DreamPotato

DreamPotato is a Dreamcast VMU emulator in currently in alpha. For the moment only Windows builds are available.

Warning

When opening a .vmu or .bin file, the emulator will modify the file on disk while running in order to persist changes, such as save file changes on the memory card or saving your progress in minigames. Since the emulator is in alpha state, please make copies of any VMU files before loading them to avoid any chance of corrupting your saves. .vms files opened in the emulator will not be modified.

See compatibility.md for the current compatibility status of various games.

Flycast Integration Prototype

Demo Video

DreamPotato can connect to Flycast to support an experience similar to using a real Dreamcast and VMU together. Currently, trying this out requires building the Flycast vmuconn branch from source.

A few caveats:

1. A controller needs to be plugged in to your PC, due to a hack/workaround being used for the Flycast prototype.
2. Make sure you launch DreamPotato before Flycast, as DreamPotato runs the server side of the TCP connection.
3. A valid VMU file must be opened manually as automatic initialization of the VMU flash storage isn't yet implemented.
4. When the VMU is ejected, the BIOS puts it to sleep, you'll have to hit Sleep again before being able to do anything.
5. In Chao Adventure, make sure you hit Mode to exit the game before inserting the VMU, or your progress may not be saved.

Usage

Download the latest bits from the Releases section.

Default Key Mappings

• W - Up
• A - Left
• S - Down
• D - Right
• K - A
• L - B
• I - Mode
• J - Sleep
• Insert - Insert/Eject VMU (experimental--simulates connecting to Dreamcast)
  • You can also hold the Sleep button for 1 second to toggle inserting/ejecting the VMU.
• F5 - Save State
• F8 - Load State
• F10 - Pause/Resume
• Tab (hold) - Fast Forward

Default Gamepad Button Mappings

Currently an unusual control scheme is used by default to make it easier to map the Dreamcast buttons and VMU buttons onto a single controller.

• Right Thumbstick - Up/Left/Down/Right
• Right Shoulder - A
• Left Shoulder - B
• Back - Sleep
• Right Stick Press - Mode

For the moment, if you build from source, you can change usages of Preset_DreamcastSimultaneous to Default, in order to use a normal button mapping. Just make sure to delete configuration.json from the output directory so that your new default is used.

Configuration

Button mapping is done by editing a json file by hand for now. Eventually a proper button mapping UI would be desirable of course.

"SourceKey"/"SourceButton" means the key/button on your keyboard/gamepad you want to use. TargetButton means the emulated VMU button or emulator command you want to perform. See Keys and Buttons for source key/button names. See enum VmuButton for target button names.

Additional configuration options include:

• AutoInitializeDate: bool, default true. If true, skips the startup beep and date setup, and uses your computer's clock to initialize the VMU's clock. Specify false if you want to use the date setup sequence as you would when installing batteries with real hardware.
• AnyButtonWakesFromSleep: bool, default true. If true, any button will wake the VMU from sleep. If false, only the sleep button will do that.

Building

• You'll need the .NET 9 SDK installed on your computer.
• Copy your american_v1.05.bin VMU ROM into src/DreamPotato.MonoGame/Data/.
• Build everything: dotnet build.
• Run the emulator: dotnet run --project src/DreamPotato.MonoGame.
• Run tests: dotnet test

Project Overview

Under src/:

• DreamPotato.Core is where the emulator implementation resides.
• DreamPotato.Tests includes unit tests for the emulator implementation.
• DreamPotato.MonoGame is the front-end, including UI and config file handling.
• dreampotato-vscode is the VS Code extension--in very barebones/"alpha" state.
  • Currently, it contains only a TextMate grammar for LC86k assembly.

Why write a new emulator?

The biggest reason was: I was interested in timing-based luck manipulation in Pinta's Quest, which requires emulating the game at a very similar speed to real VMU hardware. I found that existing emulators did not run the game at a similar enough speed to real hardware to allow the same timings to work. I thought that writing a new emulator from scratch would be a good way to learn the hardware well enough to get to the bottom of how to do that.

Acknowledgements

Thanks to the following individuals, whose invaluable work on VMU emulation and reverse engineering helped make DreamPotato possible.

• Falco Girgis, author of ElysianVMU/libevmu and extensive documentation of VMU internals.
• Dmitry Grinberg for his VMU hackery project.
• Walter Tetzner, author of the waterbear assembler/disassembler.
• Homebrew developers jvsTSX and Jahan Addison for publishing homebrew software which was very helpful for testing the emulator.