How about two pieces of paper (based on a t shirt I saw once)... paper on your front, with < BODY >, and second piece of paper with < /BODY > on your back. Made me laugh when I saw it :)
Edit: the tags keep being deleted... perhaps it is readable now...
I think one of the motivations for having separate modes like this, with (some) separate registers for each, is to reduce the time taken to switch contexts between modes. If they didn't have separate registers, the data in the user mode registers would have to be saved somewhere when making a switch into kernel mode, and then copied back again when switching back to user mode.
I really like The Mythical Man-Month by Fred Brooks. It's originally from 1975, based on his experience in managing a team that wrote the operating system for a series of IBM computers. So it doesn't talk about modern tooling. But I do like the way it gives the lay of the land, so to speak. Lots of interesting ideas, and quite a lot of wonderful illustrations and diagrams too :)
A few years ago, I used dosdude's "patcher" to install 10.13 on a pretty old MacBook Pro that couldn't run it. It's been working really well. I think "OpenCore" might the more modern version of this. Worth a look?
Apps like Halide can give RAW files, which I think gives access to data much closer to what the sensor actually recorded.
https://en.m.wikipedia.org/wiki/Heisenbug