My favorite operating system talk...

The phrase “You are Not Expected to Understand This” is probably the most famous comment in the history of Unix.

And last month, at the conference in San Francisco, systems researcher explained to an audience exactly what it was that they weren’t supposed to understand.

Computer science teacher , who was in the audience, called it “one of my favorite talks of the day,” writing that “Nothing should be a black box, even when Dennis Ritchie says it’s ok!”

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Thomas opened his talk by saying “I love systems — operating systems in particular.” But even he had to marvel at the popularity of this 42-year-old Unix source code comment: “You started seeing people wearing sweatshirts, t-shirts, and baby onesies with ‘You are not expected to understand this.’ It took off in the hacker community.” And geeks have been riffing on it .

The code comment originally appeared in the Sixth Edition Unix operating system, describing context switching — or, as Thomas put it, “the mechanism that allows for time-sharing and multi-tasking … essentially how a computer is allowed to be shared by multiple concurrent users and concurrent applications.”

Basically, a context is a mix of stack pointers and registers that allows Unix processes to be paused and resumed. “It’s a neat trick, and Unix’s time-sharing support was a big reason why it took off. It was a big leap forward in usability,” Thomas noted. “Some things have changed over the years, but the overall scheme has fundamentally remained the same.”

Thomas reminded the audience of Unix co-creator ‘s own “” web page on the subject:

“It’s often quoted as a slur on the quantity or quality of the comments in the Bell Labs research releases of Unix. Not an unfair observation in general, I fear, but in this case unjustified… we tried to explain what was going on. ‘You are not expected to understand this’ was intended as a remark in the spirit of ‘This won’t be on the exam,’ rather than as an impudent challenge.”