Time-sharing or Multitasking Operating system

Time-sharing or multitasking is a logical extension of multiprogramming. The CPU executes multiple jobs by switching among them, but the switches occur so frequently that the users can interact with each program while it is running.

Time-sharing is a mechanism to provide simultaneous interactive use of a computer system by many users in such a way that each user is given the impression that he/she has his/her own computer. It uses multiprogramming with a special CPU scheduling algorithm to achieve this. A time-sharing system has many user terminals simultaneously connected to the same computer. Using these, terminals , multiple users can simultaneously work on the system.

The multiprogramming feature allows multiple user programs to simultaneously reside in the main memory. The special CPU scheduling algorithm, used in time-sharing system allocates a very short period of CPU time one-by-one to each user process, beginning from first user process and proceeding through the last one, and again beginning from the first one. This short period of time during which a user process gets attention of the CPU is known as a time slice or time slot or quantum.

Most time-sharing systems use time-slicing scheduling. In order to prevent programs from monopolizing the processor , a program executing longer than the system-defined time-slice is interrupted by the operating system and placed at the end of the queue of waiting programs.

Memory management in time-sharing systems provides for isolation and protection of co resident programs. Some forms of controlled sharing are provided to conserve memory and to exchange data between programs.
I/O management in time-sharing systems must be sophisticated enough to cope with multiple users and devices. However , due to relatively slow speed of terminals and human users, processing of terminal interrupts need not be time-critical.
As in most multi user environments, allocation and deallocation of devices must be done in a manner that preserves system integrity and provides for good performance.

File management in a time-sharing systems must provide protection and access control due to concurrent and conflicting attempt to access files. This task is compounded by the requirement for files to be shared among certain users or classes of users.