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WASM-4 supports realtime multiplayer of up to 4 players, either locally or online.

Detecting gamepad input for Player 2 is the same as usual, but instead of GAMEPAD1, you read from GAMEPAD2. For Players 3 and 4, GAMEPAD3 and GAMEPAD4.

Local Multiplayer#

Local multiplayer is where everyone is playing on the same computer. To play local multiplayer, you can connect multiple physical USB controllers. Xbox or PlayStation controllers are known to work best.

If all you have is a keyboard, up to 3 players can be controlled from one keyboard using these keys:

Player #DirectionsX buttonZ button
1Arrows. (period), (comma)
2ESDFA or QTab or LShift
3Numpad 8456Numpad * or .Numpad - or Enter

This key layout is designed to fit many hands on one keyboard somewhat comfortably.


Netplay connects gamepad inputs over the Internet using WebRTC. WASM-4 implements rollback netcode to keep games fast and responsive.

Hosting a netplay game is as easy as copy and pasting a link.

  1. While playing any cart, press Enter to open the WASM-4 menu.
  2. Select "Copy Netplay URL". An invite link (like will be copied to your clipboard for you to send to a friend.
  3. When your friend clicks your link, they'll instantly join your game.

All WASM-4 games that support local multiplayer automatically support netplay. There are no additional steps developers need to take to make their games netplay-ready.

There are currently some caveats to keep in mind:

  • Mouse input is disabled during netplay.
  • State saving/loading and cart reset is currently disabled during netplay.
  • Netplay is new and there may be bugs, please report issues!

The NETPLAY memory register#

WASM-4 exposes the state of netplay to the cart via the NETPLAY memory register.

Reading this register is not required to implement netplay, but it can be used to implement advanced features such as non-shared screen multiplayer:

// If netplay is activeif (*NETPLAY & 0b100) {    int playerIdx = *NETPLAY & 0b011;    // Render the game from playerIdx's perspective    // ...}

This is a powerful feature enabling games with hidden information, like card games, strategy games, and first person shooters. However, caution must be taken by developers to not introduce desynchronization. In other words, the netplay player index should only be used to adjust how a frame is rendered, and not affect game logic. Each connected player must run the same game logic for all players in order to prevent desyncs.