IN  ULONG SizeOfRange,
  IN  PVOID HighestAddressToEndAt

Routine Description:

    The function examines the virtual address descriptors to locate
    an unused range of the specified size and returns the starting
    address of the range.  This routine looks from the top down.


    SectionBasedRoot - Supplies the root of the available address tree.

    SizeOfRange - Supplies the size in bytes of the range to locate.

    HighestAddressToEndAt - Supplies the virtual address to begin looking

Return Value:

    Returns the starting address of a suitable range.