if (VM_Interface.VerifyAssertions) VM_Interface._assert(MARK_BITS_ONLY);
  private final int unusedBytes(boolean prepare) {    if (VM_Interface.VerifyAssertions) VM_Interface._assert(MARK_BITS_ONLY);    int unused = 0;    for (int sizeClass = 1; sizeClass < SIZE_CLASSES; sizeClass++) {      VM_Address block = (prepare) ? currentBlock.get(sizeClass) : firstBlock.get(sizeClass);      int sets =  bitmapSets[sizeClass];      while (!block.isZero()) {	unused += cellSize[sizeClass] * (cellsInBlock[sizeClass] - markedCells(block, sets));	block = BlockAllocator.getNextBlock(block);      }    }    return unused;  }