diff --git a/src/arch.h b/src/arch.h
index 1dc335de6c..622e99a2fc 100644
--- a/src/arch.h
+++ b/src/arch.h
@@ -17,6 +17,20 @@ extern "C" void NO_RETURN
 vmJump(void* address, void* base, void* stack, void* thread,
        uintptr_t returnLow, uintptr_t returnHigh);
 
+namespace vm {
+
+inline void
+compileTimeMemoryBarrier()
+{
+#ifdef _MSC_VER
+  _ReadWriteBarrier();
+#else
+  __asm__ __volatile__("": : :"memory");
+#endif
+}
+
+} // namespace vm
+
 #if (defined ARCH_x86_32) || (defined ARCH_x86_64)
 #  include "x86.h"
 #elif defined ARCH_powerpc
diff --git a/src/compile-powerpc.S b/src/compile-powerpc.S
index c90f16c625..8936899b53 100644
--- a/src/compile-powerpc.S
+++ b/src/compile-powerpc.S
@@ -24,11 +24,11 @@
 #  define GLOBAL(x) x   
 #endif
       
-#define THREAD_CONTINUATION 100
-#define THREAD_EXCEPTION 36
-#define THREAD_EXCEPTION_STACK_ADJUSTMENT 104
-#define THREAD_EXCEPTION_OFFSET 108
-#define THREAD_EXCEPTION_HANDLER 112
+#define THREAD_CONTINUATION 112
+#define THREAD_EXCEPTION 44
+#define THREAD_EXCEPTION_STACK_ADJUSTMENT 116
+#define THREAD_EXCEPTION_OFFSET 120
+#define THREAD_EXCEPTION_HANDLER 124
 
 #define CONTINUATION_NEXT 4
 #define CONTINUATION_ADDRESS 16
diff --git a/src/compile.cpp b/src/compile.cpp
index 00fa33aa15..a695bd2367 100644
--- a/src/compile.cpp
+++ b/src/compile.cpp
@@ -588,8 +588,8 @@ print(SubroutinePath* path)
   if (path) {
     fprintf(stderr, " (");
     while (true) {
-      fprintf(stderr, "%p", reinterpret_cast<void*>
-              (path->call->returnAddress->value()));
+      fprintf(stderr, "%p", path->call->returnAddress->resolved() ?
+              reinterpret_cast<void*>(path->call->returnAddress->value()) : 0);
       path = path->stackNext;
       if (path) {
         fprintf(stderr, ", ");
@@ -603,13 +603,18 @@ print(SubroutinePath* path)
 
 class SubroutineTrace {
  public:
-  SubroutineTrace(SubroutinePath* path, SubroutineTrace* next):
+  SubroutineTrace(SubroutinePath* path, SubroutineTrace* next,
+                  unsigned mapSize):
     path(path),
-    next(next)
-  { }
+    next(next),
+    watch(false)
+  {
+    memset(map, 0, mapSize * BytesPerWord);
+  }
 
   SubroutinePath* path;
   SubroutineTrace* next;
+  bool watch;
   uintptr_t map[0];
 };
 
@@ -619,17 +624,21 @@ class TraceElement: public TraceHandler {
   static const unsigned TailCall    = 1 << 1;
   static const unsigned LongCall    = 1 << 2;
 
-  TraceElement(Context* context, object target, unsigned flags,
-               TraceElement* next):
+  TraceElement(Context* context, unsigned ip, object target, unsigned flags,
+               TraceElement* next, unsigned mapSize):
     context(context),
     address(0),
     next(next),
     subroutineTrace(0),
-    subroutineTraceCount(0),
     target(target),
+    ip(ip),
+    subroutineTraceCount(0),
     argumentIndex(0),
-    flags(flags)
-  { }
+    flags(flags),
+    watch(false)
+  {
+    memset(map, 0, mapSize * BytesPerWord);
+  }
 
   virtual void handleTrace(Promise* address, unsigned argumentIndex) {
     if (this->address == 0) {
@@ -642,10 +651,12 @@ class TraceElement: public TraceHandler {
   Promise* address;
   TraceElement* next;
   SubroutineTrace* subroutineTrace;
-  unsigned subroutineTraceCount;
   object target;
+  unsigned ip;
+  unsigned subroutineTraceCount;
   unsigned argumentIndex;
   unsigned flags;
+  bool watch;
   uintptr_t map[0];
 };
 
@@ -1587,7 +1598,7 @@ class Frame {
 
     TraceElement* e = context->traceLog = new
       (context->zone.allocate(sizeof(TraceElement) + (mapSize * BytesPerWord)))
-      TraceElement(context, target, flags, context->traceLog);
+      TraceElement(context, ip, target, flags, context->traceLog, mapSize);
 
     ++ context->traceLogCount;
 
@@ -4923,6 +4934,63 @@ printSet(uintptr_t m, unsigned limit)
   }
 }
 
+void
+calculateTryCatchRoots(Context* context, SubroutinePath* subroutinePath,
+                       uintptr_t* roots, unsigned mapSize, unsigned start,
+                       unsigned end)
+{
+  memset(roots, 0xFF, mapSize * BytesPerWord);
+
+  if (DebugFrameMaps) {
+    fprintf(stderr, "calculate try/catch roots from %d to %d", start, end);
+    if (subroutinePath) {
+      fprintf(stderr, " ");
+      print(subroutinePath);
+    }
+    fprintf(stderr, "\n");
+  }
+
+  for (TraceElement* te = context->traceLog; te; te = te->next) {
+    if (te->ip >= start and te->ip < end) {
+      uintptr_t* traceRoots = 0;
+      if (subroutinePath == 0) {
+        traceRoots = te->map;
+        te->watch = true;
+      } else {
+        for (SubroutineTrace* t = te->subroutineTrace; t; t = t->next) {
+          if (t->path == subroutinePath) {
+            traceRoots = t->map;
+            t->watch = true;
+            break;
+          }
+        }        
+      }
+
+      if (traceRoots) {
+        if (DebugFrameMaps) {
+          fprintf(stderr, "   use roots at ip %3d: ", te->ip);
+          printSet(*traceRoots, mapSize);
+          fprintf(stderr, "\n");
+        }
+
+        for (unsigned wi = 0; wi < mapSize; ++wi) {
+          roots[wi] &= traceRoots[wi];
+        }
+      } else {
+        if (DebugFrameMaps) {
+          fprintf(stderr, "  skip roots at ip %3d\n", te->ip);
+        }
+      }
+    }
+  }
+
+  if (DebugFrameMaps) {
+    fprintf(stderr, "result roots          : ");
+    printSet(*roots, mapSize);
+    fprintf(stderr, "\n");
+  }
+}
+
 unsigned
 calculateFrameMaps(MyThread* t, Context* context, uintptr_t* originalRoots,
                    unsigned eventIndex, SubroutinePath* subroutinePath = 0)
@@ -4969,7 +5037,7 @@ calculateFrameMaps(MyThread* t, Context* context, uintptr_t* originalRoots,
       eventIndex += 2;
 
       if (DebugFrameMaps) {
-        fprintf(stderr, "      roots at ip %3d: ", ip);
+        fprintf(stderr, "       roots at ip %3d: ", ip);
         printSet(*RUNTIME_ARRAY_BODY(roots), mapSize);
         fprintf(stderr, "\n");
       }
@@ -4998,7 +5066,7 @@ calculateFrameMaps(MyThread* t, Context* context, uintptr_t* originalRoots,
         }
 
         if (DebugFrameMaps) {
-          fprintf(stderr, "table roots at ip %3d: ", ip);
+          fprintf(stderr, " table roots at ip %3d: ", ip);
           printSet(*tableRoots, mapSize);
           fprintf(stderr, "\n");
         }
@@ -5022,27 +5090,27 @@ calculateFrameMaps(MyThread* t, Context* context, uintptr_t* originalRoots,
     } break;
 
     case PushExceptionHandlerEvent: {
-      unsigned reference = context->eventLog.get2(eventIndex);
+      unsigned start = context->eventLog.get2(eventIndex);
+      eventIndex += 2;
+      unsigned end = context->eventLog.get2(eventIndex);
       eventIndex += 2;
 
-      if (context->subroutineTable and context->subroutineTable[reference]) {
-        Subroutine* s = context->subroutineTable[reference];
+      if (context->subroutineTable and context->subroutineTable[start]) {
+        Subroutine* s = context->subroutineTable[start];
         unsigned originalEventIndex = eventIndex;
 
         for (SubroutineCall* c = s->calls; c; c = c->next) {
           for (SubroutinePath* p = c->paths; p; p = p->listNext) {
-            memcpy(RUNTIME_ARRAY_BODY(roots),
-                   p->rootTable + (reference * mapSize),
-                   mapSize * BytesPerWord);
+            calculateTryCatchRoots
+              (context, p, RUNTIME_ARRAY_BODY(roots), mapSize, start, end);
 
             eventIndex = calculateFrameMaps
               (t, context, RUNTIME_ARRAY_BODY(roots), originalEventIndex, p);
           }
         }
       } else {
-        memcpy(RUNTIME_ARRAY_BODY(roots),
-               context->rootTable + (reference * mapSize),
-               mapSize * BytesPerWord);
+        calculateTryCatchRoots
+          (context, 0, RUNTIME_ARRAY_BODY(roots), mapSize, start, end);
 
         eventIndex = calculateFrameMaps
           (t, context, RUNTIME_ARRAY_BODY(roots), eventIndex, 0);
@@ -5052,7 +5120,7 @@ calculateFrameMaps(MyThread* t, Context* context, uintptr_t* originalRoots,
     case TraceEvent: {
       TraceElement* te; context->eventLog.get(eventIndex, &te, BytesPerWord);
       if (DebugFrameMaps) {
-        fprintf(stderr, "trace roots at ip %3d: ", ip);
+        fprintf(stderr, " trace roots at ip %3d: ", ip);
         printSet(*RUNTIME_ARRAY_BODY(roots), mapSize);
         if (subroutinePath) {
           fprintf(stderr, " ");
@@ -5060,14 +5128,18 @@ calculateFrameMaps(MyThread* t, Context* context, uintptr_t* originalRoots,
         }
         fprintf(stderr, "\n");
       }
-
+        
+      uintptr_t* map;
+      bool watch;
       if (subroutinePath == 0) {
-        memcpy(te->map, RUNTIME_ARRAY_BODY(roots), mapSize * BytesPerWord);
+        map = te->map;
+        watch = te->watch;
       } else {
         SubroutineTrace* trace = 0;
         for (SubroutineTrace* t = te->subroutineTrace; t; t = t->next) {
           if (t->path == subroutinePath) {
             trace = t;
+            break;
           }
         }
 
@@ -5075,12 +5147,27 @@ calculateFrameMaps(MyThread* t, Context* context, uintptr_t* originalRoots,
           te->subroutineTrace = trace = new
             (context->zone.allocate
              (sizeof(SubroutineTrace) + (mapSize * BytesPerWord)))
-            SubroutineTrace(subroutinePath, te->subroutineTrace);
+            SubroutineTrace(subroutinePath, te->subroutineTrace, mapSize);
 
           ++ te->subroutineTraceCount;
         }
 
-        memcpy(trace->map, RUNTIME_ARRAY_BODY(roots), mapSize * BytesPerWord);
+        map = trace->map;
+        watch = trace->watch;
+      }
+
+      for (unsigned wi = 0; wi < mapSize; ++wi) {
+        uintptr_t v = RUNTIME_ARRAY_BODY(roots)[wi];
+
+        if (watch and map[wi] != v) {
+          if (DebugFrameMaps) {
+            fprintf(stderr, "dirty roots due to trace watch!\n");
+          }
+
+          context->dirtyRoots = true;
+        }
+
+        map[wi] = v;
       }
 
       eventIndex += BytesPerWord;
@@ -5184,14 +5271,12 @@ copyFrameMap(int32_t* dst, uintptr_t* src, unsigned mapSizeInBits,
              SubroutinePath* subroutinePath)
 {
   if (DebugFrameMaps) {
-    fprintf(stderr, " orig roots at ip %p: ", reinterpret_cast<void*>
-            (p->address->value()));
+    fprintf(stderr, "  orig roots at ip %3d: ", p->ip);
     printSet(src[0], ceiling(mapSizeInBits, BitsPerWord));
     print(subroutinePath);
     fprintf(stderr, "\n");
 
-    fprintf(stderr, "final roots at ip %p: ", reinterpret_cast<void*>
-            (p->address->value()));
+    fprintf(stderr, " final roots at ip %3d: ", p->ip);
   }
 
   for (unsigned j = 0; j < p->argumentIndex; ++j) {
@@ -5698,8 +5783,16 @@ compile(MyThread* t, Allocator* allocator, Context* context)
                         codeMaxStack(t, methodCode(t, context->method)));
           Frame frame2(&frame, RUNTIME_ARRAY_BODY(stackMap));
 
+          unsigned end = exceptionHandlerEnd(eh);
+          if (exceptionHandlerIp(eh) >= start
+              and exceptionHandlerIp(eh) < end)
+          {
+            end = exceptionHandlerIp(eh);
+          }
+
           context->eventLog.append(PushExceptionHandlerEvent);
           context->eventLog.append2(start);
+          context->eventLog.append2(end);
 
           for (unsigned i = 1;
                i < codeMaxStack(t, methodCode(t, context->method));
@@ -7211,9 +7304,9 @@ class MyProcessor: public Processor {
       compile(static_cast<MyThread*>(t),
               local::codeAllocator(static_cast<MyThread*>(t)), 0,
               resolveMethod(t, t->m->loader,
-                            "java/beans/PropertyChangeSupport",
-                            "firePropertyChange",
-                            "(Ljava/beans/PropertyChangeEvent;)V"));
+                            "com/ecovate/nat/logic/Cache",
+                            "findInCache",
+                            "(Ljava/lang/String;Ljava/lang/String;JZ)Lcom/ecovate/shared/xmlrpc/Resource;"));
       trap();
     }
 
diff --git a/src/continuations-x86.S b/src/continuations-x86.S
index 2151659ddf..aeee640388 100644
--- a/src/continuations-x86.S
+++ b/src/continuations-x86.S
@@ -10,11 +10,11 @@
 
 #ifdef __x86_64__
 
-#define THREAD_CONTINUATION 176
-#define THREAD_EXCEPTION 64
-#define THREAD_EXCEPTION_STACK_ADJUSTMENT 184
-#define THREAD_EXCEPTION_OFFSET 192
-#define THREAD_EXCEPTION_HANDLER 200
+#define THREAD_CONTINUATION 192
+#define THREAD_EXCEPTION 80
+#define THREAD_EXCEPTION_STACK_ADJUSTMENT 200
+#define THREAD_EXCEPTION_OFFSET 208
+#define THREAD_EXCEPTION_HANDLER 216
 
 #define CONTINUATION_NEXT 8
 #define CONTINUATION_ADDRESS 32
@@ -89,11 +89,11 @@ LOCAL(vmInvoke_exit):
 
 #elif defined __i386__
    
-#define THREAD_CONTINUATION 100
-#define THREAD_EXCEPTION 36
-#define THREAD_EXCEPTION_STACK_ADJUSTMENT 104
-#define THREAD_EXCEPTION_OFFSET 108
-#define THREAD_EXCEPTION_HANDLER 112
+#define THREAD_CONTINUATION 108
+#define THREAD_EXCEPTION 44
+#define THREAD_EXCEPTION_STACK_ADJUSTMENT 112
+#define THREAD_EXCEPTION_OFFSET 116
+#define THREAD_EXCEPTION_HANDLER 120
 
 #define CONTINUATION_NEXT 4
 #define CONTINUATION_ADDRESS 16
diff --git a/src/machine.cpp b/src/machine.cpp
index 8507e9c256..44247f52cb 100644
--- a/src/machine.cpp
+++ b/src/machine.cpp
@@ -1808,17 +1808,13 @@ removeMonitor(Thread* t, object o)
     hash = objectHash(t, o);
   }
 
-  object p = hashMapRemove(t, t->m->monitorMap, o, objectHash, objectEqual);
+  object m = hashMapRemove(t, t->m->monitorMap, o, objectHash, objectEqual);
 
-  expect(t, p);
+  expect(t, m);
 
   if (DebugMonitors) {
-    fprintf(stderr, "dispose monitor %p for object %x\n",
-            static_cast<System::Monitor*>(pointerValue(t, p)),
-            hash);
+    fprintf(stderr, "dispose monitor %p for object %x\n", m, hash);
   }
-
-  static_cast<System::Monitor*>(pointerValue(t, p))->dispose();
 }
 
 void
@@ -2185,9 +2181,11 @@ Thread::Thread(Machine* m, object javaThread, Thread* parent):
   parent(parent),
   peer((parent ? parent->child : 0)),
   child(0),
+  waitNext(0),
   state(NoState),
   criticalLevel(0),
   systemThread(0),
+  lock(0),
   javaThread(javaThread),
   exception(0),
   heapIndex(0),
@@ -2201,6 +2199,7 @@ Thread::Thread(Machine* m, object javaThread, Thread* parent):
   backupHeap(0),
   backupHeapIndex(0),
   backupHeapSizeInWords(0),
+  waiting(false),
   tracing(false)
 #ifdef VM_STRESS
   , stress(false)
@@ -2255,6 +2254,8 @@ Thread::init()
     parent->child = this;
   }
 
+  expect(this, m->system->success(m->system->make(&lock)));
+
   if (javaThread == 0) {
     this->javaThread = makeJavaThread(this, parent);
   }
@@ -2282,6 +2283,8 @@ Thread::exit()
 void
 Thread::dispose()
 {
+  lock->dispose();
+
   if (systemThread) {
     systemThread->dispose();
   }
@@ -3419,52 +3422,48 @@ addFinalizer(Thread* t, object target, void (*finalize)(Thread*, object))
   t->m->finalizers = f;
 }
 
-System::Monitor*
+object
 objectMonitor(Thread* t, object o, bool createNew)
 {
   assert(t, t->state == Thread::ActiveState);
 
-  object p = hashMapFind(t, t->m->monitorMap, o, objectHash, objectEqual);
+  object m = hashMapFind(t, t->m->monitorMap, o, objectHash, objectEqual);
 
-  if (p) {
+  if (m) {
     if (DebugMonitors) {
-      fprintf(stderr, "found monitor %p for object %x\n",
-              static_cast<System::Monitor*>(pointerValue(t, p)),
-              objectHash(t, o));
+      fprintf(stderr, "found monitor %p for object %x\n", m, objectHash(t, o));
     }
 
-    return static_cast<System::Monitor*>(pointerValue(t, p));
+    return m;
   } else if (createNew) {
     PROTECT(t, o);
+    PROTECT(t, m);
 
-    ENTER(t, Thread::ExclusiveState);
+    { ENTER(t, Thread::ExclusiveState);
+
+      m = hashMapFind(t, t->m->monitorMap, o, objectHash, objectEqual);
+      if (m) {
+        if (DebugMonitors) {
+          fprintf(stderr, "found monitor %p for object %x\n",
+                  m, objectHash(t, o));
+        }
+
+        return m;
+      }
+
+      object head = makeMonitorNode(t, 0, 0);
+      m = makeMonitor(t, 0, 0, 0, head, head, 0);
 
-    p = hashMapFind(t, t->m->monitorMap, o, objectHash, objectEqual);
-    if (p) {
       if (DebugMonitors) {
-        fprintf(stderr, "found monitor %p for object %x\n",
-                static_cast<System::Monitor*>(pointerValue(t, p)),
+        fprintf(stderr, "made monitor %p for object %x\n", m,
                 objectHash(t, o));
       }
 
-      return static_cast<System::Monitor*>(pointerValue(t, p));
+      hashMapInsert(t, t->m->monitorMap, o, m, objectHash);
+
+      addFinalizer(t, o, removeMonitor);
     }
 
-    System::Monitor* m;
-    System::Status s = t->m->system->make(&m);
-    expect(t, t->m->system->success(s));
-
-    if (DebugMonitors) {
-      fprintf(stderr, "made monitor %p for object %x\n",
-              m,
-              objectHash(t, o));
-    }
-
-    p = makePointer(t, m);
-    hashMapInsert(t, t->m->monitorMap, o, p, objectHash);
-
-    addFinalizer(t, o, removeMonitor);
-
     return m;
   } else {
     return 0;
diff --git a/src/machine.h b/src/machine.h
index fa8d99936d..2ea8b94bd5 100644
--- a/src/machine.h
+++ b/src/machine.h
@@ -17,6 +17,7 @@
 #include "finder.h"
 #include "processor.h"
 #include "constants.h"
+#include "arch.h"
 
 #ifdef PLATFORM_WINDOWS
 #  define JNICALL __stdcall
@@ -1345,9 +1346,11 @@ class Thread {
   Thread* parent;
   Thread* peer;
   Thread* child;
+  Thread* waitNext;
   State state;
   unsigned criticalLevel;
   System::Thread* systemThread;
+  System::Monitor* lock;
   object javaThread;
   object exception;
   unsigned heapIndex;
@@ -1360,6 +1363,7 @@ class Thread {
   uintptr_t* backupHeap;
   unsigned backupHeapIndex;
   unsigned backupHeapSizeInWords;
+  bool waiting;
   bool tracing;
 #ifdef VM_STRESS
   bool stress;
@@ -2296,7 +2300,316 @@ parameterFootprint(Thread* t, const char* s, bool static_);
 void
 addFinalizer(Thread* t, object target, void (*finalize)(Thread*, object));
 
-System::Monitor*
+inline bool
+atomicCompareAndSwapObject(Thread* t, object target, unsigned offset,
+                           object old, object new_)
+{
+  if (atomicCompareAndSwap(&cast<uintptr_t>(target, offset),
+                           reinterpret_cast<uintptr_t>(old),
+                           reinterpret_cast<uintptr_t>(new_)))
+  {
+    mark(t, target, offset);
+    return true;
+  } else {
+    return false;
+  }
+}
+
+// The following two methods (monitorAtomicAppendAcquire and
+// monitorAtomicPollAcquire) use the Michael and Scott Non-Blocking
+// Queue Algorithm: http://www.cs.rochester.edu/u/michael/PODC96.html
+
+inline void
+monitorAtomicAppendAcquire(Thread* t, object monitor)
+{
+  PROTECT(t, monitor);
+
+  object node = makeMonitorNode(t, t, 0);
+
+  while (true) {
+    object tail = monitorAcquireTail(t, monitor);
+    
+    loadMemoryBarrier();
+
+    object next = monitorNodeNext(t, tail);
+
+    loadMemoryBarrier();
+
+    if (tail == monitorAcquireTail(t, monitor)) {
+      if (next) {
+        atomicCompareAndSwapObject
+          (t, monitor, MonitorAcquireTail, tail, next);
+      } else if (atomicCompareAndSwapObject
+                 (t, tail, MonitorNodeNext, 0, node))
+      {
+        atomicCompareAndSwapObject
+          (t, monitor, MonitorAcquireTail, tail, node);
+        return;
+      }
+    }
+  }
+}
+
+inline Thread*
+monitorAtomicPollAcquire(Thread* t, object monitor, bool remove)
+{
+  while (true) {
+    object head = monitorAcquireHead(t, monitor);
+
+    loadMemoryBarrier();
+
+    object tail = monitorAcquireTail(t, monitor);
+
+    loadMemoryBarrier();
+
+    object next = monitorNodeNext(t, head);
+
+    loadMemoryBarrier();
+
+    if (head == monitorAcquireHead(t, monitor)) {
+      if (head == tail) {
+        if (next) {
+          atomicCompareAndSwapObject
+            (t, monitor, MonitorAcquireTail, tail, next);
+        } else {
+          return 0;
+        }
+      } else {
+        Thread* value = static_cast<Thread*>(monitorNodeValue(t, next));
+        if ((not remove)
+            or atomicCompareAndSwapObject
+            (t, monitor, MonitorAcquireHead, head, next))
+        {
+          return value;
+        }
+      }
+    }
+  }
+}
+
+inline bool
+monitorTryAcquire(Thread* t, object monitor)
+{
+  if (monitorOwner(t, monitor) == t
+      or (monitorAtomicPollAcquire(t, monitor, false) == 0
+          and atomicCompareAndSwap
+          (reinterpret_cast<uintptr_t*>(&monitorOwner(t, monitor)), 0,
+           reinterpret_cast<uintptr_t>(t))))
+  {
+    ++ monitorDepth(t, monitor);
+    return true;
+  } else {
+    return false;
+  }
+}
+
+inline void
+monitorAcquire(Thread* t, object monitor)
+{
+  if (not monitorTryAcquire(t, monitor)) {
+    PROTECT(t, monitor);
+
+    ACQUIRE(t, t->lock);
+
+    monitorAtomicAppendAcquire(t, monitor);
+    
+    // note that we don't try to acquire the lock until we're first in
+    // line, both because it's fair and because we don't support
+    // removing elements from arbitrary positions in the queue
+
+    while (not (t == monitorAtomicPollAcquire(t, monitor, false)
+                and atomicCompareAndSwap
+                (reinterpret_cast<uintptr_t*>(&monitorOwner(t, monitor)), 0,
+                 reinterpret_cast<uintptr_t>(t))))
+    {
+      ENTER(t, Thread::IdleState);
+      
+      t->lock->wait(t->systemThread, 0);
+    }
+
+    expect(t, t == monitorAtomicPollAcquire(t, monitor, true));
+        
+    ++ monitorDepth(t, monitor);
+  }
+
+  assert(t, monitorOwner(t, monitor) == t);
+}
+
+inline void
+monitorRelease(Thread* t, object monitor)
+{
+  expect(t, monitorOwner(t, monitor) == t);
+
+  if (-- monitorDepth(t, monitor) == 0) {
+    monitorOwner(t, monitor) = 0;
+
+    storeLoadMemoryBarrier();
+    
+    Thread* next = monitorAtomicPollAcquire(t, monitor, false);
+
+    if (next) {
+      ACQUIRE(t, next->lock);
+       
+      next->lock->notify(t->systemThread);
+    }
+  }
+}
+
+inline void
+monitorAppendWait(Thread* t, object monitor)
+{
+  assert(t, monitorOwner(t, monitor) == t);
+
+  expect(t, not t->waiting);
+  expect(t, t->waitNext == 0);
+
+  t->waiting = true;
+
+  if (monitorWaitTail(t, monitor)) {
+    static_cast<Thread*>(monitorWaitTail(t, monitor))->waitNext = t;
+  } else {
+    monitorWaitHead(t, monitor) = t;
+  }
+
+  monitorWaitTail(t, monitor) = t;
+}
+
+inline void
+monitorRemoveWait(Thread* t, object monitor)
+{
+  assert(t, monitorOwner(t, monitor) == t);
+
+  Thread* previous = 0;
+  for (Thread* current = static_cast<Thread*>(monitorWaitHead(t, monitor));
+       current; current = current->waitNext)
+  {
+    if (t == current) {
+      if (t == monitorWaitHead(t, monitor)) {
+        monitorWaitHead(t, monitor) = t->waitNext;
+      } else {
+        previous->waitNext = t->waitNext;
+      }
+
+      if (t == monitorWaitTail(t, monitor)) {
+        assert(t, t->waitNext == 0);
+        monitorWaitTail(t, monitor) = previous;
+      }
+
+      t->waitNext = 0;
+      t->waiting = false;
+
+      return;
+    } else {
+      previous = current;
+    }
+  }
+
+  abort(t);
+}
+
+inline bool
+monitorFindWait(Thread* t, object monitor)
+{
+  assert(t, monitorOwner(t, monitor) == t);
+
+  for (Thread* current = static_cast<Thread*>(monitorWaitHead(t, monitor));
+       current; current = current->waitNext)
+  {
+    if (t == current) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+inline bool
+monitorWait(Thread* t, object monitor, int64_t time)
+{
+  expect(t, monitorOwner(t, monitor) == t);
+
+  bool interrupted;
+  unsigned depth;
+
+  PROTECT(t, monitor);
+
+  { ACQUIRE(t, t->lock);
+
+    monitorAppendWait(t, monitor);
+
+    depth = monitorDepth(t, monitor);
+    monitorDepth(t, monitor) = 1;
+
+    monitorRelease(t, monitor);
+
+    ENTER(t, Thread::IdleState);
+
+    interrupted = t->lock->wait(t->systemThread, time);
+  }
+
+  monitorAcquire(t, monitor);
+
+  monitorDepth(t, monitor) = depth;
+
+  if (t->waiting) {
+    monitorRemoveWait(t, monitor);
+  } else {
+    expect(t, not monitorFindWait(t, monitor));
+  }
+
+  assert(t, monitorOwner(t, monitor) == t);
+
+  return interrupted;
+}
+
+inline Thread*
+monitorPollWait(Thread* t, object monitor)
+{
+  assert(t, monitorOwner(t, monitor) == t);
+
+  Thread* next = static_cast<Thread*>(monitorWaitHead(t, monitor));
+
+  if (next) {
+    monitorWaitHead(t, monitor) = next->waitNext;
+    next->waiting = false;
+    next->waitNext = 0;
+    if (next == monitorWaitTail(t, monitor)) {
+      monitorWaitTail(t, monitor) = 0;
+    }
+  } else {
+    assert(t, monitorWaitTail(t, monitor) == 0);
+  }
+
+  return next;
+}
+
+inline bool
+monitorNotify(Thread* t, object monitor)
+{
+  expect(t, monitorOwner(t, monitor) == t);
+  
+  Thread* next = monitorPollWait(t, monitor);
+
+  if (next) {
+    ACQUIRE(t, next->lock);
+
+    next->lock->notify(t->systemThread);
+
+    return true;
+  } else {
+    return false;
+  }
+}
+
+inline void
+monitorNotifyAll(Thread* t, object monitor)
+{
+  PROTECT(t, monitor);
+
+  while (monitorNotify(t, monitor)) { }
+}
+
+object
 objectMonitor(Thread* t, object o, bool createNew);
 
 inline void
@@ -2307,14 +2620,14 @@ acquire(Thread* t, object o)
     hash = objectHash(t, o);
   }
 
-  System::Monitor* m = objectMonitor(t, o, true);
+  object m = objectMonitor(t, o, true);
 
   if (DebugMonitors) {
     fprintf(stderr, "thread %p acquires %p for %x\n",
             t, m, hash);
   }
 
-  acquire(t, m);
+  monitorAcquire(t, m);
 }
 
 inline void
@@ -2325,14 +2638,14 @@ release(Thread* t, object o)
     hash = objectHash(t, o);
   }
 
-  System::Monitor* m = objectMonitor(t, o, false);
+  object m = objectMonitor(t, o, false);
 
   if (DebugMonitors) {
     fprintf(stderr, "thread %p releases %p for %x\n",
             t, m, hash);
   }
 
-  release(t, m);
+  monitorRelease(t, m);
 }
 
 inline void
@@ -2343,18 +2656,17 @@ wait(Thread* t, object o, int64_t milliseconds)
     hash = objectHash(t, o);
   }
 
-  System::Monitor* m = objectMonitor(t, o, false);
+  object m = objectMonitor(t, o, false);
 
   if (DebugMonitors) {
     fprintf(stderr, "thread %p waits %d millis on %p for %x\n",
             t, static_cast<int>(milliseconds), m, hash);
   }
 
-  if (m and m->owner() == t->systemThread) {
-    bool interrupted;
-    { ENTER(t, Thread::IdleState);
-      interrupted = m->wait(t->systemThread, milliseconds);
-    }
+  if (m and monitorOwner(t, m) == t) {
+    PROTECT(t, m);
+
+    bool interrupted = monitorWait(t, m, milliseconds);
 
     if (interrupted) {
       t->exception = makeInterruptedException(t);
@@ -2379,15 +2691,15 @@ notify(Thread* t, object o)
     hash = objectHash(t, o);
   }
 
-  System::Monitor* m = objectMonitor(t, o, false);
+  object m = objectMonitor(t, o, false);
 
   if (DebugMonitors) {
     fprintf(stderr, "thread %p notifies on %p for %x\n",
             t, m, hash);
   }
 
-  if (m and m->owner() == t->systemThread) {
-    m->notify(t->systemThread);
+  if (m and monitorOwner(t, m) == t) {
+    monitorNotify(t, m);
   } else {
     t->exception = makeIllegalMonitorStateException(t);
   }
@@ -2396,15 +2708,15 @@ notify(Thread* t, object o)
 inline void
 notifyAll(Thread* t, object o)
 {
-  System::Monitor* m = objectMonitor(t, o, false);
+  object m = objectMonitor(t, o, false);
 
   if (DebugMonitors) {
     fprintf(stderr, "thread %p notifies all on %p for %x\n",
             t, m, objectHash(t, o));
   }
 
-  if (m and m->owner() == t->systemThread) {
-    m->notifyAll(t->systemThread);
+  if (m and monitorOwner(t, m) == t) {
+    monitorNotifyAll(t, m);
   } else {
     t->exception = makeIllegalMonitorStateException(t);
   }
diff --git a/src/types.def b/src/types.def
index 9a704c3b91..184df800a0 100644
--- a/src/types.def
+++ b/src/types.def
@@ -111,6 +111,18 @@
   (object first)
   (object second))
 
+(type monitor
+  (void* owner)
+  (void* waitHead)
+  (void* waitTail)
+  (object acquireHead)
+  (object acquireTail)
+  (unsigned depth))
+
+(type monitorNode
+  (void* value)
+  (object next))
+
 (type continuationContext
   (object next)
   (object before)
diff --git a/src/windows.cpp b/src/windows.cpp
index 4ac44e4d05..b0146334b6 100644
--- a/src/windows.cpp
+++ b/src/windows.cpp
@@ -23,7 +23,7 @@
 #undef max
 #undef min
 
-#include "x86.h"
+#include "arch.h"
 #include "system.h"
 
 #define ACQUIRE(s, x) MutexResource MAKE_NAME(mutexResource_) (s, x)
diff --git a/src/x86.h b/src/x86.h
index f5861de2d1..a3450377a7 100644
--- a/src/x86.h
+++ b/src/x86.h
@@ -161,11 +161,7 @@ trap()
 inline void
 programOrderMemoryBarrier()
 {
-#ifdef _MSC_VER
-  _ReadWriteBarrier();
-#else
-  __asm__ __volatile__("": : :"memory");
-#endif
+  compileTimeMemoryBarrier();
 }
 
 inline void
diff --git a/test/GC.java b/test/GC.java
index eb657ef786..d04c577fc8 100644
--- a/test/GC.java
+++ b/test/GC.java
@@ -104,6 +104,46 @@ public class GC {
     System.gc();
   }
 
+  private static void stackMap7(boolean predicate) {
+    try {
+      if (predicate) {
+        Object a = null;
+      } else {
+        Object a = null;
+      }
+
+      try {
+        int a = 42;
+        throw new DummyException();
+      } finally {
+        System.gc();
+      }
+    } catch (DummyException e) {
+      e.toString();
+    }
+  }
+
+  private static void stackMap8(boolean predicate) {
+    try {
+      Object x = new Object();
+      if (predicate) {
+        Object a = null;
+      } else {
+        Object a = null;
+      }
+
+      try {
+        int a = 42;
+        throw new DummyException();
+      } finally {
+        System.gc();
+        x.toString();
+      }
+    } catch (DummyException e) {
+      e.toString();
+    }
+  }
+
   public static void main(String[] args) {
     Object[] array = new Object[1024 * 1024];
     array[0] = new Object();
@@ -139,6 +179,13 @@ public class GC {
 
     stackMap6(true);
     stackMap6(false);
+
+    stackMap7(true);
+    stackMap7(false);
+
+    stackMap8(true);
+    stackMap8(false);
   }
 
+  private static class DummyException extends RuntimeException { }
 }