@@ -1,6 +1,7 @@
 #include "gba-io.h"
 
 #include "gba-serialize.h"
+#include "gba-sio.h"
 #include "gba-video.h"
 
 static const int _isValidRegister[REG_MAX >> 1] = {
@@ -89,7 +90,7 @@ };
 
 void GBAIOInit(struct GBA* gba) {
 	gba->memory.io[REG_DISPCNT >> 1] = 0x0080;
-	gba->memory.io[REG_RCNT >> 1] = 0x8000;
+	gba->memory.io[REG_RCNT >> 1] = RCNT_INITIAL;
 	gba->memory.io[REG_KEYINPUT >> 1] = 0x3FF;
 }
 
@@ -248,6 +249,15 @@ break;
 		case REG_TM3CNT_HI:
 			value &= 0x00C7;
 			GBATimerWriteTMCNT_HI(gba, 3, value);
+			break;
+
+		// SIO
+		case REG_SIOCNT:
+			GBASIOWriteSIOCNT(&gba->sio, value);
+			break;
+		case REG_RCNT:
+			value &= 0xC1FF;
+			GBASIOWriteRCNT(&gba->sio, value);
 			break;
 
 		// Interrupts and misc

@@ -0,0 +1,29 @@
+#include "gba-sio.h"
+
+#include "gba-io.h"
+
+static void _switchMode(struct GBASIO* sio) {
+	int mode = ((sio->rcnt >> 14) & 0xC) | ((sio->siocnt >> 12) & 0x3);
+	if (mode < 8) {
+		sio->mode = (enum GBASIOMode) (mode & 0x3);
+	} else {
+		sio->mode = (enum GBASIOMode) (mode & 0xC);
+	}
+	// TODO: hangup if we have an existing connection
+}
+
+void GBASIOInit(struct GBASIO* sio) {
+	sio->rcnt = RCNT_INITIAL;
+	sio->siocnt = 0;
+	_switchMode(sio);
+}
+
+void GBASIOWriteRCNT(struct GBASIO* sio, uint16_t value) {
+	sio->rcnt = value;
+	_switchMode(sio);
+}
+
+void GBASIOWriteSIOCNT(struct GBASIO* sio, uint16_t value) {
+	sio->siocnt = value;
+	_switchMode(sio);
+}

@@ -0,0 +1,72 @@
+#ifndef GBA_SIO_H
+#define GBA_SIO_H
+
+#include <stdint.h>
+
+enum GBASIOMode {
+	SIO_NORMAL_8 = 0,
+	SIO_NORMAL_32 = 1,
+	SIO_MULTI = 2,
+	SIO_UART = 3,
+	SIO_GPIO = 8,
+	SIO_JOYBUS = 12
+};
+
+enum GBASIOMultiMode {
+	VBA_LINK_COMPAT
+};
+
+enum {
+	RCNT_INITIAL = 0x8000
+};
+
+struct GBASIODriver {
+	int (*attach)(struct GBASIODriver* driver);
+};
+
+struct GBASIO {
+	struct GBA* p;
+
+	enum GBASIOMode mode;
+	enum GBASIOMultiMode multiMode;
+	struct GBASIODriver* driver;
+
+	uint16_t rcnt;
+	union {
+		struct {
+			unsigned sc : 1;
+			unsigned internalSc : 1;
+			unsigned si : 1;
+			unsigned idleSo : 1;
+			unsigned : 4;
+			unsigned start : 1;
+			unsigned : 3;
+			unsigned length : 1;
+			unsigned : 1;
+			unsigned irq : 1;
+			unsigned : 1;
+		} normalControl;
+
+		struct {
+			unsigned baud : 2;
+			unsigned slave : 1;
+			unsigned ready : 1;
+			unsigned id : 2;
+			unsigned error : 1;
+			unsigned busy : 1;
+			unsigned : 6;
+			unsigned irq : 1;
+			unsigned : 1;
+		} multiplayerControl;
+
+		uint16_t siocnt;
+	};
+};
+
+void GBASIOInit(struct GBASIO* sio);
+
+void GBASIOWriteRCNT(struct GBASIO* sio, uint16_t value);
+void GBASIOWriteSIOCNT(struct GBASIO* sio, uint16_t value);
+void GBASIOWriteSIOMLT_SEND(struct GBASIO* sio, uint16_t value);
+
+#endif

@@ -122,6 +122,9 @@ GBAAudioInit(&gba->audio);
 
 	GBAIOInit(gba);
 
+	gba->sio.p = gba;
+	GBASIOInit(&gba->sio);
+
 	gba->timersEnabled = 0;
 	memset(gba->timers, 0, sizeof(gba->timers));
 

@@ -7,6 +7,7 @@
 #include "gba-memory.h"
 #include "gba-video.h"
 #include "gba-audio.h"
+#include "gba-sio.h"
 
 #include <stdarg.h>
 
@@ -74,6 +75,7 @@ struct GBABoard board;
 	struct GBAMemory memory;
 	struct GBAVideo video;
 	struct GBAAudio audio;
+	struct GBASIO sio;
 
 	struct GBASync* sync;