A critical region is:

On receiving an interrupt from an $${\rm I}/O$$ device the $$CPU$$:

Give minimal $$DFA$$ that performs as a Mod-$$3$$ $$1's$$ counter, i.e., outputs a $$1$$ each time the number of $$1's$$ in the input sequence is a sequence is a multiple of $$3.$$

Give the regular expression over $${\left\{ {0,\,\,1} \right\}}$$ to denote the set of proper non-null substrings of the string $$0110$$.