Programming a calulator

How can I hack this cheap solar-powered calculator that the bank gave me for Christmas? It has, like, four bytes of memory and can display seven numbers on the screen. It has no interface at all, so I'll probably have to dust off the old soldering iron and put a serial bus in. Ideas on where to begin?

>>3
I'll stay in the room forever. Having a large quantity of food and water is quite a good situation to be in indeed after the collapse.