An operating system is defined as an interface (or bridge) between the user of computer and the computer hardware. An operating creates such a friendly environment for the user in which the computer is used effectively and efficiently. An operating system performs a number inter-process communication, protection and security.

       Batch processing means executing a series of jobs all at one time. The jobs with similar requirements are batched together and run through the computer as a group. The term originated in the days when users entered programs on punch cards. They would give a batch of these programmed cards to the system operator, who would feed them into the computer. Batch jobs can be stored up during working hours and then executed during the evening or whenever the computer is idle. Batch processing is particularly useful for operations that require the computer or a peripheral device for an extended period of time. Once a batch job begins, it continues until it is done or until an error occurs. Note that batch processing implies that there is no interaction with the user while the program is being executed.
For example: The credit card companies process the billing system. The customer does not receive a bill for each separate credit card purchase but one monthly bill for all of that month’s purchases. The bill is created through batch processing, where all of the data are collected and held until the bill is processed as a batch at the end of the billing cycle.

1)    In this system data is collected for defind period of time and processed in batches.
2)    It is a simplest processing method.
3)    Information of master file is up to date only up to last execution.
4)    Magnetic tapes as well as magnetic disks are used, output reports are printed in detail.

                   Time-sharing is an approach to interactive computing in which a single computer is used to provide simultaneous interactive general-purpose computing to multiple users by sharing processor time. This is done by providing a separate terminal to each user. Each terminal is connected to main computer system. CPU time one by one. This short period of time is called time slice. The user feels that he is having his own computer. In time sharing system hundreds of users may be using the system simultaneously. It is not possible to keep the programs of all the users in the memory at the same time. So operating system keeps few programs in the main memory and the rest of the programs stored in the disk.

1)    The response time is very less in case of time sharing systems.
2)    The CPU utilization is increased.
3)    The memory management in time sharing system provides for the protection and separation of user programs.
4)    It is cost effective.

When the time requirements on the operation of processor ot the flow of data are very critical, then real time operating systems are used. Real time means a quick response from the computer. In the real time system each process is assigned a priority according to the importance of process. The processor is normally allocated to the highest priority process among those which are ready to execute.

For example:
1)  Air traffic control system
2)  Chemical process control system.
3)  Scientific experiments
4)  Real time simulations
5)  Medical equipment
6)  Automotive applications etc.

Types of real time systems:
1 Hard real time system
2 Soft real time system

                In online systems, there is direct interaction of the user and the computer and the response time is very less. Each user is provided the direct access to the CPU on time sharing basis. The transactions in the online systems are processed as soon as the data is entered into the system.

For examples:
a)      Railway reservation system
b)      Banking system
c)       Stock prices in the stock market etc.

               In offline systems, the transactions are not processed immediately rather the data is collected for some specific period of time and the processing is done after the specific time. The batch processing is the best example of offline system where the jobs are collected and processed after certain time interval.

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