Emulation programming is a good way to learn this stuff today. Just pick a platform, emulate it, and you're guaranteed to know the whole stack.

I'm having a lot of fun with this MSP430 microprocessor that cost $4.30, delivered to Australia(!). Great way to teach/refresh some skills in simple assembly or C.

Here's the original HN article where it was mentioned: https://hackertimes.com/item?id=2202737