diff --git a/interpreter.ts b/interpreter.ts
index 344870ff..32599677 100644
--- a/interpreter.ts
+++ b/interpreter.ts
@@ -108,8 +108,6 @@ module J2ME {
 
   export var onStackReplacementCount = 0;
 
-  export var onStackReplacementIsEnabled = true;
-
   /**
    * Temporarily used for fn.apply.
    */
@@ -260,56 +258,55 @@ module J2ME {
       try {
         if (frame.pc < lastPC) {
           mi.backwardsBranchCount ++;
-          if (onStackReplacementIsEnabled && mi.state === MethodState.Compiled) {
+          if (enableOnStackReplacement && mi.state === MethodState.Compiled) {
             // Just because we've jumped backwards doesn't mean we are at a loop header but it does mean that we are
             // at the beggining of a basic block. This is a really cheap test and a convenient place to perform an
             // on stack replacement.
 
-            profile && onStackReplacementCount ++;
-
-            // The current frame will be swapped out for a JIT frame, so pop it off the interpreter stack.
-            frames.pop();
-
-            // Remember the return kind since we'll need it later.
-            var returnKind = mi.getReturnKind();
-
-            // Set the global OSR frame to the current frame.
-            O = frame;
-
-            var blockMap = O.methodInfo.blockMap;
+            var blockMap = mi.blockMap;
             if (!blockMap) {
-              blockMap = new BlockMap(O.methodInfo);
+              blockMap = new BlockMap(mi);
               blockMap.build();
-              O.methodInfo.blockMap = blockMap;
+              mi.blockMap = blockMap;
             }
 
-            // Convert between PC and block ID which is what the baseline JIT expects as the PC.
-            O.pc = blockMap.getBlock(O.pc).blockID;
+            if (blockMap.getBlock(frame.pc).isLoopHeader) {
+              profile && onStackReplacementCount++;
 
-            // Set the current frame before doing the OSR in case an exception is thrown.
-            frame = frames[frames.length - 1];
+              // The current frame will be swapped out for a JIT frame, so pop it off the interpreter stack.
+              frames.pop();
 
-            // Perform OSR, the callee reads the frame stored in |O| and updates its own state.
-            returnValue = O.methodInfo.fn();
-            if (U) {
-              return;
-            }
+              // Remember the return kind since we'll need it later.
+              var returnKind = mi.getReturnKind();
 
-            // Usual code to return from the interpreter or push the return value.
-            if (Frame.isMarker(frame)) {
-              return returnValue;
-            }
-            mi = frame.methodInfo;
-            ci = mi.classInfo;
-            rp = ci.resolved_constant_pool;
-            stack = frame.stack;
-            lastPC = -1;
+              // Set the global OSR frame to the current frame.
+              O = frame;
 
-            if (returnKind !== Kind.Void) {
-              if (isTwoSlot(returnKind)) {
-                stack.push2(returnValue);
-              } else {
-                stack.push(returnValue);
+              // Set the current frame before doing the OSR in case an exception is thrown.
+              frame = frames[frames.length - 1];
+
+              // Perform OSR, the callee reads the frame stored in |O| and updates its own state.
+              returnValue = O.methodInfo.fn();
+              if (U) {
+                return;
+              }
+
+              // Usual code to return from the interpreter or push the return value.
+              if (Frame.isMarker(frame)) {
+                return returnValue;
+              }
+              mi = frame.methodInfo;
+              ci = mi.classInfo;
+              rp = ci.resolved_constant_pool;
+              stack = frame.stack;
+              lastPC = -1;
+
+              if (returnKind !== Kind.Void) {
+                if (isTwoSlot(returnKind)) {
+                  stack.push2(returnValue);
+                } else {
+                  stack.push(returnValue);
+                }
               }
             }
           }
diff --git a/runtime.ts b/runtime.ts
index 1ea206d8..fb6a195d 100644
--- a/runtime.ts
+++ b/runtime.ts
@@ -28,6 +28,11 @@ module J2ME {
    */
   export var enableRuntimeCompilation = true;
 
+  /**
+   * Turns on onStackReplacement
+   */
+  export var enableOnStackReplacement = true;
+
   /**
    * Enables more compact mangled names. This helps reduce code size but may cause naming collisions.
    */