>> Isn't Northbridge = memory controller?

> Sometimes yes. Sometimes it is:
> Northbridge = memory controller + PCIe (or AGP) graphics bridge.
> I think, that Powermac G5 PCIe is the second case.

I'm well aware of the overall PowerMac11,2 system architecture. Seems my use of the '=' operator was kind of sloppy and thus has you confused with regard to my question. "Northbridge ⊃ memory controller" would have been the correct expression.
So my question still stands: Doesn't the northbridge chip (if present) always contain the memory controller, so the first implies the second? Or why is there the need to dinstinguish between those with regard to the temperature sensor?
(Btw, I don't think there were/are any northbridge chips that contain nothing but the memory controller.)