ghsiloP (pronounced gee-hu-sai-lop) is a Reverse Polish Notation calculator. Polish Notation gets its name from Jan Lucasiewicz (1878-1956), a Polish mathematician. In Polish Notation

(1 + 2) * (3 - 4)

would be written as

* + 1 2 - 3 4

which eliminates the need for parenthesis and brackets. Reverse Polish Notation is the reverse of that. In RPN the operands come before the operators, so the pieces are reversed, but not necessarily the entire string. That expression would be

1 2 + 3 4 - *

in Reverse Polish Notation. RPN is popular in computer science and with engineering calculators (like my beloved HP48GX).

The difference of an RPN calculator is that it takes the top two values on the stack (or top one for unary operators) and performs the operation on them. So to perform the above calculation you would type:

1
(displays "1")
2
(displays "1", "2")
+
(displays "3")
3
(displays "3", "3")
4
(displays "3", "3", "4")
-
(displays "3", "-1")
*
(displays "-3")

NOTE: In ghsiloP the top of the stack is the lowest entry in the list, and the entry in the entry box becomes the top when an operation is pressed (unless it is a zero value).