I'm not sure I follow what you're talking about. Operating Systems are tangential to the point, though. When I said all useful programs halt, the kinds of progr

I'm not sure I follow what you're talking about. Operating Systems are tangential to the point, though. When I said all useful programs halt, the kinds of programs I meant are the kind used to prove things about computation. Those programs are simplified abstractions of real programs, and they generally compute one result and then halt (if they halt). Being simple makes those programs easier to use in proofs than real programs. However, the proofs still generalize to real programs; a real-world program with multiple functions is equivalent to several single-function programs glued together together. So my point in saying that all useful programs halt was really to argue that all useful programs must have the constraint that they their behavior is always predictable, or controllable, in some sense. In a trivial way, computers are always predictable, but Turing proved that there are cases where it's impossible to predict the eventual outcome of a program in advance, i.e., the halting problem. In precise terms, the outcome (halt/not halt) of some programs is said to be undecidable or uncomputable. Undecidability is a hallmark of Turing machines. But since real programs are written by humans, who can't solve undecidable problems, real programs will never be able to gain any advantage from undecidable behavior.