int start =  addr.toInt() >>> LOG_BYTES_IN_PAGE;
int start =  Conversions.addressToPagesDown(addr);
  public static void clearTag (VM_Address addr, int pages, byte v) {    int start =  addr.toInt() >>> LOG_BYTES_IN_PAGE;    for (int i=0; i<pages; i++) {        if (tagTable[start+i] != v)            VM_Interface.sysFail("VMResource.clearTag: current tag does not match expected value");        tagTable[start+i] = (byte) 0;    }  }