IN  SIZE_T SizeOfRange,
  IN  PVOID HighestAddressToEndAt,
  IN  ULONG_PTR Alignment,

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.  The function examines from the high
    addresses down and ensures that starting address is less than
    the specified address.


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

    HighestAddressToEndAt - Supplies the virtual address that limits
                            the value of the ending address.  The ending
                            address of the located range must be less
                            than this address.

    Alignment - Supplies the alignment for the address.  Must be
                 a power of 2 and greater than the page_size.

    Root - Supplies the root of the tree to search through.

Return Value:

    Returns the starting address of a suitable range.