On Tue, 10 Mar 1998, Allison J Parent wrote: It's been a long time since I've looked at a CS book, but I remember the Turing machine as a *theoretical* machine that reads and writes symbols on an *infinitely long* tape. I'm sure somebody could build an approximation of this, but the main interest in the Turing machine is that it is used as the definition of computational "power." One of theories is that no machine built today, or at any time in the future, no matter what the architecture, will be able to compute anything that a simple Turing machine cannot compute.. I think that's basically a long-winded way of saying that all computing machines are state machines, and the Turing machine was basically a device that helped visualize the mechanical realization of a state machine. Once you actually build one, however, the "tape" is no longer infinitely long, so the machine suddenly has limited computational power, and the theorem no longer applies. -- Doug