diff -r 989ad55c4aca -r d7b2b7524fe4 configs/common/Simulation.py
--- a/configs/common/Simulation.py	Sun Jun 27 23:30:26 2010 -0700
+++ b/configs/common/Simulation.py	Sun Jun 27 23:30:33 2010 -0700
@@ -190,7 +190,7 @@
             for i in xrange(np):
                 testsys.cpu[i].max_insts_any_thread = offset
 
-    m5.instantiate(root)
+    m5.instantiate()
 
     if options.checkpoint_restore != None:
         from os.path import isdir, exists
@@ -207,7 +207,7 @@
                 fatal("Unable to find checkpoint directory %s", checkpoint_dir)
 
             print "Restoring checkpoint ..."
-            m5.restoreCheckpoint(root, checkpoint_dir)
+            m5.restoreCheckpoint(checkpoint_dir)
             print "Done."
         elif options.simpoint:
             # assume workload 0 has the simpoint
@@ -224,7 +224,7 @@
                       options.bench, options.checkpoint_restore)
 
             print "Restoring checkpoint ..."
-            m5.restoreCheckpoint(root,checkpoint_dir)
+            m5.restoreCheckpoint(checkpoint_dir)
             print "Done."
         else:
             dirs = listdir(cptdir)
@@ -246,8 +246,8 @@
             maxtick = maxtick - int(cpts[cpt_num - 1])
 
             ## Restore the checkpoint
-            m5.restoreCheckpoint(root,
-                    joinpath(cptdir, "cpt.%s" % cpts[cpt_num - 1]))
+            m5.restoreCheckpoint(joinpath(cptdir,
+                                          "cpt.%s" % cpts[cpt_num - 1]))
 
     if options.standard_switch or cpu_class:
         if options.standard_switch:
@@ -314,7 +314,7 @@
 
             if exit_event.getCause() == \
                    "a thread reached the max instruction count":
-                m5.checkpoint(root, joinpath(cptdir, "cpt.%s.%d" % \
+                m5.checkpoint(joinpath(cptdir, "cpt.%s.%d" % \
                         (options.bench, checkpoint_inst)))
                 print "Checkpoint written."
                 num_checkpoints += 1
@@ -331,7 +331,7 @@
                 exit_event = m5.simulate(when - m5.curTick())
 
             if exit_event.getCause() == "simulate() limit reached":
-                m5.checkpoint(root, joinpath(cptdir, "cpt.%d"))
+                m5.checkpoint(joinpath(cptdir, "cpt.%d"))
                 num_checkpoints += 1
 
             sim_ticks = when
@@ -348,7 +348,7 @@
                     while exit_event.getCause() == "checkpoint":
                         exit_event = m5.simulate(sim_ticks - m5.curTick())
                     if exit_event.getCause() == "simulate() limit reached":
-                        m5.checkpoint(root, joinpath(cptdir, "cpt.%d"))
+                        m5.checkpoint(joinpath(cptdir, "cpt.%d"))
                         num_checkpoints += 1
 
             if exit_event.getCause() != "simulate() limit reached":
diff -r 989ad55c4aca -r d7b2b7524fe4 configs/example/memtest-ruby.py
--- a/configs/example/memtest-ruby.py	Sun Jun 27 23:30:26 2010 -0700
+++ b/configs/example/memtest-ruby.py	Sun Jun 27 23:30:33 2010 -0700
@@ -129,7 +129,7 @@
 m5.ticks.setGlobalFrequency('1ns')
 
 # instantiate configuration
-m5.instantiate(root)
+m5.instantiate()
 
 # simulate until program terminates
 exit_event = m5.simulate(options.maxtick)
diff -r 989ad55c4aca -r d7b2b7524fe4 configs/example/memtest.py
--- a/configs/example/memtest.py	Sun Jun 27 23:30:26 2010 -0700
+++ b/configs/example/memtest.py	Sun Jun 27 23:30:33 2010 -0700
@@ -181,7 +181,7 @@
 m5.ticks.setGlobalFrequency('1ns')
 
 # instantiate configuration
-m5.instantiate(root)
+m5.instantiate()
 
 # simulate until program terminates
 exit_event = m5.simulate(options.maxtick)
diff -r 989ad55c4aca -r d7b2b7524fe4 configs/example/rubytest.py
--- a/configs/example/rubytest.py	Sun Jun 27 23:30:26 2010 -0700
+++ b/configs/example/rubytest.py	Sun Jun 27 23:30:33 2010 -0700
@@ -118,7 +118,7 @@
 m5.ticks.setGlobalFrequency('1ns')
 
 # instantiate configuration
-m5.instantiate(root)
+m5.instantiate()
 
 # simulate until program terminates
 exit_event = m5.simulate(options.maxtick)
diff -r 989ad55c4aca -r d7b2b7524fe4 configs/splash2/cluster.py
--- a/configs/splash2/cluster.py	Sun Jun 27 23:30:26 2010 -0700
+++ b/configs/splash2/cluster.py	Sun Jun 27 23:30:33 2010 -0700
@@ -292,7 +292,7 @@
     root.system.mem_mode = 'timing'
 
 # instantiate configuration
-m5.instantiate(root)
+m5.instantiate()
 
 # simulate until program terminates
 if options.maxtick:
diff -r 989ad55c4aca -r d7b2b7524fe4 configs/splash2/run.py
--- a/configs/splash2/run.py	Sun Jun 27 23:30:26 2010 -0700
+++ b/configs/splash2/run.py	Sun Jun 27 23:30:33 2010 -0700
@@ -276,7 +276,7 @@
     root.system.mem_mode = 'timing'
 
 # instantiate configuration
-m5.instantiate(root)
+m5.instantiate()
 
 # simulate until program terminates
 if options.maxtick:
diff -r 989ad55c4aca -r d7b2b7524fe4 src/python/m5/simulate.py
--- a/src/python/m5/simulate.py	Sun Jun 27 23:30:26 2010 -0700
+++ b/src/python/m5/simulate.py	Sun Jun 27 23:30:33 2010 -0700
@@ -39,13 +39,30 @@
 import SimObject
 import ticks
 import objects
+from util import warn, fatal
 
 # define a MaxTick parameter
 MaxTick = 2**63 - 1
 
+# function to check for an explicitly passed root arg (for backwards
+# compatibility), warn of deprecation, and verify it is *the* root
+def check_root_arg(root):
+    warn("No need to pass root to instantiate()")
+    assert root == objects.Root.getInstance()
+    
 # The final hook to generate .ini files.  Called from the user script
 # once the config is built.
-def instantiate(root):
+def instantiate(*posargs):
+    if len(posargs) == 1:
+        check_root_arg(posargs[0])
+    elif len(posargs) > 1:
+        fatal("Bad call to instantiate: too many args")
+
+    root = objects.Root.getInstance()
+
+    if not root:
+        fatal("Need to instantiate Root() before calling instantiate()")
+
     # we need to fix the global frequency
     ticks.fixGlobalFrequency()
 
@@ -136,7 +153,18 @@
 def resume(root):
     root.resume()
 
-def checkpoint(root, dir):
+def checkpoint(*posargs):
+    if len(posargs) == 1:
+        # new args: (dir)
+        dir = posargs[0]
+    elif len(posargs) == 2:
+        # old args: (root, dir)
+        check_root_arg(posargs[0])
+        dir = posargs[1]
+    elif len(posargs) > 2:
+        fatal("Bad call to checkpoint: too many args")
+
+    root = objects.Root.getInstance()
     if not isinstance(root, objects.Root):
         raise TypeError, "Checkpoint must be called on a root object."
     doDrain(root)
@@ -144,7 +172,18 @@
     internal.core.serializeAll(dir)
     resume(root)
 
-def restoreCheckpoint(root, dir):
+def restoreCheckpoint(*posargs):
+    if len(posargs) == 1:
+        # new args: (dir)
+        dir = posargs[0]
+    elif len(posargs) == 2:
+        # old args: (root, dir)
+        check_root_arg(posargs[0])
+        dir = posargs[1]
+    elif len(posargs) > 2:
+        fatal("Bad call to restoreCheckpoint: too many args")
+
+    root = objects.Root.getInstance()
     print "Restoring from checkpoint"
     internal.core.unserializeAll(dir)
     need_resume.append(root)
diff -r 989ad55c4aca -r d7b2b7524fe4 src/python/m5/util/__init__.py
--- a/src/python/m5/util/__init__.py	Sun Jun 27 23:30:26 2010 -0700
+++ b/src/python/m5/util/__init__.py	Sun Jun 27 23:30:33 2010 -0700
@@ -66,6 +66,11 @@
     print >>sys.stderr, errorURL('fatal',fmt)
     sys.exit(1)
 
+# warn() is just a warning
+def warn(fmt, *args):
+    print >>sys.stderr, 'warn:', fmt % args
+    print >>sys.stderr, errorURL('warn',fmt)
+
 class Singleton(type):
     def __call__(cls, *args, **kwargs):
         if hasattr(cls, '_instance'):
diff -r 989ad55c4aca -r d7b2b7524fe4 src/sim/Root.py
--- a/src/sim/Root.py	Sun Jun 27 23:30:26 2010 -0700
+++ b/src/sim/Root.py	Sun Jun 27 23:30:33 2010 -0700
@@ -28,7 +28,43 @@
 
 from m5.SimObject import SimObject
 from m5.params import *
+from m5.util import fatal
 
 class Root(SimObject):
+
+    _the_instance = None
+
+    def __new__(cls, **kwargs):
+        if Root._the_instance:
+            return Root._the_instance
+
+        # first call: allocate the unique instance
+        #
+        # Don't set _the_instance yet, as we still need it to be None
+        # to signal __init__ (which will be called next) that this is
+        # the first call.
+        #
+        # If SimObject ever implements __new__, we may want to pass
+        # kwargs here, but for now this goes straight to
+        # object.__new__ which prints an ugly warning if you pass it
+        # args.  Seems like a bad design but that's the way it is.
+        return SimObject.__new__(cls)
+        
+    def __init__(self, **kwargs):
+        if Root._the_instance:
+            # could make this a warning and return _the_instance, but
+            # that seems unlikely to work correctly, so might as well
+            # just call it a fatal error
+            fatal("Attempt to allocate multiple instances of Root.")
+            return
+
+        Root._the_instance = self
+
+        SimObject.__init__(self, **kwargs)
+
+    @classmethod
+    def getInstance(cls):
+        return Root._the_instance
+
     type = 'Root'
     dummy = Param.Int(0, "We don't support objects without params")
diff -r 989ad55c4aca -r d7b2b7524fe4 tests/run.py
--- a/tests/run.py	Sun Jun 27 23:30:26 2010 -0700
+++ b/tests/run.py	Sun Jun 27 23:30:33 2010 -0700
@@ -78,7 +78,7 @@
 execfile(joinpath(tests_root, category, name, 'test.py'))
 
 # instantiate configuration
-m5.instantiate(root)
+m5.instantiate()
 
 # simulate until program terminates
 exit_event = m5.simulate(maxtick)