Maximum Value from Binary strings

You are given two binary strings $A$ and $B$, each of length $N$. Consider a non-empty substring $A'$ of $A$ and a non-empty substring $B'$ of $B$, both of the same length. Let $X = A' \oplus B'$ be the string obtained by taking the exclusive OR of $A'$ and $B'$. You are also given a function $f(X)$, defined as $$f(X) = \left \lfloor \dfrac{|X|}{2^{X_{10}}} \right \rfloor$$ where $|X|$ denotes