diff --git a/src/py65/memory.py b/src/py65/memory.py
index 2f86d8b..1f19423 100644
--- a/src/py65/memory.py
+++ b/src/py65/memory.py
@@ -1,20 +1,11 @@
-# this sparse array module only needed for large-memory CPU models
-try:
-    from blist import *
-except ImportError:
-    pass
-
 class ObservableMemory:
     def __init__(self, subject=None, addrWidth=16):
+        self.physMask = 0xffff
+        if addrWidth > 16:
+            # even with 32-bit address space, model only 256k memory
+            self.physMask = 0x3ffff
         if subject is None:
-            if addrWidth <= 16:
-                subject = 0x10000 * [0x00]
-            else:
-                try:
-                    subject = blist([0]) * (1 << addrWidth)
-                except:
-                    print "Fatal: failed to initialise large memory for this CPU"
-                    exit (1)
+            subject = (self.physMask+1) * [0x00]
 
         self._subject = subject
 
@@ -22,6 +13,7 @@ class ObservableMemory:
         self._write_subscribers = {}        
 
     def __setitem__(self, address, value):
+        address &= self.physMask
         callbacks = self._write_subscribers.get(address, [])
 
         for callback in callbacks:
@@ -32,6 +24,7 @@ class ObservableMemory:
         self._subject[address] = value
         
     def __getitem__(self, address):
+        address &= self.physMask
         callbacks = self._read_subscribers.get(address, [])
         final_result = None
 
@@ -50,15 +43,18 @@ class ObservableMemory:
 
     def subscribe_to_write(self, address_range, callback):
         for address in address_range:
+            address &= self.physMask
             callbacks = self._write_subscribers.setdefault(address, [])
             if callback not in callbacks:
                 callbacks.append(callback)
 
     def subscribe_to_read(self, address_range, callback): 
         for address in address_range:
+            address &= self.physMask
             callbacks = self._read_subscribers.setdefault(address, [])
             if callback not in callbacks:
                 callbacks.append(callback)
 
     def write(self, start_address, bytes):
+        start_address &= self.physMask
         self._subject[start_address:start_address+len(bytes)] = bytes