diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
new file mode 100644
index 00000000..93572931
--- /dev/null
+++ b/.github/workflows/tests.yml
@@ -0,0 +1,28 @@
+name: Unit tests
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python 3.8
+      uses: actions/setup-python@v2
+      with:
+        python-version: 3.8
+    - name: Install PyYAML
+      run: |
+         pip3 install -r requirements.txt
+         pip3 install coverage
+    - name: Test error outputs
+      run: coverage run -m unittest -b
+    - name: Generate coverage
+      run: coverage xml
+    - name: Upload coverage
+      uses: codecov/codecov-action@v2
+          
diff --git a/parse.py b/parse.py
index 83c1d72c..243a8bbd 100755
--- a/parse.py
+++ b/parse.py
@@ -13,6 +13,9 @@
 pp = pprint.PrettyPrinter(indent=2)
 logging.basicConfig(level=logging.INFO, format='%(levelname)s:: %(message)s')
 
+def parse_constant(string):
+    return int(string, 0)
+
 def process_enc_line(line, ext):
     '''
     This function processes each line of the encoding files (rv*). As part of
@@ -61,14 +64,23 @@ def process_enc_line(line, ext):
     temp_instr = ['-'] * 32
     entries = [
         x[0] for x in re.findall(
-            r'((\d)+\.\.(\d)+\=((0b\d+)|(0x\d+)|(\d)+))*',
+            r'((\d)+\.\.(\d)+\=((0b\d+)|(0x[0-9a-fA-F]+)|(\d)+))*',
             remaining) if x[0] != ''
     ]
     for temp_entry in entries:
         entry = temp_entry.split('=')[0]
-        f1, f2 = entry.split('..')
-        for ind in range(int(f1), int(f2)):
+        s2, s1 = entry.split('..')
+        msb = int(s2)
+        lsb = int(s1)
+
+        # check msb < lsb
+        if msb < lsb:
+            logging.error(
+                f'{line.split(" ")[0]:<10} has position {msb} less than position {lsb} in it\'s encoding'
+            )
+            raise SystemExit(1)
 
+        for ind in range(lsb, msb):
             # overlapping bits
             if temp_instr[ind] == 'X':
                 logging.error(
@@ -77,20 +89,11 @@ def process_enc_line(line, ext):
                 raise SystemExit(1)
             temp_instr[ind] = 'X'
 
-            # check x < y
-            if int(f1) < int(f2):
-                logging.error(
-                    f'{line.split(" ")[0]:<10} has position {f1} less than position {f2} in it\'s encoding'
-                )
-                raise SystemExit(1)
-
         # illegal value assigned as per bit width
-        entry_value = temp_entry.split('=')[1]
-        temp_base = 16 if 'x' in entry_value else 2 if 'b' in entry_value else 10
-        if len(str(int(entry_value,
-                       temp_base))[2:]) > (int(f1) - int(f2)):
+        entry_value = parse_constant(temp_entry.split('=')[1])
+        if entry_value >= (1 << (msb - lsb + 1)):
             logging.error(
-                f'{line.split(" ")[0]:<10} has an illegal value {entry_value} assigned as per the bit width {f1 - f2}'
+                f'{line.split(" ")[0]:<10} has an illegal value {entry_value} assigned as per the bit width {msb - lsb}'
             )
             raise SystemExit(1)
 
diff --git a/test.py b/test.py
new file mode 100644
index 00000000..89c014d4
--- /dev/null
+++ b/test.py
@@ -0,0 +1,44 @@
+#!/usr/bin/env python3
+
+from parse import *
+import logging
+import unittest
+
+class ConstantParseTest(unittest.TestCase):
+    def test_constant(self):
+        self.assertEqual(parse_constant('10'), 10)
+        self.assertEqual(parse_constant('0xa'), 10)
+        self.assertEqual(parse_constant('0xA'), 10)
+        self.assertEqual(parse_constant('0b1010'), 10)
+
+class EncodingLineTest(unittest.TestCase):
+    def setUp(self):
+        logger = logging.getLogger()
+        logger.disabled = True
+
+    def assertError(self, string):
+        self.assertRaises(SystemExit, process_enc_line, string, 'rv_i')
+
+    def test_lui(self):
+        name, data = process_enc_line('lui     rd imm20 6..2=0x0D 1..0=3', 'rv_i')
+        self.assertEqual(name, 'lui')
+        self.assertEqual(data['extension'], ['rv_i'])
+        self.assertEqual(data['match'], '0x37')
+        self.assertEqual(data['mask'], '0x7f')
+
+    def test_overlapping(self):
+        self.assertError('jol rd jimm20 6..2=0x00 3..0=7')
+
+    def test_invalid_order(self):
+        self.assertError('jol 2..6=0x1b')
+
+    def test_illegal_value(self):
+        self.assertError('jol rd jimm20 2..0=10')
+        self.assertError('jol rd jimm20 2..0=0xB')
+
+    @unittest.skip('not implemented')
+    def test_overlapping_field(self):
+        self.assertError('jol rd rs1 jimm20 6..2=0x1b 1..0=3')
+
+    def test_illegal_field(self):
+        self.assertError('jol rd jimm128 2..0=10')