HP 42s is a simple emulation of the legendary HP 42s Calculator from Hewlett Packard on the Desktop. The input of  UPN-Logic calculations are faster than the conventional way with the equals sign ( see  Wikipedia).

It is possible to operate the calculator with the mouse and the numerical keypad. It can be moved free on the desktop.

The complex operations and the matrix operations are not yet implemented. 
Also it is not possible to program the calculator.....................

The free version has no restrictions, so you can test the version before buying. The "About" Message will not been shown anymore with a registrated version.