function GameBoyAdvanceGPIO(core, rom) {
	this.core = core;
	this.rom = rom;

	this.readWrite = 0;
	this.direction = 0;

	this.device = new GameBoyAdvanceRTC(this); // TODO: Support more devices
};

GameBoyAdvanceGPIO.prototype.store16 = function(offset, value) {
	switch (offset) {
	case 0xC4:
		this.device.setPins(value & 0xF);
		break;
	case 0xC6:
		this.direction = value & 0xF;
		this.device.setDirection(this.direction);
		break;
	case 0xC8:
		this.readWrite = value & 1;
		break;
	default:
		throw new Error('BUG: Bad offset passed to GPIO: ' + offset.toString(16));
	}
	if (this.readWrite) {
		var old = this.rom.view.getUint16(offset, true);
		old &= ~this.direction;
		this.rom.view.setUint16(offset, old | (value & this.direction), true);
	}
};

GameBoyAdvanceGPIO.prototype.outputPins = function(nybble) {
	if (this.readWrite) {
		var old = this.rom.view.getUint16(0xC4, true);
		old &= this.direction;
		this.rom.view.setUint16(0xC4, old | (nybble & ~this.direction & 0xF), true);
	}
};

function GameBoyAdvanceRTC(gpio) {
	this.gpio = gpio;

	// PINOUT: SCK | SIO | CS | -
	this.pins = 0;
	this.direction = 0;

	this.totalBytes = [
		0, // Force reset
		0, // Empty
		7, // Date/Time
		0, // Force IRQ
		1, // Control register
		0, // Empty
		3, // Time
		0 // Empty
	];
	this.bytesRemaining = 0;

	// Transfer sequence:
	// == Initiate
	// > HI | - | LO | -
	// > HI | - | HI | -
	// == Transfer bit (x8)
	// > LO | x | HI | -
	// > HI | - | HI | -
	// < ?? | x | ?? | -
	// == Terminate
	// >  - | - | LO | -
	this.transferStep = 0;

	this.reading = 0;
	this.bitsRead = 0;
	this.bits = 0;
	this.command = -1;

	this.control = 0x40;
	this.time = [
		0, // Year
		0, // Month
		0, // Day
		0, // Day of week
		0, // Hour
		0, // Minute
		0 // Second
	];
};

GameBoyAdvanceRTC.prototype.setPins = function(nybble) {
	switch (this.transferStep) {
	case 0:
		if ((nybble & 5) == 1) {
			this.transferStep = 1;
		}
		break;
	case 1:
		if (nybble & 4) {
			this.transferStep = 2;
		}
		break;
	case 2:
		if (!(nybble & 1)) {
			this.bits &= ~(1 << this.bitsRead);
			this.bits |= ((nybble & 2) >> 1) << this.bitsRead;
		} else {
			if (nybble & 4) {
				// SIO direction should always != this.read
				if ((this.direction & 2) && !this.read) {
					++this.bitsRead;
					if (this.bitsRead == 8) {
						this.processByte();
					}
				} else {
					this.gpio.outputPins(5 | (this.sioOutputPin() << 1));
					++this.bitsRead;
					if (this.bitsRead == 8) {
						--this.bytesRemaining;
						if (this.bytesRemaining <= 0) {
							this.command = -1;
						}
						this.bitsRead = 0;
					}
				}
			} else {
				this.bitsRead = 0;
				this.bytesRemaining = 0;
				this.command = -1;
				this.transferStep = 0;
			}
		}
		break;
	}

	this.pins = nybble & 7;
};

GameBoyAdvanceRTC.prototype.setDirection = function(direction) {
	this.direction = direction;
};

GameBoyAdvanceRTC.prototype.processByte = function() {
	--this.bytesRemaining;
	switch (this.command) {
	case -1:
		if ((this.bits & 0x0F) == 0x06) {
			this.command = (this.bits >> 4) & 7;
			this.reading = this.bits & 0x80;

			this.bytesRemaining = this.totalBytes[this.command];
			switch (this.command) {
			case 0:
				this.control = 0;
				break;
			case 2:
			case 6:
				this.updateClock();
				break;
			}
		} else {
			this.gpio.core.WARN('Invalid RTC command byte: ' + this.bits.toString(16));
		}
		break;
	case 4:
		// Control
		this.control = this.bits & 0x40;
		break;
	}	
	this.bits = 0;
	this.bitsRead = 0;
	if (!this.bytesRemaining) {
		this.command = -1;
	}
};

GameBoyAdvanceRTC.prototype.sioOutputPin = function() {
	var outputByte = 0;
	switch (this.command) {
	case 4:
		outputByte = this.control;
		break;
	case 2:
	case 6:
		outputByte = this.time[7 - this.bytesRemaining];
		break;
	}
	var output = (outputByte >> this.bitsRead) & 1;
	return output;
};

GameBoyAdvanceRTC.prototype.updateClock = function() {
	var date = new Date();
	this.time[0] = this.bcd(date.getFullYear());
	this.time[1] = this.bcd(date.getMonth() + 1);
	this.time[2] = this.bcd(date.getDate());
	this.time[3] = date.getDay() - 1;
	if (this.time[3] < 0) {
		this.time[3] = 6;
	}
	if (this.control & 0x40) {
		// 24 hour
		this.time[4] = this.bcd(date.getHours());
	} else {
		this.time[4] = this.bcd(date.getHours() % 2);
		if (date.getHours() >= 12) {
			this.time[4] |= 0x80;
		}
	}
	this.time[5] = this.bcd(date.getMinutes());
	this.time[6] = this.bcd(date.getSeconds());
};

GameBoyAdvanceRTC.prototype.bcd = function(binary) {
	var counter = binary % 10;
	binary /= 10;
	counter += (binary % 10) << 4;
	return counter;
};