Based on the written test I attended last year, I would say
Prepare
1. GATE syllabus
2. OOPS concepts of C++, JAVA- like polymorphism, multiple inheritance and other basic stuff,,
3. You can expect some questions from SE - as last year it was there..
No aptitude and NATs are there, fully technical paper only MCQ's.
Marking : +3/-1
Total : 100 questions
Time : 120 mInutes
So, keep an eye on the watch, as the time flies by fast.
The level of questions would be same as 1 Mark Questions of GATE.
Edit: There are multiple sessions in BARC.. And cutoff varies from session to session.. depending on the performance of people in a particular session..
To be safe get 65+ correct, without much negatives.. You should definitely be through to the interviews..
Anyways try to do as much as you can :)