- Japan = Visual Memory System (VMS)
- North America = Visual Memory Unit (VMU)
- Europe = Visual Memory (VM)
The name was changed from VMS to VMU for the NA market as the name VMS was already owned and in use by the Digital Equipment Corporation for their VMS operating system. It is not known why the name was changed again to VM for the European market, however the name VMU was commonly used in the UK. Visual Memory System was first released on July 30th, 1998 in Japan without the Dreamcast branding and included Godzilla virtual pet game.
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 game console. Console-like features of the VMU include a small black and white screen, speaker, directional pad, four buttons and the ability to download additional games. Initially only available in white, colors were expanded to transparent Red, Blue, Green, Yellow, Black and many other color variations.
Minigames and in-game features
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. In other video games, statistics such as the current health condition are displayed on the VMU for convenience.
|Dreamcast game||VMU minigame||In-game features|
|Cardcaptor Sakura: Tomoyo no Video Daisakusen||?||?|
|Daytona USA||N/A||Shows a steering wheel in game while driving|
|Demolition Racer: No Exit||?||?|
|Dino Crisis||N/A||Health Condition and Ammo Count|
|Evolution: The World of Sacred Device||?||Health Condition|
|Evolution 2: Far Off Promise||12-hour clock||N/A|
|Godzilla Generations (Japanese Version)||?||?|
|Grand Theft Auto 2||?||Shows ammunition for gun player is holding|
|Jet Set Radio / Jet Grind Radio [USA]||Graffiti Transfer App||N/A|
|Marvel vs. Capcom 2: New Age of Heroes||Character, Character Color and Stage Transfer App||?|
|NBA 2K1||?||Player energy and foul stats|
|NFL 2K||?||Secret plays|
|NFL 2K1||?||Secret plays|
|NFL 2K2||?||Secret plays|
|NFL Blitz 2000||?||Secret plays|
|Power Stone||Various[note 1]||?|
|Power Stone 2||?||?|
|Quake III Arena||Maze game||?|
|Ready 2 Rumble||?||#Punches, HIT%|
|Resident Evil 2||?||Health condition|
|Resident Evil 3: Nemesis||N/A||Health condition|
|Resident Evil Code: Veronica||?||Health condition|
|Sakura Wars 3||?||?|
|Seaman||An app to organize and raise food for Seaman to eat in game||Indicates if Seaman can hear you or not when using the Sega Dreamcast Microphone to speak to him.|
|Sega GT||Pocket GT||N/A|
|Sega Rally 2||Stats and records||N/A|
|Silent Scope||?||Sniper rifle scope view|
|Skies of Arcadia||Pinta's Quest||Signaled to player that a Cham was nearby|
|Sonic Adventure||Chao Adventure||N/A|
|Sonic Adventure 2||Chao Adventure 2||N/A|
|Super Magnetic Neo||N/A||Alerted the player when a secret area was near by|
|Sword of the Berserk: Guts' Rage||?||?|
|Tokyo Bus Guide||?||?|
|Virtua Tennis||?||Shows simplified representation of match in wireframe perspective|
|Walt Disney World Quest: Magical Racing Tour||N/A||Shows the item caught|
|Who Wants To Be A Millionaire||N/A||Shows progress on the money ladder and lifelines in use|
- Powerstone includes three minigames: "Gunrock’s Gun-Gun Slot", "Falcon’s Aerial Adventure" and "Ayame’s Shuriken Training".
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. Additionally, a VMU without battery power will beep when the Dreamcast is powered on (if the VMU is inserted into a connected Dreamcast controller).
- CPU: 8-bit (Sanyo LC8670 "Potato")
- Memory: 128KB flash memory. 28KB is reserved for system use leaving 100KB for data storage, which is divided into 200 'blocks' - 1 block equaling 512 bytes.
- 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
- Sound: 1-channel PWM sound source
- Weight: 45 g (1.6 oz)
The VMU has 128KB of flash memory, however by default 28KB is reserved for system use. In recent years homebrew programs like Dream Explorer (aka VMU Tool) have allowed users to unlock an extra 44 blocks (22KB) of this reserved space increasing the overall VMU capacity to 244 blocks. However, a handful of games might be unable to detect the memory card if this is done, although none have been reported apart from DreamKey/DreamPassport and Metropolis Street Racer.
Multiple Dreamcast memory cards with no VMU features have been released. The cards therefore omit a display screen, input buttons, speaker and built-in clock. Some have the same 128 KB storage as the VMU, equivalent to 200 blocks, while others have multiple "pages" with each acting as a 128 KB memory card. In the latter case, a button or switch allows the gamer to select the desired memory card. This is due to the Dreamcast having a limit of 128 KB per individual memory card.
4x Memory Card
Late in the Dreamcast's lifespan, Sega released an official 4x Memory Card which offered four times the data storage of a standard VMU, with 800 blocks. It was released in Japan on December 14, 2000 as "Memory Card 4X", and in the USA as "4x Memory Card". A button is used to select one of four "pages", and a series of four orange LED on the card indicates which of these is currently in use. Not all games are compatible, so Sega released a list of incompatible games. 
A limited edition Phantasy Star Online 4x Memory Card was also released in Japan to coincide with the games release. It was coloured blue and featured the Phantasy Star Online logo printed onto the casing.
Since the 4x Memory Card was released very late in the Dreamcast's lifespan, it did not make it to retail in Europe despite a planned release (along with the Dreamcast Broadband Adapter which was also unreleased in Europe).
Third-party memory cards
The Nexus Memory Card is a third party version of the Visual Memory Unit that features four times the memory of a stock VMU with 800 blocks (4 megabit / 512 kilobytes) but lacks an LCD screen. The card is divided into 4 pages each with 200 blocks, each page can be selected using a button on the top left of the card. The Nexus Memory Card is slightly larger than the normal VMU and can be connected to a personal computer by USB, Parallel or Serial cable. A 3200 block (16 megabit / 2 megabytes) version of the Nexus is also available, but this model has been known to be unstable.
Nyko released two memory cards for the Dreamcast: the Jumbo Memory Pak X2 with twice the storage as a VMU, and the Hyperpak with four times the storage. The Hyperpak could also act as a Jump Pak by setting its switch to rumble mode.
The Performance Memory Card was a third-party basic memory card with the same 200 blocks of storage as a VMU. The Performance Mega Memory Card acted like a 4X Memory Card. It used a switch on its back to select the desired memory card "page". Unlike other memory cards however, the Performance model had to be removed from the controller before the "page" could be switched.
In a 10-year retrospective, GamesRadar called the VMU "one of our favorite storage devices." They offered praise for the originality of the device, as well its flexibility and commented that newer systems should have revisited the concept. However, they lamented that most games didn't find a useful purpose for it and that the device drained batteries quickly.
- Nintendo GameCube–Game Boy Advance link cable
- PocketStation, a similar device by Sony for the PlayStation.
- Wii U GamePad - game controller featuring its own screen
- "Dream Explorer". Retrieved 2009-05-31.
- "Dreamcast Memory Card 4X Details". Retrieved 2009-06-07.
- "DC Memory Card 4X Compatibility List". Retrieved 2009-06-07.
- "Review: 4X Nexus Memory Card w/PC Link". Retrieved 2009-05-31.
- Wide, Tyler (September 8, 2009). "A tribute to the Dreamcast VMU". GamesRadar. Retrieved October 11, 2012.