HP-65
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The HP-65 was the first programmable handheld calculator in the world. Introduced by Hewlett-Packard in 1974, it was considered an electronic marvel. It packed 9 storage registers and had room for 100 keystroke instructions. It also included a magnetic card reader/writer whose cards were approximately the size of a chewing gum stick. The programs used RPN to conserve memory. Thus there was no equals key, but there was an enter key to push an operator or operand onto the stack.
The HP-65 offered very rudimentary program editing capabilities, however, and its storage register R9 was corrupted whenever the user (or program) executed trigonometric functions or performed comparison tests. This problem was documented in the manual, so it's not considered a bug.
Bill Hewlett's design requirement was that the calculator should fit in his shirt pocket. That is one reason for the tapered depth of the calculator. The magnetic program cards fed in at the thick end of the calculator under the LED display. The documentation for the programs in the calculator is very complete, including algorithms for hundreds of applications, including the solutions of differential equations, stock price estimation, statistics, and so forth.