Den of Snakes is a simple 4 player snake game for the Game Boy.
Download at: https://bbbbbr.itch.io/den-of-snakes-four-player
- Graphical assets: CC-BY-NC-ND, meaning no commercial use or derivatives without explicit permission.
- Source code: Public domain
Thinking about making a game that uses the 4 Player adapter? You are welcome (and encouraged!) to use the source code in your project. The main source files of interest are 4_player_adapter.c and 4_player_adapter.h
In this game the approach to solve synchronization across four consoles uses "deterministic lockstep".
- The game runs in exact lock-step across all the consoles aside from display timing.
- All input is routed through the 4-player adapter and only used after being received from it (by all consoles at the same time).
- The game state only ticks when 4-player packets are received (~about once per frame with the configured timing, so it adds 1 frame of input lag).
As a result of this, consoles broadcast a keep-alive heart beat when not sending button events. Due to the parallel nature, if a player dies on one console then they also die in the exact same manner/location/game timing on all the other consoles.
This approach will not be suitable for all games- however the example hardware interface code is agnostic to how games are implemented.
- Hardware: In order to play this game on hardware you need at least one Game Boy (preferably more!) and a Nintendo DMG-07 Four Player Adapter.
- Emulators TODO: Double Cherry? GBE+?
Objective: Be the last snake alive!
Eating hearts grows your snake and deposits skulls. When a snake crashes into skulls or another snake then that snake/player dies and their snake is turned into skull bones. The game ends when only a single player is left alive, that player wins.
When only one player is connected to the DMG-07 then the objective is just to make as long a snake as possible.
Tip: You can enter the Konami code on the title screen of any participating console and the game will run in an alternate Snafu-like gameplay mode.
- Move Snake: D-PAD
- Pause: START
- Start Game: START (on any connected console)
When GBA-SP (and possibly original GBA) model Game Boys are connected to the DMG-07 Four Player adapter but have their power turned off and a cart inserted, they will interfere with the Transmission mode (forcing a return to Ping mode).
So, when those models are powered-off, make sure to disconnect them from the Four Player adapter.
This may apply to other Game Boy models or FPGA clones.
In this example a fixed serial ISR is used (ISR_VECTOR()) instead of the default GBDK serial interrupt dispatcher (add_SIO()) with installable ISR handlers since it has slightly lower call overhead.
The 4 Player code can be switched to the standard GBDK dispatcher by commenting out the #define FOUR_PLAYER_BARE_ISR_VECTOR toggle.
The source code is released to the public domain and may be used (please do!) for your own programs. Attribution is welcome but not required.
Additional hardware details found as a result of this example implementation have been submitted to Pandocs page about the 4 Player Adapter.
Saleae logic analyzer captures of the DMG-07 protocol on the wire are included for reference.
Many thanks to Shonumi for their original research and documentation on this device. It made implementing support tremendously easier.