diff --git a/py65/monitor.py b/py65/monitor.py
index 5d2679c..16d389c 100644
--- a/py65/monitor.py
+++ b/py65/monitor.py
@@ -43,10 +43,11 @@ class Monitor(cmd.Cmd):
                        '65Org16': V65Org16}
 
     def __init__(self, mpu_type=NMOS6502, completekey='tab', stdin=None,
-                 stdout=None, argv=None):
+                 stdout=None, argv=None, memory=None, putc_addr=0xF001, getc_addr=0xF004):
         self.mpu_type = mpu_type
-        self.putc_addr = 0xF001
-        self.getc_addr = 0xF004
+        self.memory = memory
+        self.putc_addr = putc_addr
+        self.getc_addr = getc_addr
         if argv is None:
             argv = sys.argv
         self._breakpoints = []
@@ -149,14 +150,15 @@ class Monitor(cmd.Cmd):
         return result
 
     def _reset(self, mpu_type,getc_addr=0xF004,putc_addr=0xF001):
-        self._mpu = mpu_type()
+        self._mpu = mpu_type(memory=self.memory)
         self.addrWidth = self._mpu.ADDR_WIDTH
         self.byteWidth = self._mpu.BYTE_WIDTH
         self.addrFmt = self._mpu.ADDR_FORMAT
         self.byteFmt = self._mpu.BYTE_FORMAT
         self.addrMask = self._mpu.addrMask
         self.byteMask = self._mpu.byteMask
-        self._install_mpu_observers(getc_addr,putc_addr)
+        if getc_addr and putc_addr:
+            self._install_mpu_observers(getc_addr,putc_addr)
         self._address_parser = AddressParser()
         self._disassembler = Disassembler(self._mpu, self._address_parser)
         self._assembler = Assembler(self._mpu, self._address_parser)
@@ -245,7 +247,7 @@ class Monitor(cmd.Cmd):
                 byte = 0
             return byte
 
-        m = ObservableMemory(addrWidth=self.addrWidth)
+        m = ObservableMemory(subject=self.memory, addrWidth=self.addrWidth)
         m.subscribe_to_write([self.putc_addr], putc)
         m.subscribe_to_read([self.getc_addr], getc)
 
diff --git a/py65/tests/test_monitor.py b/py65/tests/test_monitor.py
index 928c9d1..3477668 100644
--- a/py65/tests/test_monitor.py
+++ b/py65/tests/test_monitor.py
@@ -1144,6 +1144,18 @@ class MonitorTests(unittest.TestCase):
         out = stdout.getvalue()
         self.assertTrue(out.startswith("width <columns>"))
 
+    def test_external_memory(self):
+        stdout = StringIO()
+        memory = bytearray(65536)
+        memory[10] = 0xff
+        mon = Monitor(memory=memory, stdout=stdout, putc_addr=None, getc_addr=None)
+        self.assertEqual(0xff, memory[10], "memory must remain untouched")
+        mon.do_mem('0008:000c')
+        mon.do_fill('0000:0020 ab')
+        self.assertEqual(0xab, memory[10], "memory must have been modified")
+        out = stdout.getvalue()
+        self.assertTrue(out.startswith('0008:  00  00  ff  00  00'), "monitor must see pre-initialized memory")
+
 def test_suite():
     return unittest.findTestCases(sys.modules[__name__])