You have not mentioned your category in your question. I'll have you know there is a 65% minimum eligibility for general category in top institutions. However regardless of your you should be eligible to write the exam, as the eligibility differs depending on the institutions, their respective departments and the specific programs you are applying for. You'll have to check it on each department's website. Also note for direct PHD there are less restrictions in some departments of IISC.
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