diff --git a/core/bitarithm.c b/core/bitarithm.c
index 8bff87707c3c..72a4cb386e87 100644
--- a/core/bitarithm.c
+++ b/core/bitarithm.c
@@ -15,36 +15,47 @@
  *
  * @author      Kaspar Schleiser <kaspar@schleiser.de>
  * @author      Martin Lenders <mlenders@inf.fu-berlin.de>
- * @author      René Kijewski <rene.kijewski@fu-berlin.de>
  *
  * @}
  */
 
 #include <stdio.h>
 
-#include "bitarithm.h"
-
 unsigned bitarithm_msb(unsigned v)
 {
-    if ((signed) v >= 0) {
-        v &= -v;
-        return bitarithm_lsb(v);
-    }
-    else {
-        return sizeof (v) * 8 - 1;
+    register unsigned r; // result of log2(v) will go here
+
+#if ARCH_32_BIT
+    register unsigned shift;
+
+    r =     (v > 0xFFFF) << 4; v >>= r;
+    shift = (v > 0xFF  ) << 3; v >>= shift; r |= shift;
+    shift = (v > 0xF   ) << 2; v >>= shift; r |= shift;
+    shift = (v > 0x3   ) << 1; v >>= shift; r |= shift;
+                                            r |= (v >> 1);
+#else
+    r = 0;
+    while (v >>= 1) { // unroll for more speed...
+        r++;
     }
-}
 
-unsigned bitarithm_lsb(unsigned v)
+#endif
+
+    return r;
+}
+/*---------------------------------------------------------------------------*/
+unsigned bitarithm_lsb(register unsigned v)
 {
-    unsigned r = 0;
-    while ((v & 1) == 0) {
+    register unsigned r = 0;
+
+    while ((v & 0x01) == 0) {
         v >>= 1;
         r++;
-    }
+    };
+
     return r;
 }
-
+/*---------------------------------------------------------------------------*/
 unsigned bitarithm_bits_set(unsigned v)
 {
     unsigned c; // c accumulates the total bits set in v
diff --git a/tests/unittests/tests-core/tests-core-bitarithm.c b/tests/unittests/tests-core/tests-core-bitarithm.c
index 788e6317dda9..a8cc7f2e0dda 100644
--- a/tests/unittests/tests-core/tests-core-bitarithm.c
+++ b/tests/unittests/tests-core/tests-core-bitarithm.c
@@ -7,6 +7,7 @@
  */
 
 #include <limits.h>
+#include <stdint.h>
 
 #include "embUnit/embUnit.h"
 
@@ -153,14 +154,14 @@ static void test_bitarithm_msb_limit(void)
 
 static void test_bitarithm_msb_random(void)
 {
-    TEST_ASSERT_EQUAL_INT(2, bitarithm_msb(4)); /* randomized by fair
+    TEST_ASSERT_EQUAL_INT(4, bitarithm_msb(19)); /* randomized by fair
                                                            dice roll ;-) */
 }
 
-static void test_bitarithm_msb_all(void)
+static void test_bitarithm_msb_16bit(void)
 {
-    for (unsigned shift = 0; shift < sizeof(unsigned) * 8 - 1; ++shift) {
-        TEST_ASSERT_EQUAL_INT(shift, bitarithm_msb(1 << shift));
+    for (unsigned i = 1; i < UINT16_MAX; i++) {
+        TEST_ASSERT_EQUAL_INT(((sizeof(unsigned) * 8) - __builtin_clz(i) - 1), bitarithm_msb(i));
     }
 }
 
@@ -177,14 +178,14 @@ static void test_bitarithm_lsb_limit(void)
 
 static void test_bitarithm_lsb_random(void)
 {
-    TEST_ASSERT_EQUAL_INT(3, bitarithm_lsb(8)); /* randomized by fair
+    TEST_ASSERT_EQUAL_INT(3, bitarithm_lsb(24)); /* randomized by fair
                                                           dice roll ;-) */
 }
 
 static void test_bitarithm_lsb_all(void)
 {
-    for (unsigned shift = 0; shift < sizeof(unsigned) * 8 - 1; ++shift) {
-        TEST_ASSERT_EQUAL_INT(shift, bitarithm_lsb(1u << shift));
+    for (unsigned i = 1; i < UINT16_MAX; i++) {
+        TEST_ASSERT_EQUAL_INT(__builtin_ctz(i), bitarithm_lsb(i));
     }
 }
 
@@ -232,7 +233,7 @@ Test *tests_core_bitarithm_tests(void)
         new_TestFixture(test_bitarithm_msb_one),
         new_TestFixture(test_bitarithm_msb_limit),
         new_TestFixture(test_bitarithm_msb_random),
-        new_TestFixture(test_bitarithm_msb_all),
+        new_TestFixture(test_bitarithm_msb_16bit),
 
         new_TestFixture(test_bitarithm_lsb_one),
         new_TestFixture(test_bitarithm_lsb_limit),