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About the application:

The HP-21 was a scientific calculator produced by Hewlett-Packard between 1975 and 1978.[1] It was designed as a replacement for the HP-35, and was one of a set of three calculators, the others being the HP-22 and HP-25, which were similarly built but aimed at various markets.[2] 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. As was normal at the time, memory was not preserved on power-down. A physical slider switch toggled between degrees and radians modes, which was an unusual feature. It had a 12-digit LED display, which was less than the 15 digits of the HP-35. Because of these fewer digits the HP-21 (and related calculators such as the HP-25) should display 10-digit floating point numbers but only an 8-digit mantissa with a 2-digit exponent when scientific notation was used. A shift key provided access to functions whose legends were printed on the fronts of the tall trapezoidal keys.