To get computer science branch in old IITs that are namely IIT- KGP,IIT-ROORKEE,IIT-B,IIT-D,IIT-MADRAS,IIT-BHU,IIT-GUWAHATI,IIT-KANPUR you need to get a rank of somewhat under 1000 in JEE ADVANCED if you belong to the general category and if you belong to reserved categories like OBC,SC,ST you can get CSE even beyond this rank .

For OBC category you need a rank of somewhat under 1800 .

For SC category - under 5000.

For ST category - under 6000 .

Thanks for reading .