Yes. To appear for the entrances to IISER, one must have a knowledge of PCMB. There are 15 questions each of all the four subjects. You can find the detailed syllabus in this link: https://engineering.careers360.com/articles/iiser-syllabus-exam-pattern

You can further checkout the previous years' exam for the question paper pattern.

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It is not compulsory to ‘attempt’ biology but they will consider its marks in the total i.e. your marks will be out of 180. feel free to ask any doubts in the comments section and stay tuned for furhter updates. the additional subject will help you to score more.