The VMU is powered by two CR-2032
watch batteries (which will be infamous to Saturn
owners as what powers the save memory) and has an 8-bit
processor. It has two action buttons A and B, two smaller buttons labeled Select and Sleep, and a tiny D-Pad. The top can be removed exposing the connector
which allows it to be plugged into one of the sockets on a Dreamcast
controller or into another VMU to transfer data. If you're desperate, it also has a clock
and a few screensavers of fish
It's worth noting that it plugs into the DC controllers upside-down. Another thing worth noting is the horrendously short battery life. What exactly the VMU does to use the batteries up so fast isn't clear (CR-2032s aren't cheap) but one is well advised to hold on to the little plastic tab that new units have inserted into the battery compartment (which interupts the circuit and prevents the batteries draining while the things sit on shelves). This doesn't affect their operation when inserted into a controller, which makes the rapid battery consumption even more baffling.
To be honest, the 200 'blocks' of the VMU just isn't enough, rekindling memories of the tiny 125 blocks on the Sega CD's internal memory. Sega eventually released a second model with either four or eight times as much, but this was Japan only (naturally) and didn't have the buttons or even the screen.
Unfortunately the VMU idea was never fully realized. Uses of the VMU by games included:
Shenmue: Fighting moves would be displayed on the screen while you were training.
Tony Hawk's Skateboarding: Words like WEAK and LOSER would flash onscreen if you bailed.
Silent Scope: a blurry, two-colour rendition of the view through your scope would appear.
Sonic Shuffle: the numbers on each of your cards would be shown (actually useful!).
Most of the time, though, you simply got a static logo (Crazy Taxi) or an animated logo (Chu Chu Rocket!, Jet Grind Radio, Tokyo Bus Guide).
The idea of actual games to run on the VMU was underused as well.
Sonic Adventure: Let you play with the Chao creatures you raised in the main game.
Sega GT: Gave you what was basically a racing driver tamagotchi, with victory earning you exclusive bonus cars. Unfortunately, given Sega GT's full title was Sega GT: Homologation Special, the minigame was unwisely called 'Pocket Homo'.
Tokyo Bus Guide: A pointless road sign recognition test.
With any luck Nintendo will make a better effort with their Game Boy Advance/Game Cube links, although the idea of using the GBA's tiny dpad to play GC games doesn't fill my heart with glee.