In paper-III(Elective), we need to cover only one unit from the five units. It is good idea to choose Finite Automata and Formal Languages or Unix and Windows. Both units will help you to cover some other units.

Unix and Windows has three related units Operating Systems, Operating Systems with Case Study of Unix and Current Trends and Technologies.The topic YACC from this unit comes in System Software and Compilers and Programming Concepts.

Some topics from the unit Finite Automata and Formal Languages comes in Discrete Structures and Programming Concepts. Some topics in the System Software and Compilers need the knowledge of Finite Automata and Formal Languages.

The unit Finite Automata and Formal Languages requires problem solving ability. So if you choose this unit as elective, you should buy a textbook for practicing exercies. We use Peter Linz's Finite Automata and Formal Languages as reference book.