diff --git a/configs/test/fs.py b/configs/test/fs.py
index 05b38991c..55e7003a4 100644
--- a/configs/test/fs.py
+++ b/configs/test/fs.py
@@ -197,6 +197,8 @@ class LinuxAlphaSystem(LinuxAlphaSystem):
     else:
         cpu = AtomicSimpleCPU()
     cpu.mem = Parent.magicbus2
+    cpu.itb = AlphaITB()
+    cpu.dtb = AlphaDTB()
     sim_console = SimConsole(listener=ConsoleListener(port=3456))
     kernel = binary('vmlinux')
     pal = binary('ts_osfpal')
@@ -205,18 +207,15 @@ class LinuxAlphaSystem(LinuxAlphaSystem):
 #    readfile = os.path.join(test_base, 'halt.sh')
 
 
-BaseCPU.itb = AlphaITB()
-BaseCPU.dtb = AlphaDTB()
-BaseCPU.system = Parent.any
 
 class TsunamiRoot(System):
     pass
 
 
-def DualRoot(ClientSystem, ServerSystem):
+def DualRoot(clientSystem, serverSystem):
     self = Root()
-    self.client = ClientSystem()
-    self.server = ServerSystem()
+    self.client = clientSystem
+    self.server = serverSystem
 
     self.etherdump = EtherDump(file='ethertrace')
     self.etherlink = EtherLink(int1 = Parent.client.tsunami.etherint[0],
@@ -225,8 +224,8 @@ def DualRoot(ClientSystem, ServerSystem):
     self.clock = '5GHz'
     return self
 
-root = DualRoot(ClientSystem = LinuxAlphaSystem(readfile=script('netperf-stream-nt-client.rcS')),
-                ServerSystem = LinuxAlphaSystem(readfile=script('netperf-server.rcS')))
+root = DualRoot(LinuxAlphaSystem(readfile=script('netperf-stream-nt-client.rcS')),
+                LinuxAlphaSystem(readfile=script('netperf-server.rcS')))
 
 m5.instantiate(root)
 
diff --git a/src/python/m5/config.py b/src/python/m5/config.py
index 9de768d18..3eb99972f 100644
--- a/src/python/m5/config.py
+++ b/src/python/m5/config.py
@@ -211,6 +211,7 @@ class MetaSimObject(type):
         # initialize required attributes
         cls._params = multidict()
         cls._values = multidict()
+        cls._instantiated = False # really instantiated or subclassed
         cls._anon_subclass_counter = 0
 
         # We don't support multiple inheritance.  If you want to, you
@@ -225,6 +226,7 @@ class MetaSimObject(type):
         if isinstance(base, MetaSimObject):
             cls._params.parent = base._params
             cls._values.parent = base._values
+            base._instantiated = True
 
         # now process the _init_dict items
         for key,val in cls._init_dict.items():
@@ -299,6 +301,12 @@ class MetaSimObject(type):
         param = cls._params.get(attr, None)
         if param:
             # It's ok: set attribute by delegating to 'object' class.
+            if (isSimObject(value) or isSimObjSequence(value)) \
+                   and cls._instantiated:
+                raise AttributeError, \
+                  "Cannot set SimObject parameter '%s' after\n" \
+                  "    class %s has been instantiated or subclassed" \
+                  % (attr, cls.__name__)
             try:
                 cls._values[attr] = param.convert(value)
             except Exception, e:
@@ -386,6 +394,8 @@ class SimObject(object):
             # instantiated objects.
             _memo = {}
 
+        self.__class__._instantiated = True
+
         self._children = {}
         # Inherit parameter values from class using multidict so
         # individual value settings can be overridden.