VM_Extent.fromInt(bitmaps[sizeClass]<<LOG_BYTES_IN_BITMAP));
VM_Word.fromInt(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_Extent.fromInt(bitmaps[sizeClass]<<LOG_BYTES_IN_BITMAP));  };