Good question...
Microburst2002,
Good question. I don't have the answer. Obviously, you've had a good look at FCOM 1.34.25 P1 which shows the schematic. Hence, the question.
For those reading this without convenient access to the FCOMs, the inputs are: ILS1 or MMR1, ADIRU1, ADIRU3, Standby Pitot, Standby Static, DC ESS, HOT BUS 1. All these things are shot into the ISIS Accel/Gyrometer. (Whatever that is?)
Based on the description and operation, the display can 'precess' and may need to be 'set right'. This poses the valid question: What do the ADIRUs 1 and 3 have to do with it....if it has to be 'reset'?
Again, I don't know.
Further, if you'll have a look at FCOM 3.02.34 P6, which is the abnormal for loss of ADRs..(further, loss of IRs), the list of inopertive components does not include the ISIS. So, obviously, there is not a required input from the ADIRUs 1 and 3 for the thing to function.
(This makes sense....I can't image a standby attitude/airspeed system that is dependent on primary systems for operation.)
Interesting.....
Fly safe,
PantLoad