diff --git a/src/main/java/com/loomcom/symon/CommandParser.java b/src/main/java/com/loomcom/symon/CommandParser.java
deleted file mode 100644
index 5a48efc..0000000
--- a/src/main/java/com/loomcom/symon/CommandParser.java
+++ /dev/null
@@ -1,92 +0,0 @@
-package com.loomcom.symon;
-
-import java.io.*;
-import com.loomcom.symon.devices.*;
-import com.loomcom.symon.exceptions.*;
-
-
-public class CommandParser {
-
-  private BufferedReader in;
-  private BufferedWriter out;
-  private Simulator simulator;
-
-  public CommandParser(InputStream i, OutputStream o, Simulator s) {
-    this.in = new BufferedReader(new InputStreamReader(i));
-    this.out = new BufferedWriter(new OutputStreamWriter(o));
-    this.simulator = s;
-  }
-
-  public void run() throws MemoryAccessException {
-    try {
-      String command = null;
-      greeting();
-      prompt();
-      while (!shouldQuit(command = readLine())) {
-        dispatch(command);
-        prompt();
-      }
-      writeLine("\n\nGoodbye!");
-    } catch (IOException ex) {
-      System.err.println("Error: " + ex.toString());
-      System.exit(1);
-    }
-  }
-
-  public void setSimulator(Simulator sim) {
-    this.simulator = sim;
-  }
-
-  public Simulator getSimulator() {
-    return this.simulator;
-  }
-
-  /**
-   * Dispatch the command.
-   */
-  public void dispatch(String command)
-      throws MemoryAccessException, IOException {
-    // TODO: Real implementation. This first one is just
-    // for testing.
-    if ("test".equals(command)) {
-      simulator.runTest();
-    } else if ("ex".equals(command)) {
-      writeLine(simulator.getState());
-    } else {
-      writeLine("Huh?");
-    }
-  }
-
-  /*******************************************************************
-   * Private
-   *******************************************************************/
-
-  private void greeting() throws IOException {
-    writeLine("Welcome to the j6502 Simulator!");
-  }
-
-  private void prompt() throws IOException {
-    out.write("j6502> ");
-    out.flush();
-  }
-
-  private String readLine() throws IOException {
-    String line = in.readLine();
-    if (line == null) { return null; }
-    return line.trim();
-  }
-
-  private void writeLine(String line) throws IOException {
-    out.write(line);
-    out.newLine();
-    out.flush();
-  }
-
-  /**
-   * Returns true if the line is a quit.
-   */
-  private boolean shouldQuit(String line) {
-    return (line == null || "q".equals(line.toLowerCase()));
-  }
-
-}
\ No newline at end of file
diff --git a/src/main/java/com/loomcom/symon/Simulator.java b/src/main/java/com/loomcom/symon/Simulator.java
index 7bca1db..47a44a8 100644
--- a/src/main/java/com/loomcom/symon/Simulator.java
+++ b/src/main/java/com/loomcom/symon/Simulator.java
@@ -1,5 +1,15 @@
 package com.loomcom.symon;
 
+import java.io.BufferedReader;
+import java.io.BufferedWriter;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.io.OutputStream;
+import java.io.OutputStreamWriter;
+import java.util.List;
+import java.util.StringTokenizer;
+
 import com.loomcom.symon.devices.*;
 import com.loomcom.symon.exceptions.*;
 
@@ -8,11 +18,6 @@ import com.loomcom.symon.exceptions.*;
  */
 public class Simulator {
 
-  /**
-   * Command-line parser used by this simulator.
-   */
-  private CommandParser parser;
-
   /**
    * The CPU itself.
    */
@@ -23,35 +28,197 @@ public class Simulator {
    * correct IO devices.
    */
   private Bus bus;
+  
+  private BufferedReader in;
+  private BufferedWriter out;
+  
+  /* If true, trace execution of the CPU */
+  private boolean trace = false;
 
   public Simulator() throws MemoryRangeException {
     cpu = new Cpu();
     bus = new Bus(0x0000, 0xffff);
     bus.addCpu(cpu);
     bus.addDevice(new Memory(0x0000, 0x10000));
-    parser = new CommandParser(System.in, System.out, this);
-  }
-
-  public String getState() throws MemoryAccessException {
-    return cpu.toString();
+    this.in = new BufferedReader(new InputStreamReader(System.in));
+    this.out = new BufferedWriter(new OutputStreamWriter(System.out));
   }
 
   public void run() throws MemoryAccessException {
-    parser.run();
-  }
-
-  public void load(int address, int[] program)
-    throws MemoryAccessException {
-    int i = 0;
-    for (int d : program) {
-      bus.write(address + i++, d);
+    try {
+      greeting();
+      prompt();
+      String command = null;
+      while (!shouldQuit(command = readLine())) {
+        try {
+          dispatch(command);
+        } catch (CommandFormatException ex) {
+          writeLine(ex.getMessage());
+        }
+        prompt();
+      }
+      writeLine("\n\nGoodbye!");
+    } catch (IOException ex) {
+      System.err.println("Error: " + ex.toString());
+      System.exit(1);
     }
   }
-
+    
   /**
-   * A test method.
+   * Dispatch the command.
    */
-  public void runTest() throws MemoryAccessException {
+  public void dispatch(String commandLine)
+      throws MemoryAccessException, IOException, CommandFormatException {
+    Command c = new Command(commandLine);
+    String cmd = c.getCommand();
+    if (cmd != null) {
+      if ("test".equals(cmd)) {
+        doTest();
+      } else if (cmd.startsWith("s")) {
+        doGetState();
+      } else if (cmd.startsWith("r")) { 
+        doReset();
+      } else if (cmd.startsWith("e")) {
+        doExamine(c);
+      } else if (cmd.startsWith("d")) {
+        doDeposit(c);
+      } else if (cmd.startsWith("g")) {
+        doGo(c);
+      } else if (cmd.startsWith("h")) {
+        doHelp(c);
+      } else if (cmd.startsWith("t")) {
+        doToggleTrace();
+      } else if (cmd.startsWith("f")) {
+        doFill(c);
+      } else {
+        writeLine("? Type h for help");
+      }
+    }
+  }
+  
+  public void doHelp(Command c) throws IOException {
+    writeLine("Symon 6502 Simulator");
+    writeLine("");
+    writeLine("All addresses must be in hexadecimal. Commands may be short or");
+    writeLine("long (e.g. 'e' or 'ex' or 'examine'). Note that 'go' clears the");
+    writeLine("Break processor status flag");
+    writeLine("");
+    writeLine("g [address  [steps]]       Start running at address.");
+    writeLine("e [start    [end]]         Examine memory.");
+    writeLine("d [address] [data]         Deposit data into address.");
+    writeLine("f [start]   [end] [data]   Fill memory with data.");
+    writeLine("r                          Reset simulator.");
+    writeLine("s                          Show CPU state.");
+    writeLine("t                          Toggle trace.");
+    writeLine("q (or Control-D)           Quit.");
+  }
+
+  public void doGetState() throws IOException, MemoryAccessException {
+    writeLine(cpu.toString());
+    writeLine("Trace is " + (trace ? "on" : "off"));
+  }
+
+  public void doExamine(Command c) throws IOException, MemoryAccessException, CommandFormatException {
+    try {
+      if (c.numArgs() == 2) {
+        int startAddress = stringToWord(c.getArgs()[0]);
+        int endAddress   = stringToWord(c.getArgs()[1]);
+        while (startAddress < endAddress) {
+          StringBuffer line = new StringBuffer();
+          int numBytes = 0;
+          line.append(String.format("%04x  ", startAddress));
+          while (numBytes++ < 8 && startAddress <= endAddress) {
+            line.append(String.format("%02x ", bus.read(startAddress++)));
+          }
+          writeLine(line.toString());
+        }
+      } else if (c.numArgs() == 1) {
+        int address = stringToWord(c.getArgs()[0]);
+        writeLine(String.format("%04x  %02x", address, bus.read(address)));
+      } else {
+        throw new CommandFormatException("e [start [end]]");        
+      }
+    } catch (NumberFormatException ex) {
+      throw new CommandFormatException("Address not understood");
+    }
+  }
+  
+  public void doDeposit(Command c) throws MemoryAccessException, CommandFormatException {
+    if (c.numArgs() != 2) {
+      throw new CommandFormatException("d [address] [data]");
+    }
+    try {
+      int address = stringToWord(c.getArg(0));
+      int data    = stringToByte(c.getArg(1));
+      bus.write(address, data);
+    } catch (NumberFormatException ex) {
+      throw new CommandFormatException("Address not understood");
+    }
+  }
+  
+  public void doFill(Command c) throws MemoryAccessException, CommandFormatException {
+    if (c.numArgs() != 3) {
+      throw new CommandFormatException("f [start] [end] [data]");
+    }
+    try {
+      int start = stringToWord(c.getArg(0));
+      int end = stringToWord(c.getArg(1));
+      int data = stringToByte(c.getArg(2));
+      while (start < end) {
+        bus.write(start, data);
+        start++;
+      }
+    } catch (NumberFormatException ex) {
+      throw new CommandFormatException("Address not understood");
+    }
+  }
+  
+  public void doGo(Command c) throws IOException, MemoryAccessException, CommandFormatException {
+    if (c.numArgs() != 1 && c.numArgs() != 2) {
+      throw new CommandFormatException("g [address [steps]]");
+    }
+    try {
+      int start = stringToWord(c.getArg(0));
+      int steps = -1;
+      if (c.numArgs() == 2) {
+        steps = stringToWord(c.getArg(1));
+      }
+      
+      // Make a gross assumption: Restarting the CPU clears
+      // the break flag and the IRQ disable flag.
+      cpu.clearBreakFlag();
+      cpu.clearIrqDisableFlag();
+      
+      cpu.setProgramCounter(start);
+      while (!cpu.getBreakFlag() && (steps == -1 || steps-- > 0)) {
+        cpu.step();
+        if (trace) {
+          writeLine(cpu.toString());
+        }
+      }
+      if (!trace) {
+        writeLine(cpu.toString());
+      }
+    } catch (NumberFormatException ex) {
+      throw new CommandFormatException("Address not understood");
+    }
+  }
+  
+  public void doToggleTrace() throws IOException {
+    this.trace = !trace;
+    writeLine("Trace is now " + (trace ? "on" : "off"));
+  }
+  
+  public void doReset() throws MemoryAccessException {
+    cpu.reset();
+    this.trace = false;
+  }
+  
+  /**
+   * Run a very simple test program that doesn't do
+   * much other than exercise the accumulator a bit.
+   */
+  public void doTest() throws MemoryAccessException {
     int[] zpData = {
       0x39,             // $0000
       0x21,             // $0001
@@ -100,5 +267,99 @@ public class Simulator {
       System.err.println("Error: " + ex.toString());
     }
   }
+  
+  /*******************************************************************
+   * Private
+   *******************************************************************/
 
+  public void load(int address, int[] data)
+    throws MemoryAccessException {
+    int i = 0;
+    for (int d : data) {
+      bus.write(address + i++, d);
+    }
+  }
+  
+  private int stringToWord(String addrString) {
+    return Integer.parseInt(addrString, 16) & 0xffff;
+  }
+  
+  private int stringToByte(String dataString) {
+    return Integer.parseInt(dataString, 16) & 0xff;
+  }
+  
+  private void greeting() throws IOException {
+    writeLine("Welcome to the Symon Simulator!");
+  }
+
+  private void prompt() throws IOException {
+    out.write("symon> ");
+    out.flush();
+  }
+
+  private String readLine() throws IOException {
+    String line = in.readLine();
+    if (line == null) { return null; }
+    return line.trim();
+  }
+
+  private void writeLine(String line) throws IOException {
+    out.write(line);
+    out.newLine();
+    out.flush();
+  }
+
+  /**
+   * Returns true if the line is a quit.
+   */
+  private boolean shouldQuit(String line) {
+    return (line == null || "q".equals(line.toLowerCase()));
+  }
+  
+  /**
+   * Command line tokenizer class. Given a command line, tokenize
+   * it and give easy access to the command and its arguments.
+   */
+  public static final class Command {
+    private String command;
+    private String[] args;
+
+    public Command(String commandLine) {
+      StringTokenizer st = new StringTokenizer(commandLine);
+      int numTokens = st.countTokens();
+      int idx = 0;
+      args = new String[numTokens > 1 ? numTokens - 1 : 0];
+      while (st.hasMoreTokens()) {
+        if (command == null) {
+          command = st.nextToken();
+        } else {
+          args[idx++] = st.nextToken();
+        }
+      }
+    }
+    
+    public String getCommand() {
+      return command;
+    }
+    
+    public String[] getArgs() {
+      return args;
+    }
+    
+    public String getArg(int argNum) {
+      if (argNum > args.length - 1) {
+        return null;
+      } else {
+        return args[argNum];
+      }
+    }
+    
+    public int numArgs() {
+      return args.length;
+    }
+    
+    public boolean hasArgs() {
+      return args.length > 0;
+    } 
+  }
 }
diff --git a/src/main/java/com/loomcom/symon/devices/Memory.java b/src/main/java/com/loomcom/symon/devices/Memory.java
index f31480f..ef377f3 100644
--- a/src/main/java/com/loomcom/symon/devices/Memory.java
+++ b/src/main/java/com/loomcom/symon/devices/Memory.java
@@ -14,8 +14,8 @@ public class Memory extends Device {
     super(address, size, "RW Memory");
     this.readOnly = readOnly;
     this.mem = new int[size];
-    // Initialize all locations to 0xff
-    Arrays.fill(this.mem, 0xff);
+    // Initialize all locations to 0x00 (BRK)
+    Arrays.fill(this.mem, 0x00);
   }
 
   public Memory(int address, int size)
diff --git a/src/main/java/com/loomcom/symon/exceptions/CommandFormatException.java b/src/main/java/com/loomcom/symon/exceptions/CommandFormatException.java
new file mode 100644
index 0000000..2208a6b
--- /dev/null
+++ b/src/main/java/com/loomcom/symon/exceptions/CommandFormatException.java
@@ -0,0 +1,10 @@
+package com.loomcom.symon.exceptions;
+
+public class CommandFormatException extends Exception {
+  public CommandFormatException(String msg) {
+    super(msg);
+  }
+  public CommandFormatException() {
+    super();
+  }
+}
diff --git a/src/test/java/com/loomcom/symon/CommandTest.java b/src/test/java/com/loomcom/symon/CommandTest.java
new file mode 100644
index 0000000..e7b21b6
--- /dev/null
+++ b/src/test/java/com/loomcom/symon/CommandTest.java
@@ -0,0 +1,64 @@
+package com.loomcom.symon;
+
+import org.junit.*;
+import static org.junit.Assert.*;
+
+public class CommandTest {
+  
+  @Test
+  public void testCommandShouldParseCorrectNumberOfArguments() {
+    Simulator.Command c;
+    
+    c = new Simulator.Command("foo");
+    assertEquals("foo", c.getCommand());
+    assertEquals(0, c.numArgs());
+    
+    c = new Simulator.Command("foo bar");
+    assertEquals("foo", c.getCommand());
+    assertEquals(1, c.numArgs());
+    assertEquals("bar", c.getArgs()[0]);
+    
+    c = new Simulator.Command("foo bar baz quux 0 100");
+    assertEquals("foo",  c.getCommand());
+    assertEquals(5,      c.numArgs());
+    assertEquals("bar",  c.getArgs()[0]);
+    assertEquals("baz",  c.getArgs()[1]);
+    assertEquals("quux", c.getArgs()[2]);
+    assertEquals("0",    c.getArgs()[3]);
+    assertEquals("100",  c.getArgs()[4]);
+  }
+
+  @Test
+  public void testCommandShouldIgnoreWhitespaceBetweenTokens() {
+    Simulator.Command c;
+    
+    c = new Simulator.Command("foo    bar     baz");
+    assertEquals("foo", c.getCommand());
+    assertEquals(2, c.numArgs());
+    assertEquals("bar", c.getArgs()[0]);
+    assertEquals("baz", c.getArgs()[1]);
+  }
+
+  @Test
+  public void testCommandShouldIgnoreWhitespaceBeforeCommand() {
+    Simulator.Command c;
+    
+    c = new Simulator.Command("     foo bar baz");
+    assertEquals("foo", c.getCommand());
+    assertEquals(2, c.numArgs());
+    assertEquals("bar", c.getArgs()[0]);
+    assertEquals("baz", c.getArgs()[1]);
+  }
+
+  @Test
+  public void testCommandShouldIgnoreWhitespaceAfterCommand() {
+    Simulator.Command c;
+    
+    c = new Simulator.Command("foo bar baz      ");
+    assertEquals("foo", c.getCommand());
+    assertEquals(2, c.numArgs());
+    assertEquals("bar", c.getArgs()[0]);
+    assertEquals("baz", c.getArgs()[1]);
+  }
+
+}
diff --git a/src/test/java/com/loomcom/symon/CpuAbsoluteXModeTest.java b/src/test/java/com/loomcom/symon/CpuAbsoluteXModeTest.java
index c2b4a02..1546b4a 100644
--- a/src/test/java/com/loomcom/symon/CpuAbsoluteXModeTest.java
+++ b/src/test/java/com/loomcom/symon/CpuAbsoluteXModeTest.java
@@ -170,7 +170,7 @@ public class CpuAbsoluteXModeTest extends TestCase {
                     0xa9, 0xff,        // LDA #$ff
                     0x3d, 0x05, 0x1a,  // AND $1a05,X
                     0xa9, 0x01,        // LDA #$01
-                    0x3d, 0xd2, 0x1a); // AND $1ad2,X
+                    0x3d, 0xd2, 0x19); // AND $19d2,X
     cpu.step();
     assertEquals(0x00, cpu.getAccumulator());
     assertTrue(cpu.getZeroFlag());
diff --git a/src/test/java/com/loomcom/symon/CpuAbsoluteYModeTest.java b/src/test/java/com/loomcom/symon/CpuAbsoluteYModeTest.java
index ef2ffdd..da1b9ec 100644
--- a/src/test/java/com/loomcom/symon/CpuAbsoluteYModeTest.java
+++ b/src/test/java/com/loomcom/symon/CpuAbsoluteYModeTest.java
@@ -115,7 +115,7 @@ public class CpuAbsoluteYModeTest extends TestCase {
                     0xa9, 0xff,        // LDA #$ff
                     0x39, 0x05, 0x1a,  // AND $1a05,Y
                     0xa9, 0x01,        // LDA #$01
-                    0x39, 0xd2, 0x1a); // AND $1ad2,Y
+                    0x39, 0xd2, 0x19); // AND $19d2,Y
     cpu.step();
     assertEquals(0x00, cpu.getAccumulator());
     assertTrue(cpu.getZeroFlag());
diff --git a/src/test/java/com/loomcom/symon/CpuImpliedModeTest.java b/src/test/java/com/loomcom/symon/CpuImpliedModeTest.java
index 8df6fcd..1489bfa 100644
--- a/src/test/java/com/loomcom/symon/CpuImpliedModeTest.java
+++ b/src/test/java/com/loomcom/symon/CpuImpliedModeTest.java
@@ -74,7 +74,7 @@ public class CpuImpliedModeTest extends TestCase {
     cpu.setOverflowFlag();
     assertEquals(0x20|Cpu.P_CARRY|Cpu.P_OVERFLOW,
                  cpu.getProcessorStatus());
-    assertEquals(0xff, cpu.stackPeek());
+    assertEquals(0x00, cpu.stackPeek());
     assertFalse(cpu.getBreakFlag());
     assertEquals(0x0200, cpu.getProgramCounter());
     assertEquals(0xff, cpu.getStackPointer());
diff --git a/src/test/java/com/loomcom/symon/CpuIndexedIndirectModeTest.java b/src/test/java/com/loomcom/symon/CpuIndexedIndirectModeTest.java
index 72eaa76..5885ec0 100644
--- a/src/test/java/com/loomcom/symon/CpuIndexedIndirectModeTest.java
+++ b/src/test/java/com/loomcom/symon/CpuIndexedIndirectModeTest.java
@@ -1,7 +1,6 @@
 package com.loomcom.symon;
 
 import com.loomcom.symon.devices.Memory;
-import com.loomcom.symon.exceptions.MemoryAccessException;
 
 import org.junit.*;
 import static org.junit.Assert.*;
diff --git a/src/test/java/com/loomcom/symon/CpuIndirectXModeTest.java b/src/test/java/com/loomcom/symon/CpuIndirectXModeTest.java
index 45d88cf..d9bf3c9 100644
--- a/src/test/java/com/loomcom/symon/CpuIndirectXModeTest.java
+++ b/src/test/java/com/loomcom/symon/CpuIndirectXModeTest.java
@@ -115,7 +115,7 @@ public class CpuIndirectXModeTest extends TestCase {
                     0xa9, 0xff,        // LDA #$ff
                     0x3d, 0x05, 0x1a,  // AND $1a05,X
                     0xa9, 0x01,        // LDA #$01
-                    0x3d, 0xd2, 0x1a); // AND $1ad2,X
+                    0x3d, 0xd2, 0x19); // AND $19d2,X
     cpu.step();
     assertEquals(0x00, cpu.getAccumulator());
     assertTrue(cpu.getZeroFlag());
diff --git a/src/test/java/com/loomcom/symon/CpuTest.java b/src/test/java/com/loomcom/symon/CpuTest.java
index fe41322..ea86bf8 100644
--- a/src/test/java/com/loomcom/symon/CpuTest.java
+++ b/src/test/java/com/loomcom/symon/CpuTest.java
@@ -70,7 +70,7 @@ public class CpuTest extends TestCase {
 
   public void testStackPush() throws MemoryAccessException {
     assertEquals(0xff, cpu.getStackPointer());
-    assertEquals(0xff, bus.read(0x1ff));
+    assertEquals(0x00, bus.read(0x1ff));
 
     cpu.stackPush(0x06);
     assertEquals(0xfe, cpu.getStackPointer());
@@ -179,7 +179,7 @@ public class CpuTest extends TestCase {
   }
 
   public void testStackPeekDoesNotAlterStackPointer() throws MemoryAccessException {
-    assertEquals(0xff, cpu.stackPeek());
+    assertEquals(0x00, cpu.stackPeek());
     assertEquals(0xff, cpu.getStackPointer());
 
     cpu.stackPush(0x01);