configs: use SimpleMemory when using ruby in se mode

A recent changeset altered the default memory class to DRAMCtrl.  In se mode,
ruby uses the physical memory to check if a given address is within the bounds
of the physical memory.  SimpleMemory is enough for this.  Moreover,
SimpleMemory does not check whether it is connected or not, something which
DRAMCtrl does.

configs/example/se.py

@@ -157,8 +157,6 @@ else:
 (CPUClass, test_mem_mode, FutureClass) = Simulation.setCPUClass(options)
 CPUClass.numThreads = numThreads
 
-MemClass = Simulation.setMemClass(options)
-
 # Check -- do not allow SMT with multiple CPUs
 if options.smt and options.num_cpus > 1:
     fatal("You cannot use SMT with multiple CPUs!")
@@ -228,8 +226,10 @@ if options.ruby:
         print >> sys.stderr, "Ruby requires TimingSimpleCPU or O3CPU!!"
         sys.exit(1)
 
-    # Set the option for physmem so that it is not allocated any space
-    system.physmem = MemClass(range=AddrRange(options.mem_size),
+    # Use SimpleMemory with the null option since this memory is only used
+    # for determining which addresses are within the range of the memory.
+    # No space allocation is required.
+    system.physmem = SimpleMemory(range=AddrRange(options.mem_size),
                               null = True)
     options.use_map = True
     Ruby.create_system(options, system)
@@ -253,6 +253,7 @@ if options.ruby:
             system.cpu[i].itb.walker.port = ruby_port.slave
             system.cpu[i].dtb.walker.port = ruby_port.slave
 else:
+    MemClass = Simulation.setMemClass(options)
     system.membus = CoherentBus()
     system.system_port = system.membus.slave
     CacheConfig.config_cache(options, system)