diff --git a/gcksum.py b/gcksum.py
index f25d97c..efad1cd 100644
--- a/gcksum.py
+++ b/gcksum.py
@@ -1,39 +1,31 @@
 #!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 
-from binascii import hexlify
-from struct import unpack
+from grmn import ChkSum
 import sys
 
 FILE = sys.argv[1]
+BLOCKSIZE = 4 * 1024
 
-print("Opening {}".format(FILE))
+print("Reading {} ".format(FILE), end="")
 
-csum_pre = 0
-csum = 0
-last_byte = 0xff
+csum = ChkSum()
 
 with open(FILE, "rb") as f:
     while True:
-        block = f.read(1024)
-        for c in block:
-            csum_pre = csum
-            csum += c
-            csum &= 0xff
-            last_byte = c
-        if len(block) < 1024:
-            print("End reached.")
+        block = f.read(BLOCKSIZE)
+        csum.add(block)
+        print(".", end="", flush=True)
+        if len(block) < BLOCKSIZE:
+            print(" done.")
             break
+    f.close()
 
-print("Sum of all bytes: {:02x}".format(csum))
-if csum == 0:
-    print("CHECKSUM VALID.")
+print("Sum of all bytes: {:02x}".format(csum.get_sum()))
+print("Last byte: {:02x}".format(csum.get_last_byte()))
+if csum.is_valid():
+    print("☑ CHECKSUM VALID.")
 else:
-    print("CHECKSUM INVALID!!! (Or GCD or other type.)")
-
-#print("Sum without last: {:02x}".format(csum_pre))
-
-expected_cksum = ( 0x100 - csum_pre ) & 0xff
-
-print("Expected last byte: {:02x}".format(expected_cksum))
-print("Actual last byte: {:02x}".format(last_byte))
+    print("☒ CHECKSUM INVALID!!! (Or GCD or other type.)")
+    expected_cksum = csum.get_expected()
+    print("Expected last byte: {:02x}".format(expected_cksum))
diff --git a/grmn/__init__.py b/grmn/__init__.py
new file mode 100644
index 0000000..bd62d33
--- /dev/null
+++ b/grmn/__init__.py
@@ -0,0 +1,4 @@
+"""Library for firmware reading/writing."""
+
+#from .gcd import *
+from .chksum import *
diff --git a/grmn/chksum.py b/grmn/chksum.py
new file mode 100644
index 0000000..db1c468
--- /dev/null
+++ b/grmn/chksum.py
@@ -0,0 +1,34 @@
+# -*- coding: utf-8 -*-
+
+class ChkSum:
+
+    def __init__(self):
+        self.chksum = 0
+        self.chksum_without_last = 0
+        self.last_byte = 0xff
+
+    def add(self, data):
+        for c in data[:-1]:
+            self.chksum += c
+        self.chksum_without_last = self.chksum
+        self.chksum_without_last &= 0xff
+        self.last_byte = data[-1]
+        self.chksum += self.last_byte
+        self.chksum &= 0xff
+
+    def get(self):
+        remainder = ( 0x100 - self.chksum ) & 0xff
+        return remainder
+
+    def get_sum(self):
+        return self.chksum
+
+    def is_valid(self):
+        return (self.chksum == 0)
+
+    def get_expected(self):
+        expected = ( 0x100 - self.chksum_without_last ) & 0xff
+        return expected
+
+    def get_last_byte(self):
+        return self.last_byte