One common interview question I've run across is to write the code for a simple four-function calculator, the kind that you can find for a dollar or two in nearly any convenience store, right next to the pencils and erasers.

Coincidentally, I've previously posted here a detailed model/explanation of how those work: https://news.ycombinator.com/item?id=9456444

They're usually based on a 4-bit mask-ROM microcontroller running at several kHz. Very "simple" in relative terms to most computers today, and in hindsight I could probably write the code for one quite quickly, but I bet the 99% of programmers who haven't ever really given a thought to how they work in detail would struggle to come up with an accurate description in an interview.

Also worth reading: https://news.ycombinator.com/item?id=6302364

There's a wonderful online emulation and analysis of that calculator by Ken Shirriff:

http://files.righto.com/calculator/sinclair_scientific_simul...

(which was discussed previously https://news.ycombinator.com/item?id=6302364)

The ability of the designers to do more with less is reminiscent of Steve Wozniak's work on the Apple I/II and other projects