>How is re-entrancy handled when all variables have a fixed address?

The linker also knows which libaray routines are called within the ISR
which are also used by the application.

It adds a module of code to save the variables from these dual-use routines
somewhere in RAM, then proceeds to use the same addresses for the other.
This provides "true" re-entrancy.

Or, it could have a second copy of the library routine which uses different
RAM addresses.  This provides "functional" re-entrancy.

In an ideal world, the programmer should be able to select which method to
use each shared function.

Personally, I like to keep the ISR to a minimum and set flags telling the
application level what to do with the data collected during the ISR.

Andy