Family Computer Disk System

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

.FDS format

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 in 65500 byte units
• 5-15: Zero filled

References

• FDS Lister by ccovell