In case you're looking for a sensible answer: we input the info (i.e. runway, weight, temp, QNH, etc) via ACARS, the ACARS RTOW system does the calculations and returns the data to the aircraft via ACARS. The RTOW data is printed out then filed with the rest of the flight paperwork after use. There are a couple of backup systems in place in case the ACARS RTOW doesn't work, but the upshot is that a copy of the data is always retained.