diff --git a/src/main/java/com/loomcom/lm6502/Bus.java b/src/main/java/com/loomcom/lm6502/Bus.java
index 728f489..9856a93 100644
--- a/src/main/java/com/loomcom/lm6502/Bus.java
+++ b/src/main/java/com/loomcom/lm6502/Bus.java
@@ -2,33 +2,35 @@ package com.loomcom.lm6502;
 
 import java.util.*;
 import com.loomcom.lm6502.devices.*;
+import com.loomcom.lm6502.exceptions.*;
 
 public class Bus {
 
-	private int bottom = 0x0000;
-	private int top    = 0xffff;
+	/* By default, our bus starts at 0, and goes up to 64K */
+	private int startAddress = 0x0000;
+	private int endAddress   = 0xffff;
 
 	/**
 	 * Ordered list of IO devices.
 	 */
-	private List<Device> devices;
+	private SortedSet<Device> devices;
 
 	public Bus(int size) {
 		this(0, size - 1);
 	}
 
-	public Bus(int bottom, int top) {
-		this.devices = new ArrayList(8);
-		this.bottom = bottom;
-		this.top = top;
+	public Bus(int startAddress, int endAddress) {
+		this.devices = new TreeSet();
+		this.startAddress = startAddress;
+		this.endAddress = endAddress;
 	}
 
-	public int bottom() {
-		return bottom;
+	public int startAddress() {
+		return startAddress;
 	}
 
-	public int top() {
-		return top;
+	public int endAddress() {
+		return endAddress;
 	}
 
 	public void addDevice(Device device)
@@ -62,21 +64,21 @@ public class Bus {
 		for (Device d : devices) {
 			MemoryRange cur = d.getMemoryRange();
 			if (i == 0) {
-				// If the first entry doesn't start at 'bottom', return false.
-				if (cur.getStartAddress() != bottom) { return false; }
+				// If the first entry doesn't start at 'startAddress', return false.
+				if (cur.startAddress() != startAddress) { return false; }
 			}
 
 			if (prev != null && i < length - 1) {
 				// Otherwise, compare previous map's end against this map's
-				// top.  They must be adjacent!
-				if (cur.getStartAddress() - 1 != prev.getEndAddress()) {
+				// endAddress.  They must be adjacent!
+				if (cur.startAddress() - 1 != prev.endAddress()) {
 					return false;
 				}
 			}
 
 			if (i == length - 1) {
-				// If the last entry doesn't end at top, return false;
-				if (cur.getEndAddress() != top) { return false; }
+				// If the last entry doesn't end at endAddress, return false;
+				if (cur.endAddress() != endAddress) { return false; }
 			}
 
 			i++;
@@ -87,8 +89,35 @@ public class Bus {
 		return true;
 	}
 
-	public List getDevices() {
+	public int read(int address) {
+		for (Device d : devices) {
+			MemoryRange range = d.getMemoryRange();
+			if (range.includes(address)) {
+				// Compute offset into this device's address space.
+				int devAddr = address - range.startAddress();
+				return d.read(devAddr);
+			}
+		}
+		// TODO: Raise access error.
+		throw new RuntimeException("Read failed!  Device not found.");
+	}
+
+	public void write(int address, int value) {
+		for (Device d : devices) {
+			MemoryRange range = d.getMemoryRange();
+			if (range.includes(address)) {
+				// Compute offset into this device's address space.
+				int devAddr = address - range.startAddress();
+				d.write(devAddr, value);
+				return;
+			}
+		}
+		// TODO: Raise access error.
+		throw new RuntimeException("Write failed!  Device not found.");
+	}
+
+	public SortedSet getDevices() {
 		// Expose a copy of the device list, not the original
-		return new ArrayList(devices);
+		return new TreeSet(devices);
 	}
 }
diff --git a/src/main/java/com/loomcom/lm6502/MemoryRange.java b/src/main/java/com/loomcom/lm6502/MemoryRange.java
index ecf3e3e..885ba7d 100644
--- a/src/main/java/com/loomcom/lm6502/MemoryRange.java
+++ b/src/main/java/com/loomcom/lm6502/MemoryRange.java
@@ -1,7 +1,19 @@
 package com.loomcom.lm6502;
 
+import com.loomcom.lm6502.exceptions.*;
+
+/**
+ * MemoryRange is a simple container class representing a literal
+ * range of memory, with a staraddress, and an end address.  It has
+ * guards against creating impossible memory ranges, and implements
+ * some useful methods for checking address inclusion and range
+ * overlaps.
+ */
 public class MemoryRange implements Comparable<MemoryRange> {
+
+	/** The starting address of the memory range. */
 	public int startAddress;
+	/** The ending address of the memory range. */
 	public int endAddress;
 
 	public MemoryRange(int startAddress, int endAddress)
@@ -17,11 +29,17 @@ public class MemoryRange implements Comparable<MemoryRange> {
 		this.endAddress = endAddress;
 	}
 
-	public int getStartAddress() {
+	/**
+	 * @returns the starting address.
+	 */
+	public int startAddress() {
 		return startAddress;
 	}
 
-	public int getEndAddress() {
+	/**
+	 * @returns the ending address.
+	 */
+	public int endAddress() {
 		return endAddress;
 	}
 
@@ -42,8 +60,8 @@ public class MemoryRange implements Comparable<MemoryRange> {
 	 * @returns true if this range overlaps in any way with the other.
 	 */
 	public boolean overlaps(MemoryRange other) {
-		return (this.includes(other.getStartAddress()) ||
-						other.includes(this.getStartAddress()));
+		return (this.includes(other.startAddress()) ||
+						other.includes(this.startAddress()));
 	}
 
 	// Implementation of Comparable interface
@@ -54,8 +72,8 @@ public class MemoryRange implements Comparable<MemoryRange> {
 		if (this == other) {
 			return 0;
 		}
-		Integer thisStartAddr = new Integer(this.getStartAddress());
-		Integer thatStartAddr = new Integer(other.getStartAddress());
+		Integer thisStartAddr = new Integer(this.startAddress());
+		Integer thatStartAddr = new Integer(other.startAddress());
 		return thisStartAddr.compareTo(thatStartAddr);
 	}
 
diff --git a/src/main/java/com/loomcom/lm6502/Profiler.java b/src/main/java/com/loomcom/lm6502/Profiler.java
new file mode 100644
index 0000000..2c5315e
--- /dev/null
+++ b/src/main/java/com/loomcom/lm6502/Profiler.java
@@ -0,0 +1,66 @@
+/**
+ * A simple profiler, for debugging the simulator.
+ * It is safe to ignore me.
+ */
+
+package com.loomcom.lm6502;
+
+import java.util.*;
+import com.loomcom.lm6502.devices.*;
+import com.loomcom.lm6502.exceptions.*;
+
+public class Profiler {
+	public static void main(String[] args) {
+
+		try {
+
+			// Create a bus.
+			Bus b = new Bus(0, 65535);
+
+			// Create eight devices, each 8KB, to fill the bus.
+			b.addDevice(new Memory(0x0000, 0x2000, null)); // 8KB @ $0000-$1fff
+			b.addDevice(new Memory(0x2000, 0x2000, null)); // 8KB @ $2000-$3fff
+			b.addDevice(new Memory(0x4000, 0x2000, null)); // 8KB @ $4000-$5fff
+			b.addDevice(new Memory(0x6000, 0x2000, null)); // 8KB @ $6000-$7fff
+			b.addDevice(new Memory(0x8000, 0x2000, null)); // 8KB @ $8000-$9fff
+			b.addDevice(new Memory(0xa000, 0x2000, null)); // 8KB @ $a000-$bfff
+			b.addDevice(new Memory(0xc000, 0x2000, null)); // 8KB @ $c000-$dfff
+			b.addDevice(new Memory(0xe000, 0x2000, null)); // 8KB @ $e000-$ffff
+
+			// Read memory
+			long sum     = 0;
+			long average = 0;
+
+			long iters   = 500;
+
+			for (int i = 0; i < iters; i++) {
+
+				long startTime = System.nanoTime();
+
+				// Read and assign to a buffer
+				int buf = 0;
+				for (int j = 0; j < 0xffff; j++) {
+					buf = b.read(j);
+					if (buf != 0xff) {
+						System.out.println("WARNING!  MEMORY SHOULD HAVE BEEN $FF, WAS: " + buf);
+						System.exit(0);
+					}
+				}
+
+				long endTime = System.nanoTime();
+				long diff    = endTime - startTime;
+
+				// System.out.println("Read of 64KB took: " + diff + " ns (" + (diff / 1000) + " us)");
+
+				sum += diff;
+				average = sum / (i + 1);
+			}
+
+			System.out.println("Average time to read 64KB: " + average + " ns (" + (average / 1000) + " us)");
+			System.out.println("Average time to read one byte: " + sum / (64 * 1024 * iters) + " ns");
+
+		} catch (MemoryRangeException ex) {
+			System.out.println("Memory Access Exception! " + ex.getMessage());
+		}
+	}
+}
diff --git a/src/main/java/com/loomcom/lm6502/Simulator.java b/src/main/java/com/loomcom/lm6502/Simulator.java
index 391b641..0aeeac4 100644
--- a/src/main/java/com/loomcom/lm6502/Simulator.java
+++ b/src/main/java/com/loomcom/lm6502/Simulator.java
@@ -10,30 +10,26 @@ public class Simulator {
 	/**
 	 * Command-line parser used by this simulator.
 	 */
-	CommandParser m_parser;
+	CommandParser parser;
 
 	/**
 	 * The CPU itself.
 	 */
-	Cpu m_cpu;
+	Cpu cpu;
 
 	/**
 	 * The Bus responsible for routing memory read/write requests to the
 	 * correct IO devices.
 	 */
-	Bus m_bus;
+	Bus bus;
 
 	public Simulator() {
-		m_cpu = new Cpu(this);
-		m_parser = new CommandParser(System.in, System.out, this);
+		cpu = new Cpu(this);
+		parser = new CommandParser(System.in, System.out, this);
 	}
 
 	public void run() {
-		m_parser.run();
-	}
-
-	public static void main(String[] args) {
-		new Simulator().run();
+		parser.run();
 	}
 
 	public void step() {
@@ -45,4 +41,12 @@ public class Simulator {
 
 	public void write(int address, int value) {
 	}
+
+	/**
+	 * Main simulator routine.
+	 */
+	public static void main(String[] args) {
+		new Simulator().run();
+	}
+
 }
diff --git a/src/main/java/com/loomcom/lm6502/devices/Device.java b/src/main/java/com/loomcom/lm6502/devices/Device.java
index ce9d7c8..20cb6b2 100644
--- a/src/main/java/com/loomcom/lm6502/devices/Device.java
+++ b/src/main/java/com/loomcom/lm6502/devices/Device.java
@@ -1,12 +1,13 @@
 package com.loomcom.lm6502.devices;
 
 import com.loomcom.lm6502.*;
+import com.loomcom.lm6502.exceptions.*;
 
 /**
  * A memory-mapped IO Device.
  */
 
-public abstract class Device {
+public abstract class Device implements Comparable<Device> {
 
 	/** The memory range for this device. */
 	private MemoryRange memoryRange;
@@ -23,20 +24,21 @@ public abstract class Device {
 		this.cpu = cpu;
 	}
 
+	/* Methods required to be implemented by inheriting classes. */
 	public abstract void write(int address, int data);
-
 	public abstract int read(int address);
+	public abstract String toString();
 
 	public MemoryRange getMemoryRange() {
 		return memoryRange;
 	}
 
-	public int getEndAddress() {
-		return memoryRange.getEndAddress();
+	public int endAddress() {
+		return memoryRange.endAddress();
 	}
 
-	public int getStartAddress() {
-		return memoryRange.getStartAddress();
+	public int startAddress() {
+		return memoryRange.startAddress();
 	}
 
 	public String getName() {
@@ -47,4 +49,17 @@ public abstract class Device {
 		this.name = name;
 	}
 
+	/**
+	 * Compares two devices.  The sort order is defined by the sort
+	 * order of the device's memory ranges.
+	 */
+	public int compareTo(Device other) {
+		if (other == null) {
+			throw new NullPointerException("Cannot compare to null.");
+		}
+		if (this == other) {
+			return 0;
+		}
+		return getMemoryRange().compareTo(other.getMemoryRange());
+	}
 }
diff --git a/src/main/java/com/loomcom/lm6502/devices/Memory.java b/src/main/java/com/loomcom/lm6502/devices/Memory.java
index a3878c8..fc308f3 100644
--- a/src/main/java/com/loomcom/lm6502/devices/Memory.java
+++ b/src/main/java/com/loomcom/lm6502/devices/Memory.java
@@ -1,15 +1,25 @@
 package com.loomcom.lm6502.devices;
 
+import java.util.*;
+
 import com.loomcom.lm6502.*;
+import com.loomcom.lm6502.exceptions.*;
 
 public class Memory extends Device {
 
 	private boolean readOnly;
 
+	private int[] mem;
+
 	public Memory(int address, int size, Cpu cpu, boolean readOnly)
 		    throws MemoryRangeException {
 		super(address, size, "RW Memory", cpu);
+
 		this.readOnly = readOnly;
+		this.mem = new int[size];
+
+		// Init the mem to all 0xff
+		Arrays.fill(this.mem, 0xff);
 	}
 
 	public Memory(int address, int size, Cpu cpu)
@@ -18,10 +28,17 @@ public class Memory extends Device {
 	}
 
 	public void write(int address, int data) {
+		System.out.println(String.format("[write] Before write: $%04x=$%04x", address, this.mem[address]));
+		this.mem[address] = data;
+		System.out.println(String.format("[write] After write:  $%04x=$%04x", address, this.mem[address]));
 	}
 
 	public int read(int address) {
-		return 0;
+		return this.mem[address];
+	}
+
+	public String toString() {
+		return "Memory: " + getMemoryRange().toString();
 	}
 
 }
\ No newline at end of file
diff --git a/src/main/java/com/loomcom/lm6502/exceptions/MemoryAccessException.java b/src/main/java/com/loomcom/lm6502/exceptions/MemoryAccessException.java
new file mode 100644
index 0000000..951df42
--- /dev/null
+++ b/src/main/java/com/loomcom/lm6502/exceptions/MemoryAccessException.java
@@ -0,0 +1,11 @@
+package com.loomcom.lm6502.exceptions;
+
+/**
+ * Exception that will be thrown if access to memory or IO cannot be
+ * accessed.
+ */
+public class MemoryAccessException extends Exception {
+	public MemoryAccessException(String msg) {
+		super(msg);
+	}
+}
diff --git a/src/main/java/com/loomcom/lm6502/MemoryRangeException.java b/src/main/java/com/loomcom/lm6502/exceptions/MemoryRangeException.java
similarity index 83%
rename from src/main/java/com/loomcom/lm6502/MemoryRangeException.java
rename to src/main/java/com/loomcom/lm6502/exceptions/MemoryRangeException.java
index 0d8e42c..9551d1b 100644
--- a/src/main/java/com/loomcom/lm6502/MemoryRangeException.java
+++ b/src/main/java/com/loomcom/lm6502/exceptions/MemoryRangeException.java
@@ -1,4 +1,4 @@
-package com.loomcom.lm6502;
+package com.loomcom.lm6502.exceptions;
 
 /**
  * Exception that will be thrown if devices conflict in the IO map.
diff --git a/src/test/java/com/loomcom/lm6502/BusTest.java b/src/test/java/com/loomcom/lm6502/BusTest.java
index c066bc2..744e8c6 100644
--- a/src/test/java/com/loomcom/lm6502/BusTest.java
+++ b/src/test/java/com/loomcom/lm6502/BusTest.java
@@ -3,6 +3,7 @@ package com.loomcom.lm6502;
 import junit.framework.*;
 
 import com.loomcom.lm6502.devices.*;
+import com.loomcom.lm6502.exceptions.*;
 
 import java.util.*;
 
@@ -19,32 +20,32 @@ public class BusTest extends TestCase {
 		return new TestSuite(BusTest.class);
 	}
 
-	public void testCreatingWithTopAndBottom() {
+	public void testCreatingWithStartAndEndAddresses() {
 		Bus b = null;
 
 		b = new Bus(0x00, 0xff);
-		assertEquals(0x00, b.bottom());
-		assertEquals(0xff, b.top());
+		assertEquals(0x00, b.startAddress());
+		assertEquals(0xff, b.endAddress());
 
 		b = new Bus(0x20, 0xea);
-		assertEquals(0x20, b.bottom());
-		assertEquals(0xea, b.top());
+		assertEquals(0x20, b.startAddress());
+		assertEquals(0xea, b.endAddress());
 	}
 
 	public void testCreatingWithSize() {
 		Bus b = null;
 
 		b = new Bus(256);
-		assertEquals(0x00, b.bottom());
-		assertEquals(0xff, b.top());
+		assertEquals(0x00, b.startAddress());
+		assertEquals(0xff, b.endAddress());
 
 		b = new Bus(4096);
-		assertEquals(0x000, b.bottom());
-		assertEquals(0xfff, b.top());
+		assertEquals(0x000, b.startAddress());
+		assertEquals(0xfff, b.endAddress());
 
 		b = new Bus(65536);
-		assertEquals(0x0000, b.bottom());
-		assertEquals(0xffff, b.top());
+		assertEquals(0x0000, b.startAddress());
+		assertEquals(0xffff, b.endAddress());
 	}
 
 	public void testAddDevice() throws MemoryRangeException {
@@ -76,7 +77,7 @@ public class BusTest extends TestCase {
 		}
 	}
 
-	public void testIsCompleteWithFirstDeviceNotStartingAtBottom() throws MemoryRangeException {
+	public void testIsCompleteWithFirstDeviceNotStartingAtStartAddress() throws MemoryRangeException {
 		Device memory = new Memory(0x00ff, 0xff00, null, true);
 
 		Bus b = new Bus(0x0000, 0xffff);
diff --git a/src/test/java/com/loomcom/lm6502/MemoryRangeTest.java b/src/test/java/com/loomcom/lm6502/MemoryRangeTest.java
index d115626..f4fd340 100644
--- a/src/test/java/com/loomcom/lm6502/MemoryRangeTest.java
+++ b/src/test/java/com/loomcom/lm6502/MemoryRangeTest.java
@@ -5,6 +5,8 @@ import junit.framework.*;
 import java.util.HashMap;
 import java.util.Map;
 
+import com.loomcom.lm6502.exceptions.*;
+
 /**
  *
  */
@@ -36,14 +38,14 @@ public class MemoryRangeTest extends TestCase {
 		new MemoryRange(2, 10);
 	}
 
-	public void testGetStartAddress() throws MemoryRangeException {
+	public void testStartAddress() throws MemoryRangeException {
 		MemoryRange r = new MemoryRange(0x101, 0x202);
-		assertEquals(0x101, r.getStartAddress());
+		assertEquals(0x101, r.startAddress());
 	}
 
-	public void testGetEndAddress() throws MemoryRangeException {
+	public void testEndAddress() throws MemoryRangeException {
 		MemoryRange r = new MemoryRange(0x101, 0x202);
-		assertEquals(0x202, r.getEndAddress());
+		assertEquals(0x202, r.endAddress());
 	}
 
 	public void testOverlaps() throws MemoryRangeException {