diff --git a/se/sics/mspsim/core/IOPort.java b/se/sics/mspsim/core/IOPort.java
index cad5255..fc28261 100644
--- a/se/sics/mspsim/core/IOPort.java
+++ b/se/sics/mspsim/core/IOPort.java
@@ -36,7 +36,6 @@
  */
 
 package  se.sics.mspsim.core;
-import se.sics.mspsim.util.ArrayUtils;
 import se.sics.mspsim.util.Utils;
 
 public class IOPort extends IOUnit {
@@ -89,7 +88,7 @@ public class IOPort extends IOUnit {
 
     private PortReg[] portMap;
 
-    private PortListener[] listeners = new PortListener[0];
+    private PortListener portListener = null;
     // represents the direction register
 
     /* Registers for Digital I/O */
@@ -102,7 +101,7 @@ public class IOPort extends IOUnit {
     private int ifg;
     private int ies; /* edge select */
     private int ren;
-    private int ds;
+//    private int ds;
 
     private int iv; /* low / high */
 
@@ -184,9 +183,21 @@ public class IOPort extends IOUnit {
         return sel;
     }
 
-    public void setPortListener(PortListener listener) {
-    	/* 2011-09-11: XXX should now be named addPortListener() */
-        listeners = (PortListener[]) ArrayUtils.add(PortListener.class, listeners, listener);
+    public synchronized void addPortListener(PortListener newListener) {
+        portListener = PortListenerProxy.addPortListener(portListener, newListener);
+    }
+
+    public synchronized void removePortListener(PortListener oldListener) {
+        portListener = PortListenerProxy.removePortListener(portListener, oldListener);
+    }
+
+    @Deprecated
+    public synchronized void setPortListener(PortListener listener) {
+        if (listener != null) {
+            addPortListener(listener);
+        } else {
+            portListener = null;
+        }
     }
 
     public void setTimerCapture(Timer timer, int pin) {
@@ -242,25 +253,29 @@ public class IOPort extends IOUnit {
 
     void write_port(PortReg function, int data, long cycles) {
         switch(function) {
-        case OUT:
+        case OUT: {
             out = data;
-            for (PortListener listener: listeners) {
+            PortListener listener = portListener;
+            if (listener != null) {
             	listener.portWrite(this, out | (~dir) & 0xff);
             }
             break;
+        }
         case IN:
             logw("WARNING: writing to read-only " + getID() + "IN");
             throw new EmulationException("Writing to read-only " + getID() + "IN");
             //          in = data;
-        case DIR:
+        case DIR: {
             dir = data;
-            for (PortListener listener: listeners) {
+            PortListener listener = portListener;
+            if (listener != null) {
                 // Any output configured pin (pin-bit = 0) should have 1 here?! 
                 //              if (name.equals("1"))
                 //                System.out.println(getName() + " write to IOPort via DIR reg: " + Utils.hex8(data));
-                listener.portWrite(this, out | (~dir)&0xff);
+                listener.portWrite(this, out | (~dir) & 0xff);
             }
             break;
+        }
         case REN:
             ren = data;
             break;
diff --git a/se/sics/mspsim/core/PortListenerProxy.java b/se/sics/mspsim/core/PortListenerProxy.java
new file mode 100644
index 0000000..5a6f6eb
--- /dev/null
+++ b/se/sics/mspsim/core/PortListenerProxy.java
@@ -0,0 +1,95 @@
+/*
+ * Copyright (c) 2011, Swedish Institute of Computer Science.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of the Institute nor the names of its contributors
+ *    may be used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE INSTITUTE AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE INSTITUTE OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * This file is part of MSPSim.
+ *
+ * -----------------------------------------------------------------
+ *
+ * PortListenerProxy
+ *
+ * Author  : Niclas Finne
+ * Created : Tue Dec 7 18:25:00 2011
+ */
+
+package se.sics.mspsim.core;
+import se.sics.mspsim.util.ArrayUtils;
+
+public class PortListenerProxy implements PortListener {
+
+    private PortListener[] portListeners;
+
+    public PortListenerProxy(PortListener listen1, PortListener listen2) {
+        portListeners = new PortListener[] { listen1, listen2 };
+    }
+
+    public static PortListener addPortListener(PortListener portListener, PortListener listener) {
+        if (portListener == null) {
+            return listener;
+        }
+        if (portListener instanceof PortListenerProxy) {
+            return ((PortListenerProxy)portListener).add(listener);
+        }
+        return new PortListenerProxy(portListener, listener);
+    }
+
+    public static PortListener removePortListener(PortListener portListener, PortListener listener) {
+        if (portListener == listener) {
+            return null;
+        }
+        if (portListener instanceof PortListenerProxy) {
+            return ((PortListenerProxy)portListener).remove(listener);
+        }
+        return portListener;
+    }
+
+    public PortListener add(PortListener mon) {
+        portListeners = (PortListener[]) ArrayUtils.add(PortListener.class, portListeners, mon);
+        return this;
+    }
+
+    public PortListener remove(PortListener listener) {
+        PortListener[] listeners = (PortListener[]) ArrayUtils.remove(portListeners, listener);
+        if (listeners == null) {
+            return null;
+        }
+        if (listeners.length == 1) {
+            return listeners[0];
+        }
+        portListeners = listeners;
+        return this;
+    }
+
+    @Override
+    public void portWrite(IOPort source, int data) {
+        PortListener[] listeners = this.portListeners;
+        for(PortListener l : listeners) {
+            l.portWrite(source, data);
+        }
+    }
+
+}