Predicting the future is notoriously difficult. Who could have imagined, in the mid 1970s, for example, that by the end of the 20th century, computers would be as common in people's homes as TV sets? In the 1970s, computers were common enough, but only in big business, government departments, and large organizations. These were the so-called mainframe machines. Mainframe computers were very large indeed, often occupying whole air-conditioned rooms, employing  full-time technicians and run on specially-written software. Though these large machines still exist, many of their functions have been taken over by small powerful personal computers, commonly known as PCs.