VMU stands for Virtual Memory Unit, also referred to as VMS; Virtual Memory System. They are slightly larger than a matchbox and have a small LCD screen and controller pad on one side. They are primarily used for saving Dreamcast games, but also have the facility to run games on their own LCD screen.
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booyaka.com: VMU Development
Tools, saved games, mini-games, discussions, technical information and downloads for the Dreamcast Virtual Memory Unit.
The Dreamcast Browsers Group
Only place to find browser add-ons scripts (run from the VMU) for the Dreamcast Browsers. Also the home of the world's first open-source VMU file uploader.
VMU Space Invaders
The site contains the VMU Space Invaders game. Also has free source code to the game as well as useful VMU programming links.