While it's convenient technically to have unified memory and so it makes a lot of sense for your machine code, in fact the MMIO isn't just memory, and so to make this work anyway in the C abstract machine they invented the "volatile" qualifier. (I assume you weren't involved back then?)

This should be a suite of intrinsics. It's the same mistake as "register" storage, a layer violation, the actual mechanics bleeding through into the abstract machine and making an unholy mess.

If you had intrinsics it's obvious where the platform specific behaviour lives. Can we "just" do unaligned 32-bit stores to MMIO? Can we "just" write one bit of a hardware register? It depends on your platform and so as an intrinsic it's obvious how to reflect this, whereas for a type qualifier we have no idea what the compiler did and the ISO document of course has to be vague to be inclusive of everybody.