HP 80: What the heck is going on behind the curtain?

[Dave Britten]

So I hadn't realized until digging up this old thread that the HP 80 doesn't have financial registers - just four stack registers and the independent constant register. Everything is on the stack, and the machine is thus very sensitive about the order that you enter variables. Thus the only behind-the-scenes tricks appear to be the status bits that indicate which variables have been "stored" (i.e. which keys you've pressed so far).

You get exactly the same (correct) results calculating a payment amount in either of these two ways:

360 n 4.75 ENTER 12 / i 141000 PV PMT

360 i 4.75 ENTER 12 / n 141000 PV PMT

The calculator sees that you've pressed PMT without entering any new data, then since you pressed n, i, and PV before that (in no particular order), it branches off to calculate PMT under the assumption that n, i, and PV are in Z, Y, and X respectively.

I'd be really curious to know which keystrokes change the status bits, and how these bits affect the different code paths when computing a result.

Not particularly intuitive, but it's certainly a massive improvement over the old analog alternatives!