003.Evolution of operating systems.

Early computers were capable of performing only one job or task at a time. This form of computer operation is often called single-user batch processing. The computer runs a single program at a time while processing data in groups or batches. In this early systems, users generally submitted their jobs to the computer center on decks on punched cards. users often had to wait hours or even days before printouts were returned to their desks.

Software systems called operating systems were developed to help make it more convenient to use computers.Early operating systems managed the smooth transition between jobs. This minimized the time it took for computer operators to switch between jobs, and hence increased the amount of work, or throughput,computers could process.

As computers became more powerful, it became evident that single-user batch processing rarely utilized the computer's resources efficiently. Instead, it was throught that many jobs or tasks could be made to share the resources of the computer to achieve better utilization. This is called multiprogramming involves the "simultaneous" operation of many jobs on the computer_the computer shares its resources among the jobs competing for its attention. With early multiprogramming operating system, users still submitted jobson decks of punched cards and waited hours or days for results.

In the 1960s, several groups in industry and the universities pioncered itmesharing operating systems. Timesharing is a special case of multiprogramming in which users access the computer through terminals, typically devices with keyboards and screens. In a typical timesharing  computer system, there may be dozens or even hundreds of users sharing the computer at once.The computer does not actually run all the users' jobs simultancously.Rather, it runs a small portion of one user's job and then moves on to service the next user.The computer does this so quickly that it may provide service to each user several times per second. Thus the users' programs appear to be running simultaneously.An advantage of timesharing is that the user receives almost immediate responses to requests rather than having to wait long periods for results as with previous modes of computing . Also if a particular user is currently idle, the computer can continue to service other users rather than wait for one user.