The introduction of transistors in the mid-1950s made it possible to build smaller and more reliable computers. This made it possible to sell computers commercially, separating the designers and builders of the computer from the operators, programmers and maintainers. As computers were so expensive only specially trained operators were allowed into the special, air-conditioned rooms which contained the delicate equipment. Programmers had to write their programs in machine language or FORTRAN, then transfer them to punched cards and hand them to the operators to get them processed. Later they would receive back the results. The operators would load the stack of punched cards into the computer and collect the printed output when the calculations had finished. A lot of time was wasted running around with decks of cards or printouts, and ways were found to minimise the inefficient use of the computers.
Simple computers were used to transfer the punched cards to magnetic tape. The tape with several programs was then mounted into the main computer. The operator loaded a special program (the ancestor of today's operating systems) which read jobs from tape and ran them. The output of all programs on the tape was written to a special output tape instead of being printed. When all jobs were finished the operator removed the output tape and took it to a smaller simple computer which printed the contents of the output tape.