Alan Turing, the mathematician who was among the founders of computer science, showed in 1936 that it is impossible to devise an algorithm to prove that any given program will always run to completion. The essence of his argument was that such an algorithm can always trip up if it analyzes itself and finds that it is unable to stop. "It leads to a logical paradox," remarks David Schmidt, professor of computer science at Kansas State University. On a pragmatic level, the inability to "terminate," as it is called in computerese, is familiar to any user of the Windows operating system who has clicked a mouse button and then stared indefinitely at the hourglass icon indicating that the program is looping endlessly through the same lines of code.

The current version of Microsoft's operating system, known as XP, is more stable than previous ones. But manufacturers of printers, MP3 players and other devices still write faulty "driver" software that lets the peripheral interact with the operating system. So XP users have not lost familiarity with frozen hourglasses. The research arm of Microsoft has tried recently to address the long-simmering frustration by focusing on tools to check drivers for the absence of bugs.

   
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