My written test and interview was to be held on 6th June 2014 in "The Longest week" . I had been intimated by the institute almost a month in advance (12th May) so I had ample time to prepare for the interview and test. I was also made aware in the same communication that there would be a 30 minute, 10 questions written test followed by an interview if shortlisted.
CSA department at IISc has primarily 3 specializations :
• Theoretical Computer :: Algorithmic Algebra, Algorithms, Data Structures and Algorithms, etc.
• Computer Systems and Software :: Computer Architecture, Distributed Computing, High Performance Computing, Operating Systems,Power-Aware and Real-Time Software etc.
• Intelligent Systems :: Data Mining, Machine Learning, Pattern recognition etc.

We were asked to choose one among the 3 to specialize in and the interviews / research area / guide and courses that would follow would be based on the specialization you select. Given my background at work in Operating Systems / Computer Architecture and my interest aligning to this I chose Systems specialization. I mostly spent my last few weeks before the interview reading the Design of the UNIX Operating System by Maurice Bach (I simply adore this book), Computer Organization by Hennessy and Patterson and brushing up my Math and previous GATE study subjects.

On the day I showed up at the Instutite at 9:00 AM along with a handful of 25-30 others. We faced a pretty simple straight foward written test (Seemed like a warmup). The questions if I recall were:
1. a simple C code output
2. question based on Bayes Theorem
3. question based on logical inference and set theory
4. longest path problem and it being NP Hard
5. another question in probability
6. question based on a graph theory and graph algorithm

I don't quite remember the rest at this moment of time. Anyways, we waited outside the department heart-in-mouth anxious for the results. I remember them being courteous enough to provide tea and coffee, but I was too anxious to have any. After about 25-30 mins they annouced the names of students for interview and based on specialization we were directed to different panels and almost 75% of people who attempted the test were shortlisted. I was 4th inline to be interviewed by the order they annouced. The 3 who went before me completed their interviews in about 15-20 mins. Entering the room, the panel consisted of 4 profs who were warm and welcoming and I sort of remembered their names from the faces on the site. They were R C Hansdah , Uday Reddy , Murali Krishna and one other prof I didn't recognize. (EDIT: The 4th prof was Y.N Srikanth)

After a brief intro into who I was, my motivation and work-ex etc, I was asked the following questions and provided a whiteboard to write and explain if needed:
-->   How do I maintain top k numbers in a sequence of n numbers? What is the complexity (Min Heap,n log k)
-->   Given a sequence of consecutive 0's followed by 1's write an efficient program to find the location where 0 becomes 1 (I wrote my own algo that looked like a modified version of Binary search. Watch for corner cases where the algo might fail! )
-->   How do you explain mathematically big-Oh of a function and some follow up questions with examples to check if I really understand the concept
-->   Write a program for topological sort and some varitions of it later (like without destroying the adjacency list representation etc.)
-->   They asked me about my 2 favorite Math subjects - I said Linear Algebra and Set Theory.
-->   Given an integer write a C program to determine the number of 1's in its binary representation
-->   Mathematically and geometrically explain principle of Linear Independent Set of vectors, the prof continued asking me various questions on this topic to clean up my not so clear boardwork on Linear Independence.

The professors were friendly and gave hints when I was stuck on occassions. The interview lasted well over 1 hour (close to 1h 20m), and by the time I was out I was overwhelmed by the basic questions I had been asked, though on hindset they were trying to quiz me on fundamentals. I enjoyed being in that room and having that discussion with the professors that day. Most of all I enjoyed the serene lush green campus.
A word of advice to all research IISc aspirants out there, know and revise your fundamentals however they twist it, be confident and calm, do your homework of the profs and specialization you want to align yourself with and study the background subjects needed for them. Revise and be good at ATLEAST a couple of Math core topics.

A weeks later it was the most memorable day when I was shortlisted for admission into the research program. I will be joining as a student at at CSA Dept. at IISc for MSc (Engg) Research course and looking forward to my second stint in my academic career. I am hoping for a longer stretch as I try to aim for a PhD thereafter :-)

Also read the article on Preparing for GATE and beyond. My cliched 2 cents on this topic!

EDIT 1: Here is a detailed doc about CSA research interviews scope and process that I found to be extremely useful while preparing. It's possibly been taken off from the CSA servers but I fortunately managed to get a copy.

EDIT 2: I've got lots of questions via emails asking for selection process for M.E in CSA and SSA, MTech / M.Sc in SERC departments in IISc. So here is a quick summary for you reference. IISc publishes its GATE cut off scores for each department and corresponding GATE paper stream on the admissions webpage. Do refer that.
M.E at CSA Department
Direct admission for GATE CS paper. The cut off for GATE CS paper 1000 to 911.
M.E at SSA Department
Direct admission for GATE CS paper. The cut off for GATE CS paper 911 to 909.
M.Tech at SERC Department
There is a written round followed by a general interview. The cut off for GATE CS paper in 2014 was 700.
M.Sc(Engg) at SERC Department
This process is identical to the CSA M.Sc(Engg) admission process. SERC has 2 streams (a) Computational Science and (b) Computer Systems. There is a written round followed by an interview in one of the stream chosen by you. The cut off for GATE CS paper in 2014 was 700.
The research in Computational Systems includes Bio-molecular Computation, Computational Electrodynamics, Computational Photonics, Medical Imaging, Numerical Math & Scientific Computing, Scientific Computation, Structural Biology
The research in Computer Systems includes Computer Architecture, Database Systems, Distributed Systems & Applications, High Performance Computing, Image & Video Analytics, Information Systems & Security, Middleware & Runtime Systems, and Visualization & Graphics

The good thing about IISc that I have observed over the past 1 year is that students are allowed and encouraged to take courses in other departments. So students in SERC take up courses in CSA and vice-versa to complete their course credit. Many of the faculty are joint faculty of both CSA and SERC which adds to the intertwined nature of the 2 departments.

EDIT 3: Due to the overwhelming hits this article has been getting, a good friend of mine has obliged to walk us through her written tests and interviews for MTech/M.Sc admission in IISc and IIT Madras in 2015 (GATE Rank 323, Score 765).

EDIT 4: Over the year 2016, there were 2 changes in IISc.
(1) MSc(Engg) is now called MTech(Research)
(2) The academic wing of SERC is now called CDS (Computational and Data Sciences), SERC will function as the facility provider for compute to IISc while the academic degrees will be awarded from CDS.

Some more back links to similar posts regarding post GATE experiences: ankitrokdeonsns, Arpith-MSc(Engg) at CSA, 2016 batch.