Add NETPLAY docs.

site/docs/guides/multiplayer.md

@@ -42,3 +42,26 @@ There are currently some caveats to keep in mind:
 - State saving/loading and cart reset is currently disabled during netplay.
 - Netplay is new and there may be bugs, please [report
   issues](https://github.com/aduros/wasm4/issues/new)!
+
+### The `NETPLAY` memory register
+
+WASM-4 exposes the state of netplay to the cart via the [`NETPLAY`](/docs/reference/memory#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:
+
+```c
+// If netplay is active
+if (*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.

site/docs/reference/memory.md

@@ -12,7 +12,8 @@ WASM-4 uses a fixed memory layout of 64 KB.
 | `$001c` | 2            | [MOUSE_Y](#mouse_y)             |
 | `$001e` | 1            | [MOUSE_BUTTONS](#mouse_buttons) |
 | `$001f` | 1            | [SYSTEM_FLAGS](#system_flags)   |
-| `$0020` | 128          | Reserved for future use         |
+| `$0020` | 1            | [NETPLAY](#netplay)             |
+| `$0021` | 127          | Reserved for future use         |
 | `$00a0` | 6400         | [FRAMEBUFFER](#framebuffer)     |
 | `$19a0` | 58976        | Available program memory        |
 
@@ -91,6 +92,15 @@ Byte containing flags that modify WASM-4's operation. By default all flags are o
 | 0   | `SYSTEM_PRESERVE_FRAMEBUFFER` | Prevent clearing the framebuffer between frames.  |
 | 1   | `SYSTEM_HIDE_GAMEPAD_OVERLAY` | Hide the gamepad UI overlay on mobile.            |
 
+### NETPLAY
+
+Byte containing netplay multiplayer state.
+
+| Bits  | Description                         |
+| ---   | ---                                 |
+| 0 - 1 | Local player index (0 to 3).        |
+| 2     | Set if netplay is currently active. |
+
 ### FRAMEBUFFER
 
 Array of 160x160 pixels, with each pixel packed into 2 bits (colors 0 to 3).