@Lundin [To the earlier conversation about memory-mapped peripheral registers.] I came across a diagram which shows how memory mapping is done on a RISC-V microcontroller. It look like the memory-mapped peripheral space is fairly separate from the rest of the memory.
Source: chapter 9 from an undergraduate textbook on digital electronics and CPU architecture.
Like you wrote, it varies from one CPU architecture to another. But one example is better than none (for a neophyte me).