history of the 4-level stack

[jebem]

Hi Alex, welcome to our MoHPC forums.
Nice thread and question, but I guess the answer will vary depending on your view point.

My 2 cents:
Like any other technology, calculators evolved depending not only on science reasoning but also on other factors constraining the final design (budget, time to delivery, marketing, available technology,...).

So 4 levels seems to be a good number of registers to allow most of math real number operations while allowing humans to remember its contents.
Handling more than that means you are above average memorizing stack contents, or you are using a modern multi line display calculator.
Most of the machines in that era were also programmed, or should I say macro coded, in machine key coding using very basic low level control structures.
This was the beginning of portable computing after all.

It is not a coincidence that when multi line displays arrived to market, new programming paradigms also emerged, like RPL for instance, capable of structured thinking in order to get rid of the machine code spaghetti progamming used before.