I believe the x87 uses 80 bits internally, but it only runs operations until it has sufficient precision to do the rounding (which is hard to do in general, see

I believe the x87 uses 80 bits internally, but it only runs operations until it has sufficient precision to do the rounding (which is hard to do in general, see: http://perso.ens-lyon.fr/jean-michel.muller/Intro-to-TMD.htm http://perso.ens-lyon.fr/jean-michel.muller/Intro-to-TMD.htm)