diff --git a/pyasm/asm_tests/test_harness b/pyasm/asm_tests/test_harness
index 3c05d8b..1c370a7 100755
--- a/pyasm/asm_tests/test_harness
+++ b/pyasm/asm_tests/test_harness
@@ -23,17 +23,101 @@ import glob
 import getopt
 import tempfile
 import shutil
+import string
+import struct
 
 
 TestDirectory = './tests'
 TempPrefix = '_#TEMP#_'
 
+BlockloaderSize = 8 * 16        # 8 lines of 16 bytes
+
+HighBitMask = 0100000
+
+
+def oct_dec_int(s):
+    """Convert string 's' to binary integer."""
+
+    if s[0] == '0':
+        result = int(s, base=8)
+    else:
+        result = int(s)
+
+    return result
+def pyword(word):
+    """Convert a 16bit value to a real python integer.
+
+    That is, convert 0xFFFF to -1.
+    """
+
+    byte = (word >> 8) & 0xff
+    byte2 = word & 0xff
+    bstr = chr(byte) + chr(byte2)
+    return struct.unpack(">h", bstr)[0]
+
+def get_byte(handle):
+    """Read one byte from the input file."""
+
+    byte = handle.read(1)
+    val = struct.unpack("B", byte)[0]
+    return val
+
+def get_word(handle):
+    """Read one word from input file."""
+
+    val = (get_byte(handle) << 8) + get_byte(handle)
+    return val
+
+def read_block(handle, memory):
+    """Read data blocks from file, update memory dict.
+
+    Returns False if the end block was read.
+    """
+
+    # read load address, num data words and checksum
+    base_address = get_word(handle)
+    if base_address & HighBitMask:
+        return False
+
+    neg_size = get_word(handle)
+    data_size = -pyword(neg_size)
+
+    checksum = get_word(handle)
+
+    # read data words, placing into dictionary
+    for _ in range(data_size):
+        data_word = get_word(handle)
+        memory[base_address] = data_word
+        base_address += 1
+
+    return True
+
+def get_memory(ptp):
+    """Read a PTP file, return memory contents as a dictionary.
+
+    Do not return the blockloader contents.
+
+    We don't check the checksum.
+    """
+
+    memory = {}
+    with open(ptp, 'rb') as handle:
+        # skip leading zeros
+        while get_byte(handle) == 0:
+            pass
+        # then skip block loader
+        for _ in range(BlockloaderSize - 1):
+            get_byte(handle)
+
+        # now read blocks until finished
+        while read_block(handle, memory):
+            pass
+
+    return memory
 
 def test_file(filename):
     """Test one ASM test file."""
 
-    print('test_file: testing %s' % filename)
-
     # assemble file into known listing file and PTP file
     lst_file = '%s.lst' % TempPrefix
     ptp_file = '%s.ptp' % TempPrefix
@@ -45,29 +129,59 @@ def test_file(filename):
         return
     os.remove(lst_file)
 
-    print('processing %s' % ptp_file)
     # get the test instructions from the ASM file
     with open(filename, 'rb') as handle:
         lines = handle.readlines()
     lines = [line.rstrip() for line in lines]
-    print('lines=%s' % str(lines))
     tests = []
-    seen_code = False
-    for line in lines:
-        if line.startswith(';'):
-            if seen_code is True:
-                tests.append(line[1:].strip())
-        else:
-            seen_code = True
-    print('tests=%s' % str(tests))
+    for (lnum, line) in enumerate(lines):
+        if line.startswith(';!'):
+            test = line[2:].strip()
+            test = test.replace('\t', ' ')
+            if len(test.split()) < 2:
+                warn("%d: %s\nbad test comment in file '%s'"
+                     % (lnum+1, line, filename))
+                return          # finished with this file
+            tests.append((lnum+1, test))
+
     # get the generated code from the PTP file
+    # put memory locations into a dictionary
+    memory = get_memory(ptp_file)
 
     # interpret the test instructions and check generated code
+    errors = False
+    for (lnum, test) in tests:
+        (address, value) = test.split()
+        address = oct_dec_int(address)
+        value = oct_dec_int(value)
+        try:
+            mem_value = memory[address]
+        except KeyError:
+            warn("%s\n"
+                 "%2d: %s\n"
+                 "Test comment has check for address %04o which isn't in block"
+                 % (filename, lnum, test, address))
+        else:
+            if mem_value != value:
+                errors = True
+                warn("%s\n"
+                     "%2d: %s\n"
+                     "Memory at address %04o should be %06o, got %06o"
+                     % (filename, lnum, test, address, value, mem_value))
+    if errors:
+        print("File '%s' had errors" % filename)
+
+def warn(msg):
+    """Print error message and continue."""
+
+    print('---------------------------------\n'
+          '%s\n'
+          '---------------------------------' % msg)
 
 def error(msg):
     """Print error message and stop."""
 
-    print('\n%s\n\n' % msg)
+    warn(msg)
     sys.exit(10)
 
 def run_tests(directory, prefix):
diff --git a/pyasm/asm_tests/tests/aalpha b/pyasm/asm_tests/tests/aalpha
index f1a7656..b8d5bfe 100644
--- a/pyasm/asm_tests/tests/aalpha
+++ b/pyasm/asm_tests/tests/aalpha
@@ -1,7 +1,7 @@
 ; test file
         org     0100
+        law     1
         hlt
         end
-; 0100 000000
-;\t$ xxxxxx
-; .\tyyyyyy
+;! 0100 004001
+;! 0101 000000
diff --git a/pyasm/asm_tests/tests/alpha b/pyasm/asm_tests/tests/alpha
index 57c9c34..1a3289e 100644
--- a/pyasm/asm_tests/tests/alpha
+++ b/pyasm/asm_tests/tests/alpha
@@ -1,3 +1,8 @@
-; test file - smoke test for test_harness
-    end
-; 
+; test file
+        org     0100
+        law     1
+        hlt
+        end
+;! 0100 004002
+;! 0101 000000
+;! 0102 000000