As written (by me among others) in previous posts, the ISIS is a self contained instrument providing Airspeed from stby pitot, attitude from internal gyros, Altitude from STBY static ports, and slip/skid info from accelerometers,
ISIS IS the stby instuments ! 
direct inputs from the pitot and from the static ports are provided. No computer inbetween !!
Also, the FMGC provides a "backup altitude" which is provided from the GPSs on the DATA / GPS page. It's not a pressure altitude, but nevertheless gives a good idea of whether you are at 35000' or 2000' !!
The QRH checklist for "Unreliable airspeed" suggests to use this it by the way.
please read the important info provided on the thread so IT is not clogged with redundant, already asked and answered questions !
