VM_Word.fromInt(bitmaps[sizeClass]).lsh(LOG_BYTES_IN_BITMAP).toExtent());
VM_Word.fromIntZeroExtend(bitmaps[sizeClass]).lsh(LOG_BYTES_IN_BITMAP).toExtent());
  protected final void postExpandSizeClass(VM_Address block, int sizeClass) {    Memory.zeroSmall(block.add(MARK_BITMAP_BASE),                      VM_Word.fromInt(bitmaps[sizeClass]).lsh(LOG_BYTES_IN_BITMAP).toExtent());  };