HP21 scientific RPN calculator

HP21 scientific RPN calculator
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Play this online game named HP21 scientific RPN calculator.
The HP-21 was a scientific calculator produced by Hewlett-Packard between 1975 and 1978. It was designed as a replacement for the HP-35, and was the first from so called Second Generation Handheld Calculators.As with most HP calculators then and now, the HP-21 used RPN entry logic, with a four-level stack. It also had a single user-accessible memory register. A shift key provided access to functions whose legends were printed on the fronts of the tall trapezoidal keys.
To enhance practical usability, display was modified to show both X and Y register. Also I have eliminated mechanical DEG/RAD switch and moved the function under DSP button.
HP21 has few big enough buttons to be used comfortably on mobile devices.
Updates:
2.7. Y^X complex result bug fixed (Error is shown for COmplex result)2.6. touch area re-alligned to buttons properly - thank you Eric
2.5. Can enter number by EEX button directly
2.4. To improve usablity - now running in full screen
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