The 64DD disk has a large capacity of about 64 Mbytes. You can split the disk into a read-only area (ROM area) and a writable area (RAM area) with a maximum size of 38 Mbytes. With the 64DD system, it is possible to use a Game Pak and a disk at the same time, and media can be exchanged during game play. A system area is reserved on the disk for writing such information as the disk type and the disk ID.

The only way to access the 64DD with the N64 OS Leo function series is at the block level using LBAs. To make this easier to use, a file system called the MultiFileSystem (abbreviated as "MFS" henceforth) has been prepared. By using the MFS you can access the 64DD in units of files.