ISIS, stby altitude, altitude indications
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 GPS's 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.
Just a word to clarify that ISIS doesn't receive "attitude from internal gyros", it receives inputs from ADIRU1 and 3. It also receives inputs from Stand-By Pitot and Stand-By Static Probes.
If this aircraft was equipped with ISIS, it was surely also equipped with
Back-Up Speed Scale and
Back-Up Altitude, that displays on PFD's and gets information from
AoA probes and
GPS, respectively.
This is very important, once Back-Up Speed Scale and Altitude are only displayed when ALL ADR's are switched OFF. The old procedure (for the stand-by horizon equipped aircraft), would call for the trouble-shooting of the IR fault, and in case of all confirmed to be faulted, one would be advised to switch off those connected to Captn PFD and FO PFD, i.e., IR1 and 2. This would allow the aircraft to revert to Alternate Law.
This system (although brilliantly designed) seams to work fine in rather smooth air and on flight simulators, but I find it very difficult to see it properly working in a heavy turbulent environment, as seemed to be the case of AF447.