More files might exist on the disk, but the BIOS load routine will ignore them, those files are called "hidden" files. This block contains the total number of files recorded on disk. Titles manufactured or updated later than 1989 are considered legitimate (e.g. The "magic year" of 1925 comes from the Japanese Shōwa period, despite the era only lasting from 1925 until 1989. To accurately calculate they year, add 1925 to the BCD value. Date formatĪll bytes are stored in BCD format, in order of year, month, and day. 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. It must be one of the boot files, or the license message test will fail (error 20) before it proceeds to run the program. Traditionally, the first file on a disk is a nametable type file loaded into $2800, which is named KYODAKU- (きょだく or 許 諾 means approval). 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. This data contains an English licensing message stored in the SMB1/Zelda character encoding. The 224-byte text at PPU $2800-$28DF must match the data in the BIOS, starting at $ED37. The FDS also has a trademark security system similar to what Sega used on some of its consoles: However, some games make this number $00, even for the second disk to make it bootable too.Īll files with IDs smaller or equals to the boot read file code will be loaded when the game is booting. 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. If the string doesn't match, the BIOS will refuse to read the disk further. The *NINTENDO-HVC* string, stored in ASCII, proves that this is a FDS disk. Speculation is that disk version incremented with each disk received from a licensee. Disk version numbers indicate different software revisions. Unknown how this differs from game version ($14). $00 = yellow disk, $FF = blue disk, $FE = prototype, sample, or internal-use (white or pink) disk In the case of an original (non-copied) disk, this should be the same as Manufacturing date It's speculated this refers to the date the disk was formatted and rewritten by something like a Disk Writer kiosk. Speculative: some kind of game information representation? Refers to the file code/file number to load upon boot/start-up $01 for FMC blue-disk releases, and $00 otherwise $52 = "R" - Reduction in price via advertising May indicate Family Computer Network Adapter Used by BIOS to verify legitimate disk imageģ-letter ASCII code per game (e.g. The actual disk capacity is somewhat larger, and there are a few variant disk formats that may have even more capacity.īlock format Disk info block (block 1) Offset In the commonly used FDS file format, its disk image size is limited to 65500 bytes, but does not contain CRCs or gaps. The '1' bit at the end of the gap is included in the calculation. The CRC used is the common CRC-16/KERMIT algorithm. It will automatically send an error code if both CRCs doesn't match. 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. In terms of bytes, it would be $80, as the data is stored in little endian format.Īt the end of each block, a 16-bit CRC is stored. Gap between blocks : At least 480 bits, 976 bits typical.Before the start of the disc : At least 26150 bits, 28300 typical.Physically on the disc, there are "gaps" of 0 recorded between blocks and before the start of the disc. The 3, 4 pattern should be repeated once per file present on the disk.īlocks 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. Each disk side must be structured into block as follows :
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