None of those actions were visible to the virtual machine that contained the debugger. The CPU didn't even have the concept of an interrupt. The MIT Lisp Machi

None of those actions were visible to the virtual machine that contained the debugger. The CPU didn't even have the concept of an interrupt. The MIT Lisp Machine ran a simple kernel written in assembler that also included an interpreter for the user-visible instruction set, it would check periodically whether devices had raised an interrupt then return back into Lisp to handle the event.