Virtual memory and page replacement

Atlas already had to decide which page to throw out of core when a new one was needed, and it used a learning program that watched how pages were used to guess. Four years later, at IBM, László Bélády studied replacement algorithms systematically and described the optimal one, which needs to know the future.

In 1969 Bélády, Robert Nelson and Gerald Shedler reported something stranger: with first-in, first-out replacement, giving a program more memory can make it fault more. It is still called Bélády's anomaly.