 Because of the way the processor was designed, it uses Reverse Polish notation (RPN) for inputting calculations.RPN meant that the difficult implementation of brackets, and the associated recursive logic, was not necessary to implement in the hardware, but the effort was instead offloaded to the user.Instead of an "Equals" button, there is an "Enter" button that tells the calculator when a value has been entered, and then the operators are entered in after the operands.