diff --git a/code/test/threadcreate.c b/code/test/threadcreate.c
index 9861c8b..7e57fe8 100644
--- a/code/test/threadcreate.c
+++ b/code/test/threadcreate.c
@@ -6,15 +6,15 @@ void f(int c) {
      * a problem here, when executing a thread there is not 8 iterations.
      * This is like synchronisation */
     volatile int i;
-    for (i=0;i<8;i++){
+    for (i=0;i<16;i++){
         PutChar((char)c);
     }
     ThreadExit();
 }
 
 int main(){
-    int i = 64;
-    for (i=64; i < 68; i++){
+    int i;
+    for (i=65; i < 91; i++){
         ThreadCreate(f, i);
     }
     // PutString("end of main()\n");
diff --git a/code/userprog/addrspace.cc b/code/userprog/addrspace.cc
index 2aed310..fee6ced 100644
--- a/code/userprog/addrspace.cc
+++ b/code/userprog/addrspace.cc
@@ -21,7 +21,10 @@
 #include "noff.h"
 #include "syscall.h"
 #include "new"
-
+#ifdef CHANGED
+#include "synch.h"
+#include "bitmap.h"
+#endif //CHANGED
 //----------------------------------------------------------------------
 // SwapHeader
 //      Do little endian to big endian conversion on the bytes in the 
@@ -129,8 +132,17 @@ AddrSpace::AddrSpace (OpenFile * executable)
     pageTable[0].valid = FALSE;			// Catch NULL dereference
 
     #ifdef CHANGED
+    int bitmapSize = UserStacksAreaSize / UserStackSize;
     DEBUG('x', "Initialise thread counter\n");
-    Threads = 1;
+    threads = 1;
+    
+    DEBUG('x', "Initialise semaphores\n");
+    semThreadsCounter = new Semaphore("AddrSpace_thread_counter", 1);
+    semAllocateUserStack = new Semaphore("Stack Avaiable", 1);
+
+    DEBUG('x', "Initialize memory map size:%d", bitmapSize);
+    memoryMap = new BitMap(bitmapSize);
+    memoryMap->Mark(0);
     #endif //CHANGED
 
     AddrSpaceList.Append(this);
@@ -147,6 +159,11 @@ AddrSpace::~AddrSpace ()
   pageTable = NULL;
 
   AddrSpaceList.Remove(this);
+  #ifdef CHANGED
+  delete semThreadsCounter;
+  delete semAllocateUserStack;
+  delete memoryMap;
+  #endif
 }
 
 //----------------------------------------------------------------------
@@ -303,6 +320,25 @@ int
 AddrSpace::AllocateUserStack()
 {
     int memory = numPages * PageSize;
-    return memory - 256;
+    int bit;
+
+    /* If we don't have any free slot, we can wait for one to be
+     * freed by DeAllocateUserstack
+     */
+    while ((bit = memoryMap->Find()) == - 1){
+        DEBUG('x', "Can't allocate User Stack Wainting...\n");
+        semAllocateUserStack->P();
+    }
+    int addr = memory - UserStackSize * bit;
+    DEBUG('x', "Allocate User Stack bit %d, addr:%x\n", bit, addr);
+    return addr;
+}
+void
+AddrSpace::DeAllocateUserStack(int addr){
+    int memory = numPages * PageSize;
+    int bit = (memory - addr) / UserStackSize;
+    DEBUG('x', "Deallocate User Stack bit %d, addr:%x\n", bit, addr);
+    memoryMap->Clear(bit);
+    semAllocateUserStack->V();
 }
 #endif
diff --git a/code/userprog/addrspace.h b/code/userprog/addrspace.h
index bd44fdd..617b37a 100644
--- a/code/userprog/addrspace.h
+++ b/code/userprog/addrspace.h
@@ -19,6 +19,13 @@
 #include "noff.h"
 #include "list.h"
 
+
+#ifdef CHANGED
+#include "bitmap.h"
+class Semaphore;
+#define UserStackSize           256
+#endif //CHANGED
+
 #define UserStacksAreaSize		1024	// increase this as necessary!
 
 class AddrSpace:public dontcopythis
@@ -41,8 +48,12 @@ class AddrSpace:public dontcopythis
 				// Dump program layout as SVG
     unsigned NumPages() { return numPages; }
     #ifdef CHANGED
-    int Threads;            // count number of threads into address space
+    int threads;            // count number of threads into address space
+    Semaphore * semThreadsCounter;
+    Semaphore * semAllocateUserStack;
+    BitMap * memoryMap;
     int AllocateUserStack();
+    void DeAllocateUserStack(int addr);
     #endif
   private:
     NoffHeader noffH;		// Program layout
diff --git a/code/userprog/userthread.cc b/code/userprog/userthread.cc
index da97ea0..f1b0aa2 100644
--- a/code/userprog/userthread.cc
+++ b/code/userprog/userthread.cc
@@ -4,6 +4,7 @@
 #include "userthread.h"
 #include "syscall.h"
 #include "addrspace.h"
+#include "synch.h"
 
 
 static void StartUserThread( void * args ){
@@ -55,8 +56,10 @@ int do_ThreadCreate(int f, int arg){
     Thread * newThread = new Thread("new thread");
 
     // increment number of threads
-    currentThread->space->Threads++;
-    DEBUG('x', "Increase numbers of Threads:%d\n",currentThread->space->Threads);
+    currentThread->space->semThreadsCounter->P();
+    currentThread->space->threads++;
+    currentThread->space->semThreadsCounter->V();
+    DEBUG('x', "Increase numbers of Threads:%d\n",currentThread->space->threads);
 
     newThread->space = currentThread->space;
     newThread->Start(StartUserThread, args);
@@ -69,8 +72,14 @@ void do_ThreadExit(){
     // TODO: what should we do with thread space?
     //       Probalely desallocate it... if no threads remain
     // currentThread->space->Threads--;
-    DEBUG('x', "Decrease numbers of Threads:%d\n",currentThread->space->Threads);
-    if ( --currentThread->space->Threads == 0){
+    currentThread->space->semThreadsCounter->P();
+    currentThread->space->threads--;
+    currentThread->space->semThreadsCounter->V();
+    int addr = machine->ReadRegister(StackReg);
+    currentThread->space->DeAllocateUserStack(addr);
+    DEBUG('x', "Decrease numbers of Threads:%d\n",currentThread->space->threads);
+    
+    if ( currentThread->space->threads == 0 ){
         // No threads remains, desallocate addrspace
         delete currentThread->space;
         interrupt->Powerdown();