In multi-tasking or multiprogramming environments, for the purpose of increasing processing productivity, computers process several applications concurrently, switching from one to the other. This requires a powerful operating system incorporating work scheduling facilities to control the switching between programs. This means reading in data for one program while the processor is performing computations on another and printing out results on yet another.
In multi-user environments, only one user can access the system at a given moment of time. In such environments, an operating system is required to control terminal operations on a shared access basis and also the locking and unlocking of records to prevent one user attempting to read a record while another user is updating it, for instance. In such a case, one user is allowed to write to a record and all the other ones are denied access until the record is updated and unlocked.
Some environments operate in concurrent batch and real-time mode. Batch mode means that a background' job deals with programs that run without any terminal or user interaction. Real-time jobs, on the other hand, are 'foreground' jobs that deal with situations in which immediate results are necessary-such as airline seat reservations, on-line booking of hotel accommodations, automatic cash dispenser, etc. The real-time operations have priority, and the operating system: a) interrupts batch processing to deal with real-time inquiries, and bj transfers the interrupted stage of batch processing to backing storage. After the real-time operation has been dealt with, the interrupted program is transferred back to internal memory and processing recommences from a `restart point'. The operating system also copies to disk backing storage the state of the real-time system every few minutes to provide a means of 'recovering' the system in the event of a malfunction.
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