Time Sharing

It is a special type of on-line processing in which several users share the resources of an
on-line computer system to avoid turn about time, especially when results are needed quickly;
another system is available i.e., time sharing system. In this method the individual user has an
independent local terminal. The terminal is connected to a main computer, which does all the
processing work.
Under this system each person on a terminal has to log into the system. The system then
lists him in the order in which he has logged on. Once everybody has logged on, the system will
permit the first in the list to run his program. The time given for running the program is generally
10 milliseconds. It is called a time slice or a time slot or a quantum. Starting then, with the first
to log on, each user is given his time slice. Once all have made use of it, the system starts all over
again, repeating the cycle. The system maintained is called a polling system whereby each
terminal is polled on a round robin basis.
All the instructions given by the users are carried to the centrally located main computer
system and processed there and retrieve back by the user instantaneously. Since, the centrally
located main computer has high-speed processor, users may feel that they are sharing the same
computer resources.

Features

1. The processor time is shared between two or more persons.
2. The processor switches from one user's job to another at a rapid rate under the control
of a monitor program.
3. This concept is time based.

The concept of time-sharing can be explained with the help of the following diagram.

Let us suppose that there are three terminals, each person on a terminal has to log into the
system. The system then lists him the order in which he has logged on. Once everyone has
logged on, the system will permit the first on the list to run his program. The time given for
writing a program is called a time slice or time slot. Starting from the first, each user is given his
time slice. Once, all the three users have made use of it, the system starts all over again,
repeating the cycle. The sequence maintained is called Polling System. Most time sharing
systems respond to user's instructions in milli/micro seconds.

Advantages of Time-Sharing System

(1) Simultaneously several users can be served.
(2) All the users have the same priority.
(3) All the users get equal amount of processor time.
(4) The interaction with the running programs is possible.

Time-Sharing Vs Multi-programming

The difference between these two methods is that in "time, sharing" the CPU links only
one terminal at a time and its total time is shared uniformly by all the users. But in
multiprogramming, the CPU links more than one terminal or other peripherals at a time.
Therefore, multiprogramming more efficiently utilizes the main memory and the central
resources than time sharing. Time sharing is employed only for multi terminal system whereas
multi programming is meant both for single as well as the multi terminal system.