FDS file format

From NESdev Wiki
Revision as of 22:57, 28 January 2023 by Qalle (talk | contribs) (category, tidy up)
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to navigationJump to search

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 (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)
  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

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

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:

  • 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 exactly 65500 bytes is reached.