[boomlinde]

So where do we draw the line? Of course, no real computer implementation of any language is going to be able to generate fundamentally pure code because in the end they're going to be mutating memory (at the very least the program counter).

That's not a very useful distinction, and usually one talks about "purity" at a conceptual level. The language provides a layer of abstraction in which things can be considered pure (or not), even if the underlying implementation e.g. makes optimizations that require mutation. The language is just a guarantee that you can ignore these aspects of its implementation.

[ricattierger]

I think that is a good point at the end. Pure may be defined in regards to the language. So allocating memory in C may make a function unpure but something other languages may still be pure.