In 1986, after quitting my PhD in computation molecular biology and starting out on a career as a software developer, I tried to figure out what the goal for the next 5 years was. I settled on satisfying myself that I understood (enough about) every level of the computer from semiconductor physics up to GUI/UX work. Luckily I had the semiconductor stuff already covered from my undergrad days and a friend from that time who was an EE major.

The rest took a bit longer than 5 years (maybe 7), and of course the learning didn't stop there. But I do remember thinking in the early 1990s "OK, I understand the entire stack at this point, what comes next?" My understanding had reached a level I was satisfied with - it would not have compared with someone who spent their entire working life focused on that level, but it was far, far beyond any normal computer user's understanding.

In the 2 decades and more since then, not a whole lot has changed that has invalidated the understanding I think I gained at the beginning. Microcode was a bit of a shock. The biggest shock has been watching a whole new generation of programmers who barely grasp the lower 80-90% of the system. Maybe that's good, but it doesn't feel obviously right.