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Interesting! Looks like a race condition (with a 1ms window) during squawk allocation (the process that gives each aircraft a 4 digit identifier for ATC purposes) caused data corruption. They present that window as being small but 1ms is a decent chunk of time in our work lol

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More specifically: it sounds like something which was supposed to be an atomic operation was split into two by the preempting task, with corruption happening because that task also interacted with the same piece of (now-temporarily-invalid) data.

It's a decent chunk in most people's.

But we're talking about a system that is also older than most people.

It was first put into operation in 1967 in the U.S.A., and brought over to the U.K. in 1974. It's formally called 'NAS En Route Stage A', and is written in a 1950s language named JOVIAL. It originally ran on an IBM 9020. It runs on IBM 9020 compatible systems today.

For some reason, a couple of years ago someone added a lengthy unverifiable description of it to Wikipedia's IBM 9020 article, even though that very description said that NAS hadn't run on a 9020 for 34 years at the time of writing, but had been running on a 4381.

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