Thank you guys
Spannersatx
I did read that paragraph in the AMM, too, but I found it ambiguous. It says "the parameters", as if it meant several of them, like ALT, SPD, ATT...
However I think it is as you suggest: ADIRUs only provide HDG to the ISIS that include that option and all the other parameters are calculated by the ISIS by its own means.
Right now I don't recall if the ISIS is working when the ADIRUs are still OFF when entering the cockpit first flight in the morning. I think it is. They shoud be, I guess. Otherwise they wouldn't meet their STBY function. Or would they? With bat only, one ADIRU will be supplied from the HOT BUS 2, so why not?
Anyway. As usual, almost no info in the FCOMs, and even the AMM lacks it.