diff --git a/pymlac/Makefile b/pymlac/Makefile
index b7cdf67..6004cbb 100644
--- a/pymlac/Makefile
+++ b/pymlac/Makefile
@@ -18,4 +18,4 @@ lst:	$(TESTS)
 	../iasm/iasm -l $*.lst $<
 
 clean:
-	rm -Rf *.pyc *~ *.out test_*.ptp *.lst test_*.trace _#ASM#_.*
+	rm -Rf *.pyc *~ *.out test_*.ptp *.lst test_*.trace _#*#_.* CPU.test.trace.*
diff --git a/pymlac/Memory.py b/pymlac/Memory.py
index de19061..a2c53d0 100644
--- a/pymlac/Memory.py
+++ b/pymlac/Memory.py
@@ -90,15 +90,17 @@ class Memory(object):
     memory = []
     using_rom = True
     boot_rom = None
+    rom_protected = False
 
 
-    def __init__(self, boot_rom=ROM_PTR, core=None):
+    def __init__(self, boot_rom=None, core=None):
         """   """
 
         self.corefile = core
         self.boot_rom = boot_rom
-        self.memory = []
+        self.memory = [0]*MEMORY_SIZE
         self.using_rom = boot_rom in [ROM_PTR, ROM_TTY]
+        self.rom_protected = self.using_rom
 
         if self.corefile:
             try:
@@ -116,9 +118,9 @@ class Memory(object):
         If using ROM, that is unchanged.
         """
 
-        for i in range(MEMORY_SIZE):
-            self.memory.append(0)
-        self.set_ROM(self.boot_rom)
+        for address in range(MEMORY_SIZE):
+            if not self.rom_protected or not (self.ROM_START <= address <= self.ROM_END):
+                self.memory[address] = 0
 
     def loadcore(self, filename=None):
         """Load core from a file.  Read 16 bit values as big-endian."""
@@ -136,10 +138,6 @@ class Memory(object):
                     low = fd.read(1)
                     val = (ord(high) << 8) + ord(low)
 
-#                    high = struct.unpack('B', high)[0]
-#                    low = struct.unpack('B', fd.read(1))[0]
-#                    val = (high << 8) + low
-
                     self.memory.append(val)
         except struct.error:
             raise RuntimeError('Core file %s is corrupt!' % filename)
@@ -155,21 +153,19 @@ class Memory(object):
                 high = val >> 8
                 low = val & 0xff
 
-#                data = struct.pack('B', high)
-#                fd.write(data)
-#                data = struct.pack('B', low)
-#                fd.write(data)
-
                 fd.write(chr(high))
                 fd.write(chr(low))
 
     def set_PTR_ROM(self):
         """Set addresses 040 to 077 as PTR ROM."""
 
+        save_flag = self.rom_protected
+        self.rom_protected = False
         i = self.ROM_START
         for ptr_value in self.PTR_ROM_IMAGE:
             self.memory[i] = ptr_value
             i += 1
+        self.rom_protected = save_flag
 
     def set_TTY_ROM(self):
         """Set addresses 040 to 077 as TTY ROM."""
@@ -182,24 +178,16 @@ class Memory(object):
     def set_ROM(self, romtype=None):
         """Set ROM to either PTR or TTY, or disable ROM."""
 
+        save_flag = self.rom_protected
+        self.rom_protected = False
         if romtype == 'ptr':
-            self.using_rom = True
-            i = self.ROM_START
-            for ptr_value in self.PTR_ROM_IMAGE:
-                self.memory[i] = ptr_value
-                i += 1
+            self.set_PTR_ROM()
+            self.rom_protected = True
         elif romtype == 'tty':
-            self.using_rom = True
-            i = self.ROM_START
-            for ptr_value in self.TTY_ROM_IMAGE:
-                self.memory[i] = ptr_value
-                i += 1
+            self.set_TTY_ROM()
+            self.rom_protected = True
         else:
-            self.using_rom = False
-            i = self.ROM_START
-            for _ in self.PTR_ROM_IMAGE:
-                self.memory[i] = 0
-                i += 1
+            self.rom_protected = save_flag
 
     def fetch(self, address, indirect):
         """Get a value from a memory address.
@@ -261,7 +249,8 @@ class Memory(object):
         if indirect:
             address = self.memory[address] & ADDRMASK
 
-        if self.using_rom and self.ROM_START <= address <= self.ROM_END:
+        if self.rom_protected and self.ROM_START <= address <= self.ROM_END:
+            print('Attempt to write to ROM address %07o' % address)
             Trace.comment('Attempt to write to ROM address %07o' % address)
             return
 
diff --git a/pymlac/Trace.py b/pymlac/Trace.py
index dbdbd7a..d2351c2 100644
--- a/pymlac/Trace.py
+++ b/pymlac/Trace.py
@@ -21,7 +21,7 @@ def init(filename, maincpu, displaycpu):
     global tracing, tracefile, cpu, dcpu
 
     tracing = True
-    tracefile = open(filename, 'w')
+    tracefile = open(filename, 'wa')
     trace('%s trace\n%s\n' % (PYMLAC_VERSION, '-'*60))
     tracing = False
     comment = None
@@ -30,7 +30,7 @@ def init(filename, maincpu, displaycpu):
     dcpu = displaycpu
 
 def close():
-    import tracing, tracefile
+    global tracing, tracefile
 
     tracefile.close()
     tracing = False
diff --git a/pymlac/test_CPU.py b/pymlac/test_CPU.py
index c435268..f31d042 100644
--- a/pymlac/test_CPU.py
+++ b/pymlac/test_CPU.py
@@ -147,7 +147,7 @@ class TestCPU(object):
         self.memory.put(value, addr, False)
         log.debug('setmem: After, Memory at %07o is %07o' % (addr, self.memory.fetch(addr, False)))
 
-    def allmem(self, value):
+    def allmem(self, value, ignore=None):
         """Set all of memory to a value.
 
         Remember value to check later.
@@ -183,6 +183,8 @@ class TestCPU(object):
                                   % (mem, value, self.mem_values[mem]))
             else:
                 if value != self.mem_all_value:
+                    print('mem: %s, value: %s, self.mem_all_value: %s'
+                          % (str(type(mem)), str(type(value)), str(type(self.mem_all_value))))
                     result.append('Memory at %07o changed, is %07o, should be %07o'
                                   % (mem, value, self.mem_all_value))
 
@@ -317,8 +319,10 @@ class TestCPU(object):
         # set globals
         self.reg_values = {}
         self.mem_values = {}
-        self.reg_all_value = {}
-        self.mem_all_value = {}
+        #self.reg_all_value = {}
+        #self.mem_all_value = {}
+        self.reg_all_value = 0
+        self.mem_all_value = 0
 
         result = []
 
@@ -327,10 +331,10 @@ class TestCPU(object):
         self.cpu.running = True
         self.display_state = False
 
-        Trace.init(filename+'.trace', self.cpu, None)
+        trace_filename = filename + '.trace'
+        Trace.init(trace_filename, self.cpu, None)
 
-        # clear memory and registers to 0 first
-        self.allmem(0)
+        # clear registers to 0 first
         self.allreg(0)
 
         # interpret the test instructions