Topsy VM Subsytem:

Topsy VM is a virtual memory implementation with these goals:
   * protection domains for kernel and user
   * hierarchical structure of protection domains
   * sharing of regions (map entry passing) and loose pages (page loan out)
   * transfering of regions and loose pages (page transfer)
   * non complex region splitting and partial freeing
   * possibility of copy-on-write and snapshot
   
Topsy VM is not designed for demand paging and swapping!

Architecture:
 the vm space of a protection domain is hierarchicaly built out of following 
 structures:
     - vm_domain:    root of the vm tree. This is just a part of the 
										 protection domain descriptor. All memory information of a
                     protection domain is stored under this node.
     - vm_map:       the high-level address map data structure. A vm map 
		 								 contains multiple regions that point to a object or a 
										 sub-vm_map if the memory is shared with an other 
										 protection domain.
     - vm_pmap:      the physical address map data structure. This structure 
		 								 is used to access the physical layer of the memory 
										 management. 
     - vm_region:    an entry in a address map. Here are the start and end 
		 								 address of a virtual memory block stored with protection 
										 info and a reference to a vm object.
     - vm_oref:      reference to a vm_object and its handler. It is poosible 
		 								 to have different offsets to the vm_object from different 
										 vm_region.
     - vm_object:    abstract object that describes the memory mapped by a 
		 								 region. All pages under control by this object have to be 
										 stored somehow here.
		 - vm_mananager: handler for a specified object type. For each object type a
                     different vm_manager has to be defined and used.
     - vm_pref:      reference to a vm_page. vm_pref are object dependent, 
		 								 different object may use different pref structures with 
										 specialiced linking structures.
     - vm_page:      represents a page of physical memory. For each physical 
		 								 page one vm_page exists. This structures are allocated on 
										 startup and never freed.

See the coresponding include file for the usage of those structures.

