When later calculators used a one-line display, leaving results in an invisible stack register would have been a nuisance so RPN was altered. (Since calculator displayed the entire stack, the user always knew exactly where each number was and whether it needed to be copied up with ↑.) The ↑ wouldn't be needed on later RPN calculators.
For example, to calculate √(25)+5, the user would press: Keying numbers into the stack did not cause the automatic stack lift that occurred on later calculators. As mentioned above, the stack had only three levels and two operand functions left their result in the Y register. HP's first implementation of RPN was somewhat different than later versions. When HP later used 4 level stacks, the temporary name stuck and the new level became T. (The entire stack was displayed.) Finally, Z was called the temporary register. Y was called the accumulator since the results of arithmetic on numbers in X and Y ended up in this register. X was called the keyboard register since numbers were keyed into it. The HP 9100 had a three-level stack with registers X, Y, and Z. When people first encounter HP's RPN calculators, they often wonder about the stack labels X, Y, Z, and T. Hewlett-Packard had entered the young electronic calculator market in a big way. The HP 9100, built with magnetic core memory, printed circuit board ROM, a CRT display (and not a single digital IC chip) provided industrial strength calculating in a machine that weighed 40 pounds and cost just under $5000. Options such as a printer and a plotter.A logic system that could handle complex expressions ( RPN).Trigonometric (including hyperbolic) functions and inverses.Floating-point math with a range of 10 -98 to 10 99.In 1968 HP introduced the HP 9100A featuring: In the mid to late 1960's electronic four function fixed-point calculators were brand new and typically cost $1000-$2500. The HP 9100A was Hewlett-Packard's first calculator.