FDS file format

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fwNES was an NES emulator developed by Fan Wan Yang and Shu Kondo. Its most lasting contribution to the NES scene was its disk image file format, an image of the Quick Disk media.

fwNES FDS file format

The FDS format (filename suffix .fds) is a way to store Famicom Disk System disk data. It consists of the following sections, in order:

  1. Header (16 bytes), sometimes omitted
  2. Disk data (65500 * x bytes)

The format of the header is as follows:

Bytes Description
0-3 Constant $46 $44 $53 $1A ("FDS" followed by MS-DOS end-of-file)
4 Number of disk sides
5-15 Zero filled padding

Some .FDS images may omit the header.

Disk data

The disk data follows the FDS disk format, but gaps and CRCs are not included in the .FDS image.

Most games are an even number of sides. Ports from NROM were one side. No commercial FDS game had an odd number of sides greater than 1. Disk sides comes in the following order:

  • Disk 1 Side A
  • Disk 1 Side B
  • Disk 2 Side A
  • Disk 2 Side B
  • etc...

After the last file block, fill a side with all 0 so that the disk side reaches exactly 65500 bytes.