diff --git a/src/py65/memory.py b/src/py65/memory.py
index cddfc94..3cf42b1 100644
--- a/src/py65/memory.py
+++ b/src/py65/memory.py
@@ -12,22 +12,26 @@ class ObservableMemory:
         self._observers = []
 
     def __setitem__(self, address, value):
-        self._notify(self.WRITE, address, value)
+        for oper, addr_range, callback in self._observers:
+            if address in addr_range:
+                if (oper == self.RW) or (oper == self.WRITE):
+                    result = callback(self.WRITE, address, value)        
+                    if result is not None:
+                        value = result
         self._subject[address] = value
 
     def __getitem__(self, address):
-        self._notify(self.READ, address)
+        for oper, addr_range, callback in self._observers:
+            if address in addr_range:
+                if (oper == self.RW) or (oper == self.READ):
+                    result = callback(self.READ, address, None)
+                    if result is not None:
+                        return result
         return self._subject[address]
     
     def __getattr__(self, address):
         return getattr(self._subject, address)
 
-    def _notify(self, operation, address, value=None):
-        for oper, addr_range, callback in self._observers:
-            if address in addr_range:
-                if (oper == self.RW) or (oper == operation):
-                    callback(operation, address, value)
-
     def subscribe(self, operation, addr_range, callback):
         if operation not in (self.READ, self.WRITE, self.RW):
             raise ValueError("Unsupported operation")
diff --git a/src/py65/monitor.py b/src/py65/monitor.py
index 51985b0..2a4af30 100644
--- a/src/py65/monitor.py
+++ b/src/py65/monitor.py
@@ -5,8 +5,9 @@ import os
 import re
 import shlex
 import asyncore
+import sys
 from py65.mpu6502 import MPU
-from py65.util import itoa, AddressParser
+from py65.util import itoa, AddressParser, getch
 from py65.memory import ObservableMemory
 
 class Monitor(cmd.Cmd):
@@ -54,11 +55,16 @@ class Monitor(cmd.Cmd):
         return line
 
     def _install_mpu_observers(self):
-        def printit(operation, address, value):
+        def putc(operation, address, value):
             self.stdout.write(chr(value))
+            self.stdout.flush()
+
+        def getc(operation, address, value):
+            return getch(self.stdin)
 
         m = ObservableMemory()
-        m.subscribe(m.WRITE, [0xE001], printit)
+        m.subscribe(m.WRITE, [0xF001], putc)
+        m.subscribe(m.READ,  [0xF004], getc)
         
         self._mpu.memory = m
 
diff --git a/src/py65/util.py b/src/py65/util.py
index e05d126..2d6bd5f 100644
--- a/src/py65/util.py
+++ b/src/py65/util.py
@@ -1,4 +1,8 @@
 import re
+import select
+import termios
+import fcntl
+import os
 
 class AddressParser:
     """Parse user input into addresses or ranges of addresses.
@@ -123,3 +127,31 @@ bin2bcd = [
   0x80,0x81,0x82,0x83,0x84,0x85,0x86,0x87,0x88,0x89,
   0x90,0x91,0x92,0x93,0x94,0x95,0x96,0x97,0x98,0x99
 ]
+
+def getch(stdin):
+    """ Performs a nonblocking read of one byte from stdin and returns 
+    its ordinal value.  If no byte is available, 0 is returned.
+    """
+    
+    fd = stdin.fileno()
+
+    oldterm = termios.tcgetattr(fd)
+    newattr = oldterm[:]
+    newattr[3] = newattr[3] & ~termios.ICANON & ~termios.ECHO
+    termios.tcsetattr(fd, termios.TCSANOW, newattr)
+
+    oldflags = fcntl.fcntl(fd, fcntl.F_GETFL)
+    fcntl.fcntl(fd, fcntl.F_SETFL, oldflags | os.O_NONBLOCK)
+
+    try:
+        byte = 0
+        r, w, e = select.select([fd], [], [], 0.1)
+        if r:
+            c = stdin.read(1)
+            byte = ord(c)
+            if byte == 0x0a:
+                byte = 0x0d
+    finally:
+        termios.tcsetattr(fd, termios.TCSAFLUSH, oldterm)
+        fcntl.fcntl(fd, fcntl.F_SETFL, oldflags)
+    return byte