int start =  addr.toInt() >>> LOG_BYTES_IN_PAGE;
int start =  Conversions.addressToPagesDown(addr);
  public static void setTag (VM_Address addr, int pages, byte v) {    int start =  addr.toInt() >>> LOG_BYTES_IN_PAGE;    for (int i=0; i<pages; i++)        tagTable[start+i] = v;  }