CMBitMapRO::CMBitMapRO(ReservedSpace rs, int shifter):

  {- -------------------------------------------
  (1) 
      ---------------------------------------- -}

      _bm((uintptr_t*)NULL,0),
      _shifter(shifter) {
      _bmStartWord = (HeapWord*)(rs.base());
      _bmWordSize  = rs.size()/HeapWordSize;    // rs.size() is in bytes
      ReservedSpace brs(ReservedSpace::allocation_align_size_up(
                         (_bmWordSize >> (_shifter + LogBitsPerByte)) + 1));

      guarantee(brs.is_reserved(), "couldn't allocate CMS bit map");
      // For now we'll just commit all of the bit map up fromt.
      // Later on we'll try to be more parsimonious with swap.
      guarantee(_virtual_space.initialize(brs, brs.size()),
                "couldn't reseve backing store for CMS bit map");
      assert(_virtual_space.committed_size() == brs.size(),
             "didn't reserve backing store for all of CMS bit map?");
      _bm.set_map((uintptr_t*)_virtual_space.low());
      assert(_virtual_space.committed_size() << (_shifter + LogBitsPerByte) >=
             _bmWordSize, "inconsistency in bit map sizing");
      _bm.set_size(_bmWordSize >> _shifter);
    }