diff --git a/pymlac/DisplayCPU.py b/pymlac/DisplayCPU.py
index 7182de6..c65ccbd 100644
--- a/pymlac/DisplayCPU.py
+++ b/pymlac/DisplayCPU.py
@@ -60,7 +60,6 @@ class DisplayCPU(object):
         return result
 
     def doDEIMByte(self, byte):
-        print('doDEIMByte')
         if byte & 0x80:			# increment?
             prevDX = self.DX
             prevDY = self.DY
diff --git a/pymlac/MainCPU.py b/pymlac/MainCPU.py
index 88dd279..873bb07 100644
--- a/pymlac/MainCPU.py
+++ b/pymlac/MainCPU.py
@@ -8,13 +8,6 @@ The Imlac main CPU.
 import sys
 
 from Globals import *
-#import DisplayCPU
-#import Memory
-#import Ptr
-#import Ptp
-#import TtyIn
-#import TtyOut
-#import Kbd
 import Trace
 
 
@@ -228,7 +221,6 @@ class MainCPU(object):
     def i_DAC(self, indirect, address, instruction):
         address = self.memory.eff_address(address, indirect)
         self.memory.put(self.AC, address, False)
-        print('DAC: stored %07o at address %07o' % (self.AC, address))
         Trace.itrace('DAC', indirect, address)
         return 3 if indirect else 2
 
diff --git a/pymlac/Memory.py b/pymlac/Memory.py
index 5e9a0df..66f6b28 100644
--- a/pymlac/Memory.py
+++ b/pymlac/Memory.py
@@ -185,7 +185,6 @@ class Memory(object):
             self.using_rom = True
             i = self.ROM_START
             for ptr_value in self.PTR_ROM_IMAGE:
-                print('Storing %07o at address %07o' % (ptr_value, i))
                 self.memory[i] = ptr_value
                 i += 1
         elif romtype == 'tty':
@@ -265,8 +264,6 @@ class Memory(object):
             Trace.comment('Attempt to write to ROM')
             return
 
-        print('Storing value %07o at address %07o' % (MASK_16(value), address))
-
         try:
             self.memory[address] = MASK_16(value)
         except IndexError:
diff --git a/pymlac/Ptr.py b/pymlac/Ptr.py
index 12461ca..4c7e282 100644
--- a/pymlac/Ptr.py
+++ b/pymlac/Ptr.py
@@ -83,7 +83,6 @@ class Ptr(object):
     def read(self):
         """Read papertape value."""
 
-        print('Ptr: value=%04o' % self.value)
         return self.value
 
     def eof(self):
diff --git a/pymlac/pymlac b/pymlac/pymlac
index 0c0f7cc..8041fbd 100755
--- a/pymlac/pymlac
+++ b/pymlac/pymlac
@@ -403,7 +403,6 @@ class PymlacFrame(wx.Frame):
 
         # now perform operations
         for (operation, args) in ops:
-            print('%s: operation=%s, args=%s' % ('*'*60, operation, str(args)))
             if operation == 'boot':
                 self.memory.set_ROM(args)
                 Trace.comment('Bootstrap ROM set to %s' % args.upper())
@@ -517,27 +516,34 @@ class PymlacFrame(wx.Frame):
             Trace.tracing = False
 
     def execute_once(self):
+        # decide if trace should be on/off
         if self.traceend is None:
             if self.cpu.PC == self.tracestart:
                 Trace.settrace(True)
         else:
             Trace.settrace(self.cpu.PC >= self.tracestart and
-                           self.cpu.PC <= traceend)
+                           self.cpu.PC <= self.traceend)
 
+        # execute and trace the Display CPU
         if self.display_cpu.ison():
             Trace.trace('%6.6o' % self.display_cpu.DPC)
         Trace.trace('\t')
 
         instruction_cycles = self.display_cpu.execute_one_instruction()
 
-        Trace.trace('%6.6o\t' % self.cpu.PC)
-
-        instruction_cycles += self.cpu.execute_one_instruction()
-
-        if instruction_cycles > 0:
-            Trace.itraceend(self.display_cpu.ison())
+        # execute and trace the Main CPU
+        if self.cpu.running:
+            Trace.trace('%6.6o\t' % self.cpu.PC)
         else:
+            Trace.trace('\t')
 
+        cpu_cycles = self.cpu.execute_one_instruction()
+        instruction_cycles += cpu_cycles
+
+        if cpu_cycles < 1:
+            Trace.trace('\t\t')
+
+        Trace.itraceend(self.display_cpu.ison())
 
         self.__tick_all(instruction_cycles)