The comment was put there by Dennis Ritchie to state that the context switching code was hard to understand. It depended on the hardware of the PDP-11. He wrot

The comment was put there by Dennis Ritchie to state that the context switching code was hard to understand. It depended on the hardware of the PDP-11. He wrote that comment to indicate that it was complex, buggy, and that they didn't quite fully understand the code either. https://thenewstack.io/not-expected-understand-explainer/ https://thenewstack.io/not-expected-understand-explainer/