starting to clear up slots

mostly docs

lib/virtual/slots/message_slot.rb

@@ -1,5 +1,20 @@
 module Virtual
-  # The message that is being processed has a layout as per the constant above
+
+  MESSAGE_CALLER = 0
+  MESSAGE_RETURN_ADDRESS = 1
+  MESSAGE_EXCEPTION_ADDRESS = 2
+  MESSAGE_SELF = 3
+  MESSAGE_NAME = 4
+  MESSAGE_RETURN_VALUE = 5
+  MESSAGE_FRAME = 6
+  MESSAGE_PAYLOAD = 7
+
+  # The current Message is one of four objects the virtual machine knows
+  #
+  # Slots represent instance variables of objects, so MessageSlots
+  # represent instance variables of Message objects.
+  # The Message has a layout as per the constant above
+
   class MessageSlot < Slot
     def initialize index , type = Unknown , value = nil
       super(index ,type , value )