_xvalloc()         Allocate a VM segment
 _xvfree()          Free an allocated VM segment
 _xvheapalloc()     Allocate a memory block from a segment heap
 _xvheapdestroy()   Free the segment allocated for a segment heap
 _xvheapfree()      Free an allocated block of segment heap memory
 _xvheaplock()      Lock an allocated block of segment heap memory
 _xvheapnew()       Allocate a VM segment for use as a segment heap
 _xvheapresize()    Resize a segment heap
 _xvheapunlock()    Unlock an allocated block of segment heap memory
 _xvlock()          Lock a VM segment
 _xvlockcount()     Determine the number of locks on a VM segment
 _xvrealloc()       Change the size of a VM segment
 _xvsize()          Determine the size of a VM segment
 _xvunlock()        Unlock a VM segment
 _xvunwire()        Unlock a wired VM segment
 _xvwire()          Obtain a long term lock on a VM segment