Most of the PSU now take the admission through GATE. For the rest, it depends. Like Coal India conducted exam a few years back and it gave most of the questions from the previous GATE question papers and most of them from compiler design. ISRO provides its own past question papers. As I gave exams of various PSUs, I can say that it is difficult to retrieve those questions as they don't let the candidate to take the question paper. So spreparing previous GATE questions, MCQ by Timothy Williams and questions from ISRO question paper will serve as good mix of questions. If I have any question paper with me, I will share the questions or at least the scan copy.