 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 . 
