diff --git a/asm/creole.py b/asm/creole.py
index 8e87b58..c1045c5 100644
--- a/asm/creole.py
+++ b/asm/creole.py
@@ -24,16 +24,32 @@ class ArgType(Enum):
 	def typecheck(self, s):
 		t = ArgType.gettype(s)
 		if self == ArgType.VAL:
-			return t[0] == ArgType.REG or t[0] == ArgType.IMM
+			if t[0] == ArgType.REG or t[0] == ArgType.IMM:
+				return t
+			else:
+				return None
+		elif t[0] == self:
+			return t
 		else:
-			return t[0] == self
+			return None
 
 class OpcodeException(Exception):
 	pass
 class TypecheckLenException(Exception):
-	pass
+	def __init__(self, opcode, insargs, argtypelen):
+		self.opcode = opcode
+		self.insargs = insargs
+		self.argtypelen = argtypelen
+	def __str__(self):
+		return f'arguments {insargs} to opcode {self.opcode} not length {self.argtypelen}'
 class TypecheckException(Exception):
-	pass
+	def __init__(self, argtype, sarg, i, opcode):
+		self.argtype = argtype
+		self.sarg = sarg
+		self.i = i
+		self.opcode = opcode
+	def __str__(self):
+		return f'opcode {self.opcode} has invalid value {self.sarg} (expected {self.argtype} in position {self.i}'
 class Instruction:
 	def __init__(self, opcode, argtypes):
 		if opcode > 0x7F or opcode < 0:
@@ -44,12 +60,15 @@ class Instruction:
 	def typecheck(self, sargs):
 		rargs = []
 		if len(sargs) != len(self.argtypes):
-			raise TypecheckLenException(sargs, self.argtypes)
+			raise TypecheckLenException(self.opcode, sargs,
+			                            len(self.argtypes))
 		for i in range(0, len(sargs)):
-			if not self.argtypes[i].typecheck(sargs[i]):
+			t = self.argtypes[i].typecheck(sargs[i])
+			if t is None:
 				raise TypecheckException(self.argtypes[i],
-				                         sargs[i])
-			rargs.append(ArgType.gettype(sargs[i]))
+				                         sargs[i],
+				                         i, self.opcode)
+			rargs.append(t)
 		return rargs
 
 instructions = {
diff --git a/asm/ffi.py b/asm/ffi.py
index 87f6275..7468996 100644
--- a/asm/ffi.py
+++ b/asm/ffi.py
@@ -1,6 +1,18 @@
 from ctypes import *
+from enum import Enum
 dll = CDLL("./libcreole.so")
 
+class CompileRet(Enum):
+	COMPILE_OK = 0
+	OPCODE_READ_ERROR = 1
+	OPCODE_MALFORMED = 2
+	ARG_READ_ERROR = 3
+	ARG_MALFORMED = 4
+	LAST_READ_ERROR = 5
+	LAST_MALFORMED = 6
+	LABEL_OVERFLOW = 7
+	TYPE_ERROR = 8
+
 class CIns(Structure):
 	_fields_ = [("opcode", c_int),
 	            ("w_flags", c_ubyte * 3),
diff --git a/asm/test.py b/asm/test.py
index dc07655..b9d6520 100644
--- a/asm/test.py
+++ b/asm/test.py
@@ -2,17 +2,57 @@ from creole import *
 import unittest
 import ffi
 
-class ProgramTest(unittest.TestCase):
-	def test_oneline(self):
+class PushTest(unittest.TestCase):
+	def test_parse_push_reg(self):
 		p = Program()
-		p.parse_asm_line("PUSH r0")
+		p.parse_asm_line("push r5")
 		b = p()
 		self.assertEqual(b, b'\x01\xC2\x80\x00')
+		ins = ffi.parse_line(b)
+		self.assertEqual(ins[0], instructions["push"].opcode)
+		self.assertEqual(ins[1][0], (1,5))
+
+	def test_parse_push_catch_typecheck_push_imm(self):
+		p = Program()
+		with self.assertRaises(TypecheckException) as cm:
+			p.parse_asm_line("push 0")
+		self.assertEqual(cm.exception.argtype, ArgType.REG)
+		self.assertEqual(cm.exception.sarg, '0')
+		self.assertEqual(cm.exception.i, 0)
+		self.assertEqual(cm.exception.opcode, 1)
+
+	def test_parse_push_catch_typecheck_push_lab(self):
+		p = Program()
+		with self.assertRaises(TypecheckException) as cm:
+			p.parse_asm_line("push l0")
+		self.assertEqual(cm.exception.argtype, ArgType.REG)
+		self.assertEqual(cm.exception.sarg, 'l0')
+		self.assertEqual(cm.exception.i, 0)
+		self.assertEqual(cm.exception.opcode, 1)
+
+	def test_parse_push_catch_typecheck_argument_overflow(self):
+		p = Program()
+		with self.assertRaises(TypecheckLenException) as cm:
+			p.parse_asm_line("push r1 r2")
+		self.assertEqual(cm.exception.opcode, 1)
+		self.assertEqual(cm.exception.insargs, ["r1", "r2"])
+		self.assertEqual(cm.exception.argtypelen, 1)
+
+	def test_parse_push_catch_typecheck_argument_underflow(self):
+		p = Program()
+		with self.assertRaises(TypecheckLenException) as cm:
+			p.parse_asm_line("push")
+		self.assertEqual(cm.exception.opcode, 1)
+		self.assertEqual(cm.exception.insargs, [])
+		self.assertEqual(cm.exception.argtypelen, 1)
+
 	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')
+
+class ProgramTest(unittest.TestCase):
 	def test_two(self):
 		p = Program()
 		p.parse_asm_line("PUSH r1")
@@ -39,12 +79,14 @@ class ProgramTest(unittest.TestCase):
 		b = p()
 		self.assertEqual(b, b_ex)
 
-	def test_parse_reg(self):
+	def test_parse_imm(self):
 		p = Program()
-		p.parse_asm_line("push r5")
+		p.parse_asm_line("add r1 23 3648")
 		ins = ffi.parse_line(p())
-		self.assertEqual(ins[0], instructions["push"].opcode)
-		self.assertEqual(ins[1][0], (1,5))
+		self.assertEqual(ins[0], instructions["add"].opcode)
+		self.assertEqual(ins[1][0], (1,1))
+		self.assertEqual(ins[1][1], (0,23))
+		self.assertEqual(ins[1][2], (0,3648))
 
 if __name__ == "__main__":
 	unittest.main()