Evolution of Conditional Tests

[Thomas Okken]

My guess would be that they provided eight tests on most of the early calculators because that number could be placed on the keyboard reasonably elegantly, and the choice of which tests were provided probably went something like this: for the comparisons between X and Y, provide one strict and one non-strict inequality, so the missing two can be easily synthesized by swapping X and Y; for the comparisons between X and 0, provide only strict inequalities, because those are used more often than the non-strict ones. N.B. I don't have any inside information on this, I'm just guessing... the differences between the comparison operators are something that I have wondered about many times, and of course they are something one tends to run into when porting programs from one calculator to another!

The HP-25 is the odd one out with its choice of inequalities, because they were running out of ROM space. Making the X vs. Y comparisons exactly match the X vs. 0 comparisons saved a few bytes of code in their implementation.