This calculator works in reverse Polish notation, or RPN. It is a stack-based calculator.
The operands are first pushed onto the stack by the user, and then any operator (whether it be addition, or something more complex like calculating the mean of every item on the stack) pops arguments off the stack in a determined order. To see how it works for yourself, hit the following key sequence: 5, Enter, 3, Enter, +, 6, Enter, 4, Enter, -, /. ((5+3)/(6-4)) should be displayed in the stack window. You can hit any of the buttons in the rightmost column to see the expression in a different representation. To evaluate the expression, hit the Eval button. The Drop button discards the bottom item on the stack, the Swap button swaps the two bottom items on the stack. Clear clears the immediate value, and Reset clears both the immediate value and the stack.