A generic calculator has its own chip which is designed to perform calculation (simple and floating point). A generic 8051 however, only includes simple 8 bit calculations (hence, the name is microcontroller, not microprocessor), so it is a difficult task for an 8051 to calculate -for example- pi * 2. You may browse for Code Library to see how hard it is to perform floating point calculations (many of them is assembly language off course). I suggest you also learn a little of 8051's assembly language even you write your code in C, it will be useful in finding out how the chip works. Don't forget to read the datasheet of the chip you will be using completely, including hardware informations, so you will not encounter many dead microcontrollers in your room (yay... I like the open-drain feature of 8051... it minimizes dead uC)