diff --git a/idasm/check_sum.py b/idasm/check_sum.py
new file mode 100644
index 0000000..6958bd7
--- /dev/null
+++ b/idasm/check_sum.py
@@ -0,0 +1,127 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+
+"""
+Test code to determine the checksum algorithm used in Imlac PTP blocks.
+"""
+
+MASK = 0177777
+
+# actual block from 40tp_simpleDisplay.ptp
+block = [065,       # word count
+         0000100,   # load address
+         0001012, 0102010, 0010101, 0002200, 0010103, 0001071, 0100041, 0044164,    # data words
+         0020111, 0000000, 0004116, 0001003, 0003100, 0010101, 0015000, 0025000,    # data words
+         0004005, 0006000, 0050132, 0050141, 0050151, 0050151, 0050156, 0015000,    # data words
+         0025000, 0000000, 0030303, 0141702, 0114230, 0143707, 0143203, 0100770,    # data words
+         0174171, 0030303, 0141702, 0154330, 0127655, 0174350, 0103605, 0154330,    # data words
+         0074571, 0030303, 0141702, 0000600, 0154330, 0074571, 0030202, 0141723,    # data words
+         0150327, 0143766, 0170362, 0074571, 0100000,                               # data words
+         0105226]   # checksum word
+
+def checksum1(block):
+    """Checksum is sum of data words, mask at end."""
+
+    checksum = 0
+    for word in block[2:-1]:
+        checksum += word
+    return checksum & MASK
+
+def checksum2(block):
+    """Checksum is sum of data words, mask at each sum and end."""
+
+    checksum = 0
+    for word in block[2:-1]:
+        checksum = (checksum + word) & MASK
+    return checksum & MASK
+
+def checksum3(block):
+    """Checksum is sum of load address and data words, mask at end."""
+
+    checksum = block[1]
+    for word in block[2:-1]:
+        checksum += word
+    return checksum & MASK
+
+def checksum4(block):
+    """Checksum is sum of load address and data words, mask at each sum and end."""
+
+    checksum = block[1]
+    for word in block[2:-1]:
+        checksum = (checksum + word) & MASK
+    return checksum & MASK
+
+def checksum5(block):
+    """Checksum is sum of word count, load address and data words,
+    mask at end."""
+
+    checksum = block[0]
+    checksum += block[1]
+    for word in block[2:-1]:
+        checksum += word
+    return checksum & MASK
+
+def checksum6(block):
+    """Checksum is sum of word count, load address and data words,
+    mask at at each sum and end."""
+
+    checksum = block[0]
+    checksum = (checksum + block[1]) & MASK
+    for word in block[2:-1]:
+        checksum = (checksum + word) & MASK
+    return checksum & MASK
+
+def checksum7(block):
+    """Checksum is sum of data words, mask at end.  Sum is +1 on overflow"""
+
+    checksum = 0
+    for word in block[2:-1]:
+        checksum += word
+        if checksum & MASK:
+            checksum += 1
+    return checksum & MASK
+
+def checksum8(block):
+    """Checksum is sum of load + data words, mask at end.  Sum is +1 on overflow"""
+
+    checksum = block[1]
+    for word in block[2:-1]:
+        checksum += word
+        if checksum & MASK:
+            checksum += 1
+    return checksum & MASK
+
+def checksum9(block):
+    """Checksum is sum of count+load+data words, mask at end.  Sum is +1 on overflow"""
+
+    checksum = block[0]
+    checksum += block[1]
+    if checksum & MASK:
+        checksum += 1
+    for word in block[2:-1]:
+        checksum += word
+        if checksum & MASK:
+            checksum += 1
+    return checksum & MASK
+
+csum1 = checksum1(block)
+csum2 = checksum2(block)
+csum3 = checksum3(block)
+csum4 = checksum4(block)
+csum5 = checksum5(block)
+csum6 = checksum6(block)
+csum7 = checksum7(block)
+csum8 = checksum8(block)
+csum9 = checksum9(block)
+
+print('csum1=%06o' % csum1)
+print('csum2=%06o' % csum2)
+print('csum3=%06o' % csum3)
+print('csum4=%06o' % csum4)
+print('csum5=%06o' % csum5)
+print('csum6=%06o' % csum6)
+print('csum7=%06o' % csum7)
+print('csum8=%06o' % csum8)
+print('csum9=%06o' % csum9)
+
+print('checksum should be 105226')