diff --git a/platform/cooja/sys/cooja_mtarch.c b/platform/cooja/sys/cooja_mtarch.c
index 38755b470..1e23c0449 100644
--- a/platform/cooja/sys/cooja_mtarch.c
+++ b/platform/cooja/sys/cooja_mtarch.c
@@ -28,15 +28,39 @@
  *
  * This file is part of the Contiki operating system.
  *
- * @(#)$Id: cooja_mtarch.c,v 1.5 2007/04/02 08:47:28 fros4943 Exp $
+ * @(#)$Id: cooja_mtarch.c,v 1.6 2008/11/28 16:41:26 fros4943 Exp $
  */
 
+#include <stddef.h>
+
+#include <limits.h>
 #include <stdio.h>
 #include <string.h>
 #include "sys/cooja_mt.h"
 
+#ifndef __WORDSIZE
+#define __WORDSIZE 32
+#endif /* __WORDSIZE */
+
+#ifndef ON_64BIT_ARCH
+#if __WORDSIZE == 64
+#define ON_64BIT_ARCH 1
+#else /* ON_64BIT_ARCH */
+#define ON_64BIT_ARCH 0
+#endif /* __WORDSIZE == 64 */
+#endif /* ON_64BIT_ARCH */
+
 struct frame {
   unsigned long flags;
+#if ON_64BIT_ARCH
+  unsigned long rbp;
+  unsigned long rdi;
+  unsigned long rsi;
+  unsigned long rdx;
+  unsigned long rcx;
+  unsigned long rbx;
+  unsigned long rax;
+#else /* ON_64BIT_ARCH */
   unsigned long ebp;
   unsigned long edi;
   unsigned long esi;
@@ -44,6 +68,7 @@ struct frame {
   unsigned long ecx;
   unsigned long ebx;
   unsigned long eax;
+#endif /* ON_64BIT_ARCH */
   unsigned long retaddr;
   unsigned long retaddr2;
   unsigned long data;
@@ -56,20 +81,24 @@ cooja_mtarch_init(void)
 /*--------------------------------------------------------------------------*/
 void
 cooja_mtarch_start(struct cooja_mtarch_thread *t,
-		   void (*function)(void *), void *data)
+    void (*function)(void *), void *data)
 {
   struct frame *f = (struct frame *)&t->stack[COOJA_MTARCH_STACKSIZE - sizeof(struct frame)/4];
   int i;
-  
+
   for(i = 0; i < COOJA_MTARCH_STACKSIZE; ++i) {
     t->stack[i] = i;
   }
-  
+
   memset(f, 0, sizeof(struct frame));
   f->retaddr = (unsigned long)function;
   f->data    = (unsigned long)data;
   t->sp      = (unsigned long)&f->flags;
+#if ON_64BIT_ARCH
+  f->rbp     = (unsigned long)&f->rax;
+#else /* ON_64BIT_ARCH */
   f->ebp     = (unsigned long)&f->eax;
+#endif /* ON_64BIT_ARCH */
 }
 /*--------------------------------------------------------------------------*/
 static struct cooja_mtarch_thread *cooja_running_thread;
@@ -77,39 +106,78 @@ static struct cooja_mtarch_thread *cooja_running_thread;
 void cooja_sw(void)
 {
   /* Store registers */
+#if ON_64BIT_ARCH
   __asm__ (
-	   "pushl %eax\n\t"
-	   "pushl %ebx\n\t"
-	   "pushl %ecx\n\t"
-	   "pushl %edx\n\t"
-	   "pushl %esi\n\t"
-	   "pushl %edi\n\t"
-	   "pushl %ebp\n\t"
-	   "pushl %ebp\n\t");
-  
+      "pushq %rax\n\t"
+      "pushq %rbx\n\t"
+      "pushq %rcx\n\t"
+      "pushq %rdx\n\t"
+      "pushq %rsi\n\t"
+      "pushq %rdi\n\t"
+      "pushq %rbp\n\t"
+      "pushq %rbp\n\t");
+#else /* ON_64BIT_ARCH */
+  __asm__ (
+      "pushl %eax\n\t"
+      "pushl %ebx\n\t"
+      "pushl %ecx\n\t"
+      "pushl %edx\n\t"
+      "pushl %esi\n\t"
+      "pushl %edi\n\t"
+      "pushl %ebp\n\t"
+      "pushl %ebp\n\t");
+#endif /* ON_64BIT_ARCH */
+
   /* Switch stack pointer */
+#if ON_64BIT_ARCH
+  __asm__ ("movq %0, %%rax\n\t" : : "m" (cooja_running_thread));
+  __asm__ (
+      "movq 0(%rax), %rbx\n\t"
+      "movq %rsp, 0(%rax)\n\t"
+      "movq %rbx, %rsp\n\t"
+  );
+#else /* ON_64BIT_ARCH */
   __asm__ ("movl %0, %%eax\n\t" : : "m" (cooja_running_thread));
   __asm__ (
-	   "movl 0(%eax), %ebx\n\t"
-	   "movl %esp, 0(%eax)\n\t"
-	   "movl %ebx, %esp\n\t"
-	   );
-  
-  /* Restore stored registers and return "our" way */
+      "movl 0(%eax), %ebx\n\t"
+      "movl %esp, 0(%eax)\n\t"
+      "movl %ebx, %esp\n\t"
+  );
+#endif /* ON_64BIT_ARCH */
+
+  /* Restore previous registers */
+#if ON_64BIT_ARCH
   __asm__ (
-	   "popl %ebp\n\t"
-	   "popl %ebp\n\t"
-	   "popl %edi\n\t"
-	   "popl %esi\n\t"
-	   "popl %edx\n\t"
-	   "popl %ecx\n\t"
-	   "popl %ebx\n\t"
-	   "popl %eax\n\t"
-	   
-	   "leave\n\t"
-	   "ret\n\t"							
-	   );
+      "popq %rbp\n\t"
+      "popq %rbp\n\t"
+      "popq %rdi\n\t"
+      "popq %rsi\n\t"
+      "popq %rdx\n\t"
+      "popq %rcx\n\t"
+      "popq %rbx\n\t"
+      "popq %rax\n\t"
+
+      "leave\n\t"
+      "ret\n\t"
+  );
+#else /* ON_64BIT_ARCH */
+  __asm__ (
+      "popl %ebp\n\t"
+      "popl %ebp\n\t"
+      "popl %edi\n\t"
+      "popl %esi\n\t"
+      "popl %edx\n\t"
+      "popl %ecx\n\t"
+      "popl %ebx\n\t"
+      "popl %eax\n\t"
+
+      "leave\n\t"
+      "ret\n\t"
+  );
+#endif /* ON_64BIT_ARCH */
+
 }
+
 /*--------------------------------------------------------------------------*/
 void
 cooja_mtarch_exec(struct cooja_mtarch_thread *t)