diff -r 4c0f7929ee33 -r fec1d60c0594 src/sim/root.cc
--- a/src/sim/root.cc	Tue Jan 18 01:27:04 2011 -0800
+++ b/src/sim/root.cc	Tue Jan 18 04:18:40 2011 -0800
@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2002-2005 The Regents of The University of Michigan
+ * Copyright (c) 2011 Advanced Micro Devices
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -27,17 +28,51 @@
  *
  * Authors: Nathan Binkert
  *          Steve Reinhardt
+ *          Gabe Black
  */
 
 #include "base/misc.hh"
-#include "params/Root.hh"
-#include "sim/sim_object.hh"
+#include "sim/root.hh"
 
-// Dummy Object
-struct Root : public SimObject
+Root *Root::_root = NULL;
+
+void
+Root::timeSync()
 {
-    Root(RootParams *params) : SimObject(params) {}
-};
+    Time cur_time, diff;
+
+    do {
+        cur_time.setTimer();
+        diff = cur_time - lastTime;
+        Time remainder = period() - diff;
+        if (diff < period() && remainder > _spinThreshold) {
+            DPRINTF(TimeSync, "Sleeping to sync with real time.\n");
+            // Sleep until the end of the period, or until a signal.
+            sleep(remainder);
+            // Refresh the current time.
+            cur_time.setTimer();
+        }
+    } while (diff < period());
+    lastTime = cur_time;
+    schedule(&syncEvent, curTick() + _periodTick);
+}
+
+void
+Root::enable(bool en)
+{
+    if (en == _enabled)
+        return;
+    _enabled = en;
+    if (_enabled) {
+        // Get event going.
+        Tick periods = ((curTick() + _periodTick - 1) / _periodTick);
+        Tick nextPeriod = periods * _periodTick;
+        schedule(&syncEvent, nextPeriod);
+    } else {
+        // Stop event.
+        deschedule(&syncEvent);
+    }
+}
 
 Root *
 RootParams::create()
diff -r 4c0f7929ee33 -r fec1d60c0594 src/sim/Root.py
--- a/src/sim/Root.py	Tue Jan 18 01:27:04 2011 -0800
+++ b/src/sim/Root.py	Tue Jan 18 04:18:40 2011 -0800
@@ -57,4 +57,9 @@
         return 'root'
 
     type = 'Root'
-    dummy = Param.Int(0, "We don't support objects without params")
+
+    # Time syncing prevents the simulation from running faster than real time.
+    time_sync_enable = Param.Bool(False, "whether time syncing is enabled")
+    time_sync_period = Param.Clock("100ms", "how often to sync with real time")
+    time_sync_spin_threshold = \
+            Param.Clock("100us", "when less than this much time is left, spin")
diff -r 4c0f7929ee33 -r fec1d60c0594 src/sim/SConscript
--- a/src/sim/SConscript	Tue Jan 18 01:27:04 2011 -0800
+++ b/src/sim/SConscript	Tue Jan 18 04:18:40 2011 -0800
@@ -73,6 +73,7 @@
 TraceFlag('Loader')
 TraceFlag('Stack')
 TraceFlag('SyscallVerbose')
+TraceFlag('TimeSync')
 TraceFlag('TLB')
 TraceFlag('Thread')
 TraceFlag('Timer')
diff -r 4c0f7929ee33 -r fec1d60c0594 src/sim/root.hh
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/sim/root.hh	Tue Jan 18 04:18:40 2011 -0800
@@ -0,0 +1,106 @@
+/*
+ * Copyright (c) 2011 Advanced Micro Devices
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * 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;
+ * neither the name of the copyright holders 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 COPYRIGHT HOLDERS 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 COPYRIGHT
+ * OWNER 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.
+ *
+ * Authors: Gabe Black
+ */
+
+#ifndef __SIM_ROOT_HH__
+#define __SIM_ROOT_HH__
+
+#include "base/time.hh"
+#include "params/Root.hh"
+#include "sim/core.hh"
+#include "sim/eventq.hh"
+#include "sim/sim_object.hh"
+
+class Root : public SimObject
+{
+  private:
+    static Root *_root;
+
+  protected:
+    bool _enabled;
+    Time _period;
+    Tick _periodTick;
+    Time _spinThreshold;
+
+    Time lastTime;
+
+    void timeSync();
+    EventWrapper<Root, &Root::timeSync> syncEvent;
+    friend class EventWrapper<Root, &Root::timeSync>;
+
+  public:
+    static Root *
+    root()
+    {
+        assert(_root);
+        return _root;
+    }
+
+  public:
+
+    bool enabled() const { return _enabled; }
+    const Time period() const { return _period; }
+    const Time spinThreshold() const { return _spinThreshold; }
+
+    void enable(bool en);
+
+    void
+    period(long ns)
+    {
+        bool en = enabled();
+        _period = ns;
+        _periodTick = _period * SimClock::Int::ns;
+        enable(en);
+    }
+
+    void
+    spinThreshold(long ns)
+    {
+        bool en = enabled();
+        _spinThreshold = ns;
+        enable(en);
+    }
+
+    Root(RootParams *p) : SimObject(p), _enabled(false),
+        _periodTick(p->time_sync_period), syncEvent(this)
+    {
+        uint64_t nsecs = p->time_sync_period / SimClock::Int::ns;
+        _period.set(nsecs / Time::NSEC_PER_SEC, nsecs % Time::NSEC_PER_SEC);
+        nsecs = p->time_sync_spin_threshold / SimClock::Int::ns;
+        _spinThreshold.set(nsecs / Time::NSEC_PER_SEC,
+                nsecs % Time::NSEC_PER_SEC);
+
+        assert(_root == NULL);
+        _root = this;
+        lastTime.setTimer();
+        enable(p->time_sync_enable);
+    }
+};
+
+#endif
diff -r 4c0f7929ee33 -r fec1d60c0594 configs/example/fs.py
--- a/configs/example/fs.py	Tue Jan 18 01:27:04 2011 -0800
+++ b/configs/example/fs.py	Tue Jan 18 04:18:40 2011 -0800
@@ -65,6 +65,10 @@
 
 parser = optparse.OptionParser()
 
+# Simulation options
+parser.add_option("--timesync", action="store_true",
+        help="Prevent simulated time from getting ahead of real time")
+
 # System options
 parser.add_option("--kernel", action="store", type="string")
 parser.add_option("--script", action="store", type="string")
@@ -187,4 +191,7 @@
     print "Error I don't know how to create more than 2 systems."
     sys.exit(1)
 
+if options.timesync:
+    root.time_sync_enable = True
+
 Simulation.run(options, root, test_sys, FutureClass)