diff --git a/cpu/x86/mtarch.c b/cpu/x86/mtarch.c
index 1b606530a..6dd72c61f 100644
--- a/cpu/x86/mtarch.c
+++ b/cpu/x86/mtarch.c
@@ -2,8 +2,29 @@
 #include <stdio.h>
 #include "sys/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;
@@ -11,24 +32,22 @@ struct frame {
   unsigned long ecx;
   unsigned long ebx;
   unsigned long eax;
+#endif /* ON_64BIT_ARCH */
   unsigned long retaddr;
   unsigned long retaddr2;
   unsigned long data;
 };
-
 /*--------------------------------------------------------------------------*/
 void
 mtarch_init(void)
 {
-
 }
 /*--------------------------------------------------------------------------*/
 void
 mtarch_start(struct mtarch_thread *t,
-         void (*function)(void *), void *data)
+    void (*function)(void *), void *data)
 {
-
-  struct frame *f = (struct frame *)&t->stack[MTARCH_STACKSIZE - sizeof(struct frame)/4];
+  struct frame *f = (struct frame *)&t->stack[MTARCH_STACKSIZE - sizeof(struct frame)/sizeof(unsigned long)];
   int i;
 
   for(i = 0; i < MTARCH_STACKSIZE; ++i) {
@@ -39,38 +58,89 @@ mtarch_start(struct mtarch_thread *t,
   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 unsigned long spsave, sptmp;
 static struct mtarch_thread *running_thread;
-
+/*--------------------------------------------------------------------------*/
 static void
 sw(void)
 {
+  /* Store registers */
+#if ON_64BIT_ARCH
+  __asm__ (
+      "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 */
 
-  asm("pushl %eax");
-  asm("pushl %ebx");
-  asm("pushl %ecx");
-  asm("pushl %edx");
-  asm("pushl %esi");
-  asm("pushl %edi");
-  asm("pushl %ebp");
-  asm("pushl %ebp"); /* XXX: should push FPU flags here. */
-  asm("movl %esp, spsave");
+  /* Switch stack pointer */
+#if ON_64BIT_ARCH
+  __asm__ ("movq %0, %%rax\n\t" : : "m" (running_thread));
+  __asm__ (
+      "movq (%rax), %rbx\n\t"
+      "movq %rsp, (%rax)\n\t"
+      "movq %rbx, %rsp\n\t"
+  );
+#else /* ON_64BIT_ARCH */
+  __asm__ ("movl %0, %%eax\n\t" : : "m" (running_thread));
+  __asm__ (
+      "movl (%eax), %ebx\n\t"
+      "movl %esp, (%eax)\n\t"
+      "movl %ebx, %esp\n\t"
+  );
+#endif /* ON_64BIT_ARCH */
 
-  sptmp = running_thread->sp;
-  running_thread->sp = spsave;
+  /* Restore previous registers */
+#if ON_64BIT_ARCH
+  __asm__ (
+      "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 */
 
-  asm("movl sptmp, %esp");
-  asm("popl %ebp"); /* XXX: should pop FPU flags here. */
-  asm("popl %ebp");
-  asm("popl %edi");
-  asm("popl %esi");
-  asm("popl %edx");
-  asm("popl %ecx");
-  asm("popl %ebx");
-  asm("popl %eax");
 }
 
 /*--------------------------------------------------------------------------*/
@@ -85,7 +155,6 @@ mtarch_exec(struct mtarch_thread *t)
 void
 mtarch_remove(void)
 {
-
 }
 /*--------------------------------------------------------------------------*/
 void
@@ -97,13 +166,11 @@ mtarch_yield(void)
 void
 mtarch_pstop(void)
 {
-
 }
 /*--------------------------------------------------------------------------*/
 void
 mtarch_pstart(void)
 {
-
 }
 /*--------------------------------------------------------------------------*/
 int
@@ -115,5 +182,6 @@ mtarch_stack_usage(struct mt_thread *t)
       return MTARCH_STACKSIZE - i;
     }
   }
+  return -1;
 }
 /*--------------------------------------------------------------------------*/