diff --git a/src/mem/bus.cc b/src/mem/bus.cc
index 86a148f87..7ae41e11e 100644
--- a/src/mem/bus.cc
+++ b/src/mem/bus.cc
@@ -42,13 +42,14 @@
 Port *
 Bus::getPort(const std::string &if_name, int idx)
 {
-    if (if_name == "default")
+    if (if_name == "default") {
         if (defaultPort == NULL) {
             defaultPort = new BusPort(csprintf("%s-default",name()), this,
-                    defaultId);
+                                      defaultId);
             return defaultPort;
         } else
             fatal("Default port already set\n");
+    }
 
     // if_name ignored?  forced to be empty?
     int id = interfaces.size();
@@ -269,7 +270,16 @@ Bus::findPort(Addr addr, int id)
                 return defaultPort;
             }
         }
-        panic("Unable to find destination for addr: %#llx", addr);
+
+        if (responderSet) {
+            panic("Unable to find destination for addr (user set default "
+                  "responder): %#llx", addr);
+        } else {
+            DPRINTF(Bus, "Unable to find destination for addr: %#llx, will use "
+                    "default port", addr);
+
+            return defaultPort;
+        }
     }
 
 
@@ -392,12 +402,15 @@ Bus::recvStatusChange(Port::Status status, int id)
 
     if (id == defaultId) {
         defaultRange.clear();
-        defaultPort->getPeerAddressRanges(ranges, snoops);
-        assert(snoops.size() == 0);
-        for(iter = ranges.begin(); iter != ranges.end(); iter++) {
-            defaultRange.push_back(*iter);
-            DPRINTF(BusAddrRanges, "Adding range %#llx - %#llx for default range\n",
-                    iter->start, iter->end);
+        // Only try to update these ranges if the user set a default responder.
+        if (responderSet) {
+            defaultPort->getPeerAddressRanges(ranges, snoops);
+            assert(snoops.size() == 0);
+            for(iter = ranges.begin(); iter != ranges.end(); iter++) {
+                defaultRange.push_back(*iter);
+                DPRINTF(BusAddrRanges, "Adding range %#llx - %#llx for default range\n",
+                        iter->start, iter->end);
+            }
         }
     } else {
 
@@ -503,18 +516,20 @@ BEGIN_DECLARE_SIM_OBJECT_PARAMS(Bus)
     Param<int> bus_id;
     Param<int> clock;
     Param<int> width;
+    Param<bool> responder_set;
 
 END_DECLARE_SIM_OBJECT_PARAMS(Bus)
 
 BEGIN_INIT_SIM_OBJECT_PARAMS(Bus)
     INIT_PARAM(bus_id, "a globally unique bus id"),
     INIT_PARAM(clock, "bus clock speed"),
-    INIT_PARAM(width, "width of the bus (bits)")
+    INIT_PARAM(width, "width of the bus (bits)"),
+    INIT_PARAM(responder_set, "Is a default responder set by the user")
 END_INIT_SIM_OBJECT_PARAMS(Bus)
 
 CREATE_SIM_OBJECT(Bus)
 {
-    return new Bus(getInstanceName(), bus_id, clock, width);
+    return new Bus(getInstanceName(), bus_id, clock, width, responder_set);
 }
 
 REGISTER_SIM_OBJECT("Bus", Bus)
diff --git a/src/mem/bus.hh b/src/mem/bus.hh
index 7ec7e6830..619720a79 100644
--- a/src/mem/bus.hh
+++ b/src/mem/bus.hh
@@ -240,6 +240,9 @@ class Bus : public MemObject
     /** Port that handles requests that don't match any of the interfaces.*/
     BusPort *defaultPort;
 
+    /** Has the user specified their own default responder? */
+    bool responderSet;
+
   public:
 
     /** A function used to return the port associated with this bus object. */
@@ -247,9 +250,11 @@ class Bus : public MemObject
 
     virtual void init();
 
-    Bus(const std::string &n, int bus_id, int _clock, int _width)
+    Bus(const std::string &n, int bus_id, int _clock, int _width,
+        bool responder_set)
         : MemObject(n), busId(bus_id), clock(_clock), width(_width),
-        tickNextIdle(0), busIdle(this), inRetry(false), defaultPort(NULL)
+          tickNextIdle(0), busIdle(this), inRetry(false), defaultPort(NULL),
+          responderSet(responder_set)
     {
         //Both the width and clock period must be positive
         if (width <= 0)
diff --git a/src/python/m5/objects/Bus.py b/src/python/m5/objects/Bus.py
index 6710111e5..e7019f3ac 100644
--- a/src/python/m5/objects/Bus.py
+++ b/src/python/m5/objects/Bus.py
@@ -1,10 +1,18 @@
+from m5 import build_env
 from m5.params import *
+from m5.proxy import *
 from MemObject import MemObject
+from Tsunami import BadAddr
 
 class Bus(MemObject):
     type = 'Bus'
     port = VectorPort("vector port for connecting devices")
-    default = Port("Default port for requests that aren't handeled by a device.")
     bus_id = Param.Int(0, "blah")
     clock = Param.Clock("1GHz", "bus clock speed")
     width = Param.Int(64, "bus width (bytes)")
+    responder_set = Param.Bool(False, "Did the user specify a default responder.")
+    if build_env['FULL_SYSTEM']:
+        default = Port(Self.responder.pio, "Default port for requests that aren't handled by a device.")
+        responder = BadAddr(pio_addr=0x0, pio_latency="1ps")
+    else:
+        default = Port("Default port for requests that aren't handled by a device.")
diff --git a/src/python/m5/objects/Tsunami.py b/src/python/m5/objects/Tsunami.py
index 0b53153a0..42bcab089 100644
--- a/src/python/m5/objects/Tsunami.py
+++ b/src/python/m5/objects/Tsunami.py
@@ -15,6 +15,9 @@ class IsaFake(BasicPioDevice):
     type = 'IsaFake'
     pio_size = Param.Addr(0x8, "Size of address range")
 
+class BadAddr(BasicPioDevice):
+    type = 'BadAddr'
+
 class TsunamiIO(BasicPioDevice):
     type = 'TsunamiIO'
     time = Param.UInt64(1136073600,
@@ -70,6 +73,7 @@ class Tsunami(Platform):
         self.cchip.pio = bus.port
         self.pchip.pio = bus.port
         self.pciconfig.pio = bus.default
+        bus.responder_set = True
         self.fake_sm_chip.pio = bus.port
         self.fake_uart1.pio = bus.port
         self.fake_uart2.pio = bus.port