It has a 32-bit co-processor to help it read magnetic disks and transfer the data to the main console. It was designed due to the high cost of cartridges for the main Nintendo 64 system, and their low storage capacity. For example, Super Mario 64 was an 8 Mb cartridge. The magnetic disks had 64 Mb of storage space. By the end of the N64 era, however, cartridges had matched the 64 Mb storage capacity of the magnetic disks, obviating the need for the magnetic drive.
The drive works almost like a Zip drive, and has an enhanced audio library for the games to use. The main N64 deck uses its RCP and MIPS4300i to process data from the top cartridge slot and the I/O devices. To hook up with the 64DD it needed an extra 4 Mb of RAM for a total of 8 Mb. The 64DD can boot up on its own, without the need of a cartridge on the top deck. This is because it has a standard OS, unlike the N64 on which every game has its own. The games on normal cartridges could hook up with DD expansions, for extra levels, minigames, even saving personal data.
The 64DD had its own development kit that worked in conjunction with the N64 development kit.
The released version of 64DD included a modem for connecting to the testing network RANDnet, an audio-video (female RCA jack, and line in) adaptor to plug into the main cartridge slot, a mouse that plugged into the controller inputs, and some games: Sim City 64, F-zero Xpansion, Doshin The Giant, and the Maker Trilogy.
Games that were intended to be great hits on the system were: Ura Zelda (Master Quest), the expansion disk to Ocarina Of Time (released on Gamecube); Pokemon Stadium 2 (only released in Japan); Cabbage, and others.
The 64DD was finally released with a high price, and was mostly available through the internet, retailing at around 800 USD. It is very rare to find one with all of the above included, but it is a collector's item for every Nintendo fan.