diff --git a/src/cpu/BaseCPU.py b/src/cpu/BaseCPU.py
index 697be87e1..957203150 100644
--- a/src/cpu/BaseCPU.py
+++ b/src/cpu/BaseCPU.py
@@ -95,6 +95,7 @@ class BaseCPU(MemObject):
         code('''
     void switchOut();
     void takeOverFrom(BaseCPU *cpu);
+    bool switchedOut();
 ''')
 
     def takeOverFrom(self, old_cpu):
diff --git a/src/cpu/base.cc b/src/cpu/base.cc
index 2b1df6696..b8cd60f63 100644
--- a/src/cpu/base.cc
+++ b/src/cpu/base.cc
@@ -119,6 +119,7 @@ BaseCPU::BaseCPU(Params *p, bool is_checker)
       _instMasterId(p->system->getMasterId(name() + ".inst")),
       _dataMasterId(p->system->getMasterId(name() + ".data")),
       _taskId(ContextSwitchTaskId::Unknown), _pid(Request::invldPid),
+      _switchedOut(p->defer_registration),
       interrupts(p->interrupts), profileEvent(NULL),
       numThreads(p->numThreads), system(p->system)
 {
@@ -356,6 +357,8 @@ BaseCPU::findContext(ThreadContext *tc)
 void
 BaseCPU::switchOut()
 {
+    assert(!_switchedOut);
+    _switchedOut = true;
     if (profileEvent && profileEvent->scheduled())
         deschedule(profileEvent);
 }
@@ -365,8 +368,11 @@ BaseCPU::takeOverFrom(BaseCPU *oldCPU)
 {
     assert(threadContexts.size() == oldCPU->threadContexts.size());
     assert(_cpuId == oldCPU->cpuId());
+    assert(_switchedOut);
+    assert(oldCPU != this);
     _pid = oldCPU->getPid();
     _taskId = oldCPU->taskId();
+    _switchedOut = false;
 
     ThreadID size = threadContexts.size();
     for (ThreadID i = 0; i < size; ++i) {
diff --git a/src/cpu/base.hh b/src/cpu/base.hh
index 6552be0d6..633b7f2a7 100644
--- a/src/cpu/base.hh
+++ b/src/cpu/base.hh
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2011 ARM Limited
+ * Copyright (c) 2011-2012 ARM Limited
  * All rights reserved
  *
  * The license below extends only to copyright in the software and shall
@@ -114,6 +114,9 @@ class BaseCPU : public MemObject
      * used to generate a taskId */
     uint32_t _pid;
 
+    /** Is the CPU switched out or active? */
+    bool _switchedOut;
+
     /**
      * Define a base class for the CPU ports (instruction and data)
      * that is refined in the subclasses. This class handles the
@@ -320,6 +323,13 @@ class BaseCPU : public MemObject
      */
     virtual void takeOverFrom(BaseCPU *cpu);
 
+    /**
+     * Determine if the CPU is switched out.
+     *
+     * @return True if the CPU is switched out, false otherwise.
+     */
+    bool switchedOut() const { return _switchedOut; }
+
     /**
      *  Number of threads we're actually simulating (<= SMT_MAX_THREADS).
      * This is a constant for the duration of the simulation.
diff --git a/src/python/m5/simulate.py b/src/python/m5/simulate.py
index 8ad273225..a30648929 100644
--- a/src/python/m5/simulate.py
+++ b/src/python/m5/simulate.py
@@ -228,11 +228,21 @@ def switchCpus(cpuList):
         if not isinstance(item, tuple) or len(item) != 2:
             raise RuntimeError, "List must have tuples of (oldCPU,newCPU)"
 
+    old_cpu_set = set([old_cpu for old_cpu, new_cpu in cpuList])
     for old_cpu, new_cpu in cpuList:
         if not isinstance(old_cpu, objects.BaseCPU):
             raise TypeError, "%s is not of type BaseCPU" % old_cpu
         if not isinstance(new_cpu, objects.BaseCPU):
             raise TypeError, "%s is not of type BaseCPU" % new_cpu
+        if new_cpu in old_cpu_set:
+            raise RuntimeError, \
+                "New CPU (%s) is in the list of old CPUs." % (old_cpu,)
+        if not new_cpu.switchedOut():
+            raise RuntimeError, \
+                "New CPU (%s) is already active." % (new_cpu,)
+        if old_cpu.switchedOut():
+            raise RuntimeError, \
+                "Old CPU (%s) is inactive." % (new_cpu,)
 
     # Now all of the CPUs are ready to be switched out
     for old_cpu, new_cpu in cpuList: