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Operating system controls and coordinates the use of the hardware among the various applications programs for various uses.
Operating system acts as resource allocator and manager. Since there are many possibly conflicting requests for resources the operating system must decide which requests are allocated resources to operating the computer system efficiently and fairly .
Operating system is control program which controls the user programs to prevent errors and improper use of the computer. It is especially concerned with the operation and control of I/O devices.
Multiprogramming – In multiprogramming more than one program reside in the main memory. When one job is unable to execute because of I/O operation to complete, it switches to other program and allow that program to run the main motive of multiprogramming is to utilize the CPU efficiently.
Multitasking- In multitasking operating system more than one program is assigned to the CPU or switches between the programs occurs so fast that it seems like more than one program is running. The main motive of multitasking is to utilize the CPU efficiently and reduce the response time.
Multiprocessing-In multiprocessing operating system a task is divided between more than one processor (CPU) so that parallelism can be achieved.
Multithreading -Sometime a program need to do multiple task at the same time. It is an extension of multiprogramming. In multithreading the program is divided into multiple task.