[siqnkckp] World's slowest useful computer

What are the features of a modern processor that are needed for functionality (as opposed to performance), especially for functionality of the operating system, particularly for things like security, process separation, memory protection, preemptive multitasking, debuggers with breakpoints, multiprocessors, virtual memory, and virtualization (for the hypervisor)?

"Needed" is a gray scale because of Turing completeness, but the general idea is that a hardware feature is needed for functionality if emulating it in software would encounter a cost comparable to trapping every instruction or emulating every instruction.

And then, of course, build such a computer.  Probably a toy, or an educational tool for operating systems implementation.  Though it might be useful for embedded systems.