Example of overflow error7/27/2023 ![]() “There’s always an engineering trade-off between the cost of having a more precise representation and the benefit of the efficiency.” “We have to recognise that in software we are always approximating reality,” explains Bill Scherlis, a software expert at Carnegie Mellon University. ![]() The problem with that, as the Ariane 5 proved, is that such limitations aren’t always foreseen as problematic. Rocket guidance systems do a lot of critical number crunching very quickly, so these overheads certainly matter. If you kept to certain limits, software was expected to run more smoothly. Storage space used to be much more costly than it is today and processing larger values took longer. Numbers are infinite, so why choose such limited storage spaces for them? The answer is that computers have traditionally demanded efficiency in all things. On Ariane, by comparison, the software was using a "16-bit" space, which is much smaller and only capable of storing a maximum value of 32,767. How so? It just so happens that 2,147,483,647 is the maximum positive value that can be stored by a “32-bit signed register”, commonly installed on many computer systems. Although scant details have been released – the FAA and Boeing declined to comment for this article – some amateur observers have pointed out that 248 days (when counted in 100ths of a second) is equal to the number 2,147,483,647 – which is significant. The Federal Aviation Administration’s directive on the matter states that a counter in the control unit’s software will “overflow” after this specific period of time, causing an error. Hypothetically, the engines could suddenly halt even in mid-flight. The control unit managing the delivery of power to the plane’s engines will automatically enter a failsafe mode – and shut down the engines – if it has been left on for over 248 days. It’s suspected that the reason why Nasa lost contact with the Deep Impact space probe in 2013 was an integer limit being reached.Īnd just last week it was reported that Boeing 787 aircraft may suffer from a similar issue. Such glitches emerge with surprising frequency. A couple of seconds later, the rocket was history, as the video below shows. ![]() More technically, it’s called “integer overflow”, essentially meaning that numbers are too big to be stored in a computer system, and sometimes this can cause malfunction.Ī full investigation of the Ariane incident found that a process left over from software in the previous generation of rockets, Ariane 4, had captured an unexpectedly high reading for the sideways velocity of the newer, faster vehicle – and the Ariane 5 rocket’s software couldn’t handle this high figure. This is the same species of inaccuracy that doomed the 1996 Ariane 5 launch. The counter would “roll over” to 00,000 and then count up to 5,350, the remaining value. Imagine trying to represent a value of, say, 105,350 miles on an odometer that has a maximum value of 99,999. So what are these bugs, and why do they happen? How is it possible that computers get befuddled by numbers in this way? It turns out such errors are answerable for a series of disasters and mishaps in recent years, destroying rockets, making space probes go missing, and sending missiles off-target.
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