diff --git a/monitor.py b/monitor.py
new file mode 100644
index 0000000..cb40c59
--- /dev/null
+++ b/monitor.py
@@ -0,0 +1,295 @@
+#!/usr/bin/env python -u
+
+import cmd
+import os
+import re
+import shlex
+from mpu import MPU
+
+class Monitor(cmd.Cmd):
+
+    def __init__(self, options, completekey='tab', stdin=None, stdout=None):
+        self.options = options
+        self._mpu = MPU()
+        self._update_prompt()
+        self._radix = 16
+        cmd.Cmd.__init__(self, completekey, stdin, stdout)
+
+    def onecmd(self, line):
+        matches = re.match('^~\s*', line)
+        if matches:
+            start, end = matches.span()
+            line = "tilde %s" % line[end:]
+        
+        result = cmd.Cmd.onecmd(self, line)
+        self._update_prompt()
+        return result
+
+    def _update_prompt(self):
+        self.prompt = "\n%s\n." % repr(self._mpu)
+        
+    def _output(self, stuff):
+        if stuff is not None:
+            self.stdout.write(stuff + '\n')
+
+    def help_version(self):
+        self._output("version\t\tDisplay Py65 version information.")
+
+    def do_version(self, args):
+        self._output("\nPy65 Monitor")
+
+    def help_help(self):
+        self._output("help\t\tPrint a list of available actions.")
+        self._output("help <action>\tPrint help for <action>.")
+
+    def help_reset(self):
+        self._output("reset\t\tReset the microprocessor")
+
+    def do_reset(self):
+        self._mpu = MPU()
+
+    def do_EOF(self, args):
+        self._output('')
+        return 1
+
+    def help_EOF(self):
+        self._output("To quit, type ^D or use the quit command.")
+
+    def do_quit(self, args):
+        return self.do_EOF
+    
+    def help_quit(self):
+        return self.help_EOF()
+
+    def help_step(self):
+        self._output("Single-step through instructions.")
+
+    def do_step(self, args):
+        self._mpu.step()
+    
+    def help_return(self):
+        self._output("Continues execution and returns to the monitor just")
+        self._output("before the next RTS or RTI is executed.")
+    
+    def do_return(self, args):
+        last_instruct = None
+        while last_instruct != 0x60:  # RTS
+            self._mpu.step()
+            last_instruct = self._mpu.memory[self._mpu.pc]
+    
+    def help_radix(self):
+        self._output("radix [H|D|O|B]")
+        self._output("Set the default radix to hex, decimal, octal, or binary.")
+        self._output("With no argument, the current radix is printed.")
+    
+    def help_cycles(self):
+        self._output("Display the total number of cycles executed.")
+    
+    def do_cycles(self, args):
+        self._output(str(self._mpu.processorCycles))
+    
+    def do_radix(self, args):
+        radixes = {'Hexadecimal': 16, 'Decimal': 10, 'Octal': 8, 'Binary': 2}
+
+        if args != '':
+            new = args[0].lower()
+            changed = False
+            for name, radix in radixes.iteritems():
+                if name[0].lower() == new:
+                    self._radix = radix
+                    changed = True
+            if not changed:
+                self._output("Illegal radix: %s" % args)
+
+        for name, radix in radixes.iteritems():
+            if self._radix == radix:
+                self._output("Default radix is %s" % name)
+    
+    def help_tilde(self):
+        self._output("~ <number>")
+        self._output("Display the specified number in decimal, hex, octal, and binary.")
+    
+    def do_tilde(self, args):
+        try:
+            num = self._parsenum(args)
+        except ValueError:
+            self._output("Syntax error: %s" % args)
+            return
+
+        self._output("+%u" % num)
+        self._output("$%02x" % num)
+        self._output("%04o" % num)
+        self._output(self._itoa(num, 2).zfill(8))
+    
+    def help_registers(self):
+        self._output("registers[<reg_name> = <number> [, <reg_name> = <number>]*]")
+        self._output("Assign respective registers.  With no parameters,")
+        self._output("display register values.")
+    
+    def do_registers(self, args):
+        if args == '':
+            return
+        
+        pairs = re.findall('([^=,\s]*)=([^=,\s]*)', args)
+        if pairs == []:
+            return self._output("Syntax error: %s" % args)
+
+        for register, value in pairs:
+            if register not in ('pc', 'sp', 'a', 'x', 'y'):
+                self._output("Invalid register: %s" % register)
+            else:
+                try:
+                    intval = self._parsenum(value) & 0xFFFF
+                    if len(register) == 1:
+                        intval &= 0xFF
+                    setattr(self._mpu, register, intval)
+                except ValueError:
+                    self._output("Syntax error: %s" % value)
+    
+    def help_cd(self, args):
+        self._output("cd <directory>")
+        self._output("Change the working directory.")
+    
+    def do_cd(self, args):
+        try:
+            os.chdir(args)
+        except OSError, why:
+            msg = "Cannot change directory: [%d] %s" % (why[0], why[1])
+            self._output(msg)
+        self.do_pwd()
+
+    def help_pwd(self):
+        self._output("Show the current working directory.")
+        
+    def do_pwd(self, args=None):
+        cwd = os.getcwd()
+        self._output(cwd)
+
+    def help_load(self):
+        self._output("load \"filename\" <address>")
+        self._output("Load the specified file into memory at the specified address.")
+        self._output("Commodore-style load address bytes are ignored.")
+
+    def do_load(self, args):
+        split = shlex.split(args)
+        if len(split) > 2:
+            self._output("Syntax error: %s" % args)
+            return
+
+        filename = split[0]
+        if len(split) == 2:
+            start = self._parsenum(split[1])
+        else:
+            start = self._mpu.pc
+
+        try:
+            f = open(filename, 'rb')
+            bytes = f.read()
+            f.close()
+        except (OSError, IOError), why:
+            msg = "Cannot load file: [%d] %s" % (why[0], why[1])
+            self._output(msg)
+            return
+
+        address = start
+        for byte in bytes:
+            address &= 0xFFFF
+            self._mpu.memory[address] = ord(byte)
+            address += 1
+
+        fmt = (address - start, start, address)
+        self._output("Loaded %d bytes from %d to %d" % fmt)
+    
+    def help_fill(self):
+        self.output("fill <address_range> <data_list>")
+        self.output("Fill memory in the specified address range with the data in")
+        self.output("<data_list>.  If the size of the address range is greater")
+        self.output("than the size of the data_list, the data_list is repeated.")
+    
+    def do_fill(self, args):
+        split = shlex.split(args)
+        if len(split) != 2:
+            self._output("Syntax error: %s" % args)
+            return
+        addresses, fillbyte = split
+
+        start, end = self._parserange(addresses)
+        fillbyte   = self._parsenum(fillbyte)
+        
+        self._fill(start, end, fillbyte)
+
+    def _fill(self, start, end, byte):
+        byte &= 0xFF
+        address = start
+        while address <= end:
+            address &= 0xFFFF
+            self._mpu.memory[address] = byte
+            address += 1
+        fmt = (end - start + 1, start, end)
+        self._output("Filled +%d bytes from %d to %d" % fmt)
+    
+    def help_mem(self):
+        self._output("mem <address_range>")
+        self._output("Display the contents of memory.")
+    
+    def do_mem(self, args):
+        start, end = self._parserange(args)
+
+        out = self._itoa(start, 16) + ":  "
+        for byte in self._mpu.memory[start:end]:
+            out += "%02x  " % byte
+        self._output(out)
+         
+    def _parsenum(self, num):
+        if num.startswith('%'):
+            return int(num[1:], 2)
+
+        elif num.startswith('$'):
+            return int(num[1:], 16)
+
+        elif num.startswith('+'):
+            return int(num[1:], 10)
+
+        else:
+            return int(num, self._radix)
+
+    def _parserange(self, addresses):
+        matches = re.match('^([^,]+)\s*[:,]+\s*([^,]+)$', addresses)
+        if not matches:
+            raise ValueError
+        start, end = map(self._parsenum, matches.groups(0))
+
+        if start > end:
+            start, end = end, start            
+        return (start, end)
+
+    def _itoa(self, num, base=10):
+       negative = num < 0
+       if negative:
+          num = -num
+       digits = []
+       while num > 0:
+          num, last_digit = divmod(num, base)
+          digits.append('0123456789abcdefghijklmnopqrstuvwxyz'[last_digit])
+       if negative:
+          digits.append('-')
+       digits.reverse()
+       return ''.join(digits) 
+
+def main(args=None, options=None):
+    c = Monitor(options)
+
+    try:
+        import readline
+    except ImportError:
+        pass
+
+    try:
+        c.onecmd('version')
+        c.cmdloop()
+    except KeyboardInterrupt:
+        c._output('')
+        pass
+
+if __name__ == "__main__":
+    main()