This is still wrong though, since UB doesn't exist at an assembly level, and yet C has it. Therefore there is no clear mapping from C to assembly (even considering hypothetical ISAs).
There are plenty of undefined things in ISAs, writing to some internal ARM registries also springs to mind, but that's a bit of a red herring. Just because code with undefined behaviour doesn't map to some abstract machine model nicely, doesn't imply that code that doesn't invoke UB can't have a simple mapping to some abstract machine semantics.
Yes, things like illegal opcodes, or other things the hardware _does_ but isn't officially part of the spec... those things are almost as old as computers. Certainly as old as home computers.
