diff --git a/MicSim/components/caretaker.py b/MicSim/components/caretaker.py
index 82ac63d..cb92cbb 100644
--- a/MicSim/components/caretaker.py
+++ b/MicSim/components/caretaker.py
@@ -15,20 +15,28 @@ class Caretaker:
 		if key=="MBRU": # If we ask for unsigned
 			return(abs(self.objects["MBR"]))
 		elif key== "MBR":
-			if abs(self.objects[key]>>7)==1: # If it a negative number (2 complement)
-				return(-((self.objects[key]-1)^0xFF)) # transforme bin negative number to python negative number
+			if self.objects[key] < 0:
+				return(self.objects[key])
+			elif self.objects[key]>>7==1: # If it a negative number (2 complement)
+				return(-(self.objects[key]&0x7F))
 			else:
 				return(self.objects[key])
 		return(self.objects[key])
 
 	def __setitem__(self,key,value):
 		if key!="RAM":
-			if value > (2**32) and key!="MBR" and key!="MBRU": # Check value fit in word
-				print("Warning word overflow: value {} on register {}".format(value,key))
-				value=value%(2**32) # Force to fit in word
-			elif value > (2**8) and key=="MBR" and key=="MBRU": # Check value fit in byte
-				print("Warning byte overflow: value {} on register {}".format(value,key))
-				value=value%256 # Force to fit in byte
+			if (-(2**7-1))>value and (key=="MBR" or key=="MBRU"):
+				raise RuntimeError("Value {} cannot fit in MBR register (-2^7 minimum value)".format(value))
+			elif value>(2**32-1) and (key!="MBR" and key!="MBRU"):
+				raise RuntimeError("Value {} cannot fit in MBR register (2^32 minimum value)".format(value))
+
+#		if key!="RAM":
+#			if (value > (2**32) or value <(2**32)/2)  and key!="MBR" and key!="MBRU": # Check value fit in word
+#				print("Warning word overflow: value {} on register {}".format(value,key))
+#				value=value%(2**32) # Force to fit in word
+#			elif (value > (2**8) or value <126) and key=="MBR" or key=="MBRU": # Check value fit in byte
+#				print("Warning byte overflow: value {} on register {}".format(value,key))
+#				value=abs(value)%256 # Force to fit in byte
 		self.objects[key]=value
 		
 	def items(self):
diff --git a/MicSim/test/test_caretaker.py b/MicSim/test/test_caretaker.py
index 5cc7483..e4b8dfb 100644
--- a/MicSim/test/test_caretaker.py
+++ b/MicSim/test/test_caretaker.py
@@ -16,13 +16,16 @@ class CaretakerTest(unittest.TestCase):
 		"""
 			Test if getitem operation follow Mic-1 rules
 		"""
-		toWrite=randint(0,126) # Only 7 bit for signed MBR (2^7=127)
-		self.c["MBR"]=-toWrite
-		self.assertEqual(self.c["MBRU"],toWrite,"Tested with {}".format(-toWrite))
-		self.assertEqual(self.c["MBR"],-(-((toWrite-1)^0xFF)),"Tested with {}".format(-toWrite))
-		self.c["MBR"]=toWrite
-		self.assertEqual(self.c["MBRU"],toWrite,"Tested with {}".format(toWrite))
-		self.assertEqual(self.c["MBR"],toWrite,"Tested with {}".format(toWrite))
+		for toWrite in range(0,127):# Only 7 bit for signed MBR (2^7=127)
+			self.c["MBR"]=-toWrite
+			self.assertEqual(self.c["MBRU"],toWrite,"Tested with {}".format(-toWrite))
+			self.assertEqual(self.c["MBR"],-toWrite,"Tested with {}".format(-toWrite))
+			
+		for toWrite in range(0,255):# Only 2^8 value for unsigned
+			self.c["MBR"]=toWrite
+			self.assertEqual(self.c["MBRU"],toWrite,"Tested with {}".format(toWrite))
+			if toWrite>127: # We enter in the zone of negative number at 127
+				self.assertEqual(self.c["MBR"],-(toWrite&0x7F),"Tested with {}".format(toWrite))
 
 		with self.assertRaises(KeyError): # Check it returns a KeyError
 			self.c["kjhkjhkoih"+str(randint(0,7698))]