Computer based Instrumentation & Control (I&C) systems are extensively deployed in the 500 MWe Prototype Fast Breeder Reactor (PFBR), which is under construction at Kalpakkam. They are employed in safety critical and safety related systems. The hardware and software for these systems have been designed in accordance with Atomic Energy Regulatory board (AERB) safety guides. Further, the hardware designed and developed for PFBR has been already deployed in Fast Breeder Test Reactor (FBTR) at Kalpakkam as part of refurbishment of aging Computerised Data Processing System (CDPS) based on Unipower-30 computer system. The operational performance feedback of the refurbished CDPS system is excellent.