diff --git a/asm_test.py b/asm_test.py
new file mode 100644
index 0000000..481875b
--- /dev/null
+++ b/asm_test.py
@@ -0,0 +1,17 @@
+from creole_asm import *
+import unittest
+
+class ProgramTest(unittest.TestCase):
+	def test_oneline(self):
+		p = Program()
+		p.parse_asm_line("PUSH r0")
+		b = p()
+		self.assertEqual(b, b'\x01\xC2\x80\x00')
+	def test_large_reg(self):
+		p = Program(regnum=0x8000000)
+		p.parse_asm_line("PUSH r134217727")
+		b = p()
+		self.assertEqual(b, b'\x01\xFC\x87\xbf\xbf\xbf\xbf\x00')
+
+if __name__ == "__main__":
+	unittest.main()
diff --git a/creole_asm.py b/creole_asm.py
index 06e470f..8f7ab22 100644
--- a/creole_asm.py
+++ b/creole_asm.py
@@ -2,56 +2,66 @@
 
 from enum import Enum
 
+class MalformedArgument(Exception):
+	pass
+
 class ArgType(Enum):
-	TYPE_IMM = 1
-	TYPE_REG = 2
-	TYPE_VAL = 3
-	TYPE_LAB = 4
+	IMM = 1
+	REG = 2
+	VAL = 3
+	LAB = 4
 
 	def gettype(s):
 		if s.isnumeric():
-			return (TYPE_IMM, int(s))
+			return (ArgType.IMM, int(s))
 		elif s[0] == 'r' and s[1:].isnumeric():
-			return (TYPE_REG, int(s[1:]))
+			return (ArgType.REG, int(s[1:]))
 		elif s[0] == 'l' and s[1:].isnumeric():
-			return (TYPE_LAB, int(s[1:]))
+			return (ArgType.LAB, int(s[1:]))
 		else:
-			return None
+			raise MalformedArgument(s)
 
 	def typecheck(self, s):
 		t = ArgType.gettype(s)
-		if t is None:
-			return None
-		if self == TYPE_VAL:
-			return t[0] == TYPE_REG or t[0] == TYPE_IMM
+		if self == ArgType.VAL:
+			return t[0] == ArgType.REG or t[0] == ArgType.IMM
 		else:
 			return t[0] == self
 
+class OpcodeException(Exception):
+	pass
+class TypecheckLenException(Exception):
+	pass
+class TypecheckException(Exception):
+	pass
 class Instruction:
-	def __init__(self,  opcode, argtypes):
+	def __init__(self, opcode, argtypes):
+		if opcode > 0x7F or opcode < 0:
+			raise OpcodeException(opcode)
+
 		self.opcode = opcode
-		assert self.opcode < 0x80 and self.opcode >= 0
 		self.argtypes = argtypes
 	def typecheck(self, sargs):
 		rargs = []
 		if len(sargs) != len(self.argtypes):
-			return None
+			raise TypecheckLenException(sargs, self.argtypes)
 		for i in range(0, len(sargs)):
 			if not self.argtypes[i].typecheck(sargs[i]):
-				return None	
+				raise TypecheckException(self.argtypes[i],
+				                         sargs[i])
 			rargs.append(ArgType.gettype(sargs[i]))
 		return rargs
 
 instructions = {
-"NOP" : Instruction(0, []),
-"PUSH" : Instruction(1, [ArgType.TYPE_REG]),
-"POP" : Instruction(2, [ArgType.TYPE_REG]),
-"ADD" : Instruction(3, [ArgType.TYPE_VAL, ArgType.TYPE_VAL, ArgType.TYPE_VAL]),
-"MUL" : Instruction(4, [ArgType.TYPE_VAL, ArgType.TYPE_VAL, ArgType.TYPE_VAL]),
-"DIV" : Instruction(5, [ArgType.TYPE_VAL, ArgType.TYPE_VAL, ArgType.TYPE_VAL]),
-"JL" : Instruction(6, [ArgType.TYPE_LAB, ArgType.TYPE_VAL, ArgType.TYPE_VAL]),
-"CLB" : Instruction(7, [ArgType.TYPE_LAB]),
-"SYS" : Instruction(8, [ArgType.TYPE_VAL])
+"nop" : Instruction(0, []),
+"push" : Instruction(1, [ArgType.REG]),
+"pop" : Instruction(2, [ArgType.REG]),
+"add" : Instruction(3, [ArgType.VAL, ArgType.VAL, ArgType.VAL]),
+"mul" : Instruction(4, [ArgType.VAL, ArgType.VAL, ArgType.VAL]),
+"div" : Instruction(5, [ArgType.VAL, ArgType.VAL, ArgType.VAL]),
+"jl" : Instruction(6, [ArgType.LAB, ArgType.VAL, ArgType.VAL]),
+"clb" : Instruction(7, [ArgType.LAB]),
+"sys" : Instruction(8, [ArgType.VAL])
 }
 
 encoding_types = {
@@ -66,18 +76,31 @@ encoding_types = {
 #	B : Total number of bits excluding high bits
 }
 
+class InvalidNumberException(Exception):
+	pass
+class InvalidLengthException(Exception):
+	pass
 def encode_pseudo_utf8(n, high_bits, to):
+	if n < 0:
+		raise InvalidNumberException(n)
+	if to is None or to < 0:
+		for k in sorted(encoding_types):
+			if n <= encoding_types[k][0]:
+				to = k
+				break
+		if to is None:
+			raise InvalidNumberException(n)
 	if to > 8 or to < 0:
-		return None
+		raise InvalidLengthException(to)
 	elif to == 1:
 		if n < 0x80:
 			return bytes([n])
 		else:
-			return None
+			raise InvalidNumberException(n,to)
 
 	(maxval, start_byte, n_tot) = encoding_types[to]
 	if n > maxval or high_bits > 15:
-		return None
+		raise InvalidNumberException(n, high_bits)
 	n = n | (high_bits << n_tot)
 	all_bytes = []
 	for i in range(0, to - 1):
@@ -86,32 +109,47 @@ def encode_pseudo_utf8(n, high_bits, to):
 	all_bytes.append(start_byte | n)
 	return bytes(reversed(all_bytes))
 
+class RangeCheckException(Exception):
+	pass
 class Line:
-	def __init__(self, opcode, args):
+	def __init__(self, opcode, args, labnum, regnum):
 		self.opcode = opcode
 		self.args = args
+		for a in args:
+			if a[0] == ArgType.REG:
+				if a[1] < 0 or a[1] >= regnum:
+					raise RangeCheckException(a[0],
+					                          a[1],
+					                          regnum)
+			elif a[0] == ArgType.LAB:
+				if a[1] < 0 or a[1] >= labnum:
+					raise RangeCheckException(a[0],
+					                          a[1],
+					                          regnum)
+
 	def __call__(self):
 		b = bytes([self.opcode])
-		for a in args:
-			if a[0] == TYPE_REG:
+		for a in self.args:
+			if a[0] == ArgType.REG:
 				b = b + encode_pseudo_utf8(a[1],1,None)
 			else:
 				b = b + encode_pseudo_utf8(a[1],0,None)
 		return b + bytes([0])
 
+class InstructionNotFoundException(Exception):
+	pass
 class Program:
 	def asm_push_line(self, ins, args):
-		self.asm.append(Line(ins, args))
+		self.asm.append(Line(ins, args, self.labnum, self.regnum))
 
 	def parse_asm_line(self, line):
 		line = line.split()
+		line[0] = line[0].casefold()
 		if line[0] not in instructions:
-			raise Exception
+			raise InstructionNotFoundException(line[0])
 		else:
 			ins = instructions[line[0]]
 			args_w_type = ins.typecheck(line[1:])
-			if r is None:
-				raise Exception
 			self.asm_push_line(ins.opcode, args_w_type)
 
 	def __call__(self):
@@ -120,5 +158,7 @@ class Program:
 			b = b + line()
 		return b
 
-	def __init__(self):
+	def __init__(self, labnum=16, regnum=16):
 		self.asm = []
+		self.labnum = labnum
+		self.regnum = regnum