FDS file format

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.FDS format

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

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

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

The format of the header is as follows:

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

Note: 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 :

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

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.

From the last file, fill the side with all 0 so that the side has exactly 65500 is reached.

Data not stored in the FDS image

In addition to data stored in the FDS image, there is other data stored on the disc, but removed when the disc is dumped because such data wasn't meaningful for emulation.