Thousands of ZX81 programs were published, either as printed listings that could be typed in or as ready-made applications that could be loaded from cassette tape. Many computer magazines featured ZX81 program listings – some, such as "Sinclair Programs", were dedicated entirely to listings – while many individuals became the archetypal "bedroom programmers", producing games and applications which they produced, marketed, recorded, and sold from their own homes. Some went on to found their own software houses, employing teams of programmers – some of whom were still at school – to produce programs for the ZX81 and other computers. Existing companies also sold software; Psion produced a series of ZX81 programs in close association with Sinclair, including a flight simulator, while ICL's range of ZX81 programs sold over 100,000 cassettes in less than three months. Psion's success with the ZX81 had a profound effect on the future of the company. Its work on the ZX81 database program "Vu-File" led to Psion switching its focus to the development of personal digital assistants, which resulted in the launch in 1984 of the Psion Organiser, the world's first handheld personal computer. Some of the most popular ZX81 games (Psion's "Flight Simulation" being an example) were rewritten for the Spectrum to take advantage of the newer machine's colour and sound capabilities.