represent November ($11) 28th ($28), 1988 ($63 + 1925). The "magic year" of 1925 comes from the Japanese Shōwa period, despite the era only lasting from 1925 until 1989. Titles manufactured or updated later than 1989 are considered legitimate (e.g. $83 + 1925 = 2008).

File amount block (block 2)

This block contains the total number of files recorded on disk.

Offset Length (bytes) Description Details
$00 1 Block code Raw byte: $02
$01 1 File Amount
$02 2 CRC (Omitted from .FDS files)

More files might exist on the disk, but the BIOS load routine will ignore them, those files are called "hidden" files. Some games have a simple copy protection this way: They have their own loading routine similar to the one from the BIOS but hard-code the file amount to a higher number, which will allow for loading hidden files. This also allows the game to load faster because the BIOS will stop reading the disc after the last non-hidden file.

File header block (block 3)

Offset Length (bytes) Description Details
$00 1 Block code Raw byte: $03
$01 1 File Number
$02 1 File ID The ID specified at disk-read function call.
$03 8 File Name
$0B 2 File Address 16-bit little-endian. The destination address when loading.
$0D 2 File Size 16-bit little-endian.
$0F 1 File Type 0: Program (PRAM)
1: Character (CRAM)
2: Name table (VRAM)
$10 2 CRC (Omitted from .FDS files)

The file Number must go in increasing order, first file is 0. File IDs can be freely assigned, and this is the number which will decide which file is loaded from the disk (instead of the file number). An ID smaller than the boot number means the file is a boot file, and will be loaded on first start up.

File names are uppercase ASCII.

File data block (block 4)

Offset Length (bytes) Description Details
$00 1 Block code Raw byte: $04
$01 -- Disk data
-- 2 CRC (Omitted from .FDS files)

See also

References

  1. FamicomWorld: FMC and FSC product codes
  2. Forum post: Skipping FDS license screen?