> I have a function that seemed to be trashing a global variable.

I would start by digging through the assembly code produced by the
compiler.  Knowing the address assigned by the compiler for the global,
see if you can determine where in the function the variable gets changed.
Then see if you managed to fool the compiler somehow (perhaps you ran
past the end of an array?), or if it is an actual bug (bank bits not
set properly, etc.).
Is the list file for this function and the symbol table postable?

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