VMU

The VMU, initialism of Visual Memory Unit (called VMS, Visual Memory System in Japan or Virtual Memory Unit), is a memory card peripheral for the Sega Dreamcast. While its most basic function is as a removable storage device the VMU may also serve as an auxiliary display during normal gameplay and, through use of additional software, act as a handheld. Console-like features of the VMU include a small black and white screen, speaker, d-pad, and four buttons. They were initially only available in white, but expanded to transparent Blue, Green, Black, and Red.

Mini-games
Several titles for the Dreamcast included mini-games that could be downloaded onto a VMU. The Sonic Adventure series, for instance, included the Chao Adventure mini-game. In the game players could transfer Chao eggs to the VMU and play to increase the stats of their hatched Chao, whereupon they could upload their improved Chao back into the Dreamcast game. It looks fun :)

Dreamcast titles that include VMU games and extra features

 * Evolution 2 (12 Hour Clock)
 * Godzilla Generations (Japanese Version)
 * Power Stone
 * Power Stone 2
 * Marvel Vs. Capcom 2
 * Namco Museum
 * NFL 2K
 * NFL 2K1
 * NFL 2K2
 * Quake III Arena
 * Resident Evil 2
 * Sega GT
 * Shenmue
 * Skies of Arcadia
 * Sonic Adventure
 * Sonic Adventure 2
 * Soulcalibur (Japanese Version)
 * Tech Romancer
 * Time Stalkers
 * TrickStyle
 * Zombie Revenge
 * Tokyo Bus Guide

Hardware
The VMU is plugged directly into one of two slots on a Dreamcast controller. Two VMUs may be plugged in each controller. When operated independently of the Dreamcast console, the VMU acts as a file manager with limited game console functionality. VMUs may also connect to each other directly to facilitate file transfer or multiplayer gaming.

Upon initial use the player is prompted to select from a group of simple bitmap images to serve as the default background for the VMU. This image is displayed while the Dreamcast console is in the operating system menu; it is usually replaced by a simplified logo during regular gameplay. Some titles display custom animations on the VMU's screen during gameplay, while some titles also offer new background images for the unit.

The VMU runs on two CR-2032 lithium batteries which are inserted into the rear of the VMU under a screw-secured lid. Without battery power, the VMU still works as a memory card and auxiliary display, but cannot play downloaded mini-games.

Technical specifications

 * CPU: 8-bit (Sanyo LC8670 "Potato")
 * Memory: 128 KiB (200 blocks of 512 bytes each)
 * Display: 48 dot width × 32 dot height, LCD
 * Display size (width×height): 37 mm × 26 mm (1.46 in × 1.02 in)
 * Case dimensions (width×height×depth): 47 mm × 80 mm × 16 mm (1.85 in × 3.15 in × 0.63 in)
 * Power source: 2 × CR2032 batteries with auto-off function, Lasts From 20-30 Minutes
 * Sound: 1-channel PWM sound source
 * Weight: 45 g (1.6 oz)

Extra blocks
The VMU has 41 blocks of unused space, but with programs like Dream Explorer (aka VMU Tool) users are able to unlock the extra space increasing the overall capacity to 241 blocks. However it has been said that a handful of games might be unable to detect the memory card if this is done, although none has been reported apart from DreamKey/DreamPassport and Metropolis Street Racer. Dream Explorer official site

Dream Explorer 0.8.5 hosted image

Nexus Memory Card
The Nexus Memory Card is a third party version of the Visual Memory Unit that features four times the memory (4 mebibits / 800 blocks). The Nexus Memory Card is physically larger than the normal VMU and can be connected to a personal computer by USB, Parallel or Serial cable. A 16 mebibit (3200 block) version of the Nexus is also available, but this model has been known to be unstable.

Nyko Hyperpak
The Nyko Hyperpak was a basic memory card without the screen and VMU buttons. However, it was four times the size of a normal memory unit and also had a switch that would switch the device from memory mode to rumble mode. It was larger than a standard VMU but had no way to connect to a personal computer.