m of the doubly linked list layout in DAS integer address
   space is:
<P>
 
<PRE>
 
                 +------------------------------+
                 |  Pointer to first descriptor | ----+
                 +------------------------------+     |
   +-------------|  Pointer to last descriptor  |     |
   |             +------------------------------+     |
   |                                                  |
   |             +------------------------------+     |
   |  NULL &lt;---- |  Backward pointer  