Introduction:- An oprerating system (OS) is basic interface between software and hardware. Hardware without software has no meaning and is of no use. It is like a policeman which controls the traffic around its area. Similar way an operating system manages the computer. Whenever we switch on the computer, the first thing computer looks for is operating system.
               The operating system is the manager among the software. Its main functiin is managing the physical resources of the system, physical resources means all input/output devices, storage devices and even CPU of the computer. Operating system manages that what is required and is requested by the user at a particular moment. It may be the printer or monitor or keyboard or floppy drive or the CPU.
              The American National Standard Institute (ANSI) has defined operating system as: The software which controls the execution of computer programs and which may provide scheduling, debugging, input/output control, accounting, compilation, storage assignment, data management and related services.
Operating system as resource manager:- As a hotel manager has to take care of restaurant, kitchen, bar and room service also, similarly the OS must keep track of the status of each resource, decides which process is to get the resource, how much and when, allocate it and eventually reclaim it i.e. when to get it back.
As a resource manager it has to do following functions:-
1. Keep trach of all the resources.
2. Find and implement policy that determines who gets what, when and how much. 
3. Allocate the resource.
4. Get it back.
As operating system performs following functions:-
1. Memory management function.
2. Processor management function.
3. Device management function.
4. Informative/file management function.
Operating system components:-A computer operating system is comprisedof an integrated set of specialised programs which varies from one type of operating system to another. However all the programs can be classified as belonging to one of the three groups.
1. Control programs.
2. System service programs.
3. Processing programs.
TYPES OF OPERATING SYSTEM
1. Batch prcessing:-In such a type of operating system the processing is done in batches with the increase in the number of programs to be run we put them in the batches into the storage. CPU picks up the programs one by one at a time in a strictly sequential manner in the order jn which the batch of jobs is fed to the computr. The next job in the batch is taken up by the CPU only after the first job is terminated either in completion or because of an error.
         In batch processing the programs are picked from memory into primary memory one by one by the CPU. When there is only one program with CPU, two of its important parts are under utilized. Parts of main memory may be lying vacant and only some programs may be using ALU because in general in electronic data processing most of the time programs take for input and output. So to use all the capabilities of the CPU a new concept evolve and that is called as Multi programming.
                 Multi programming means using more than one program at a time. It enables more than one program to reside in main memory at the same time so that OS may direct different programs to different resources. For example, when program A is reading or writing means using input/output device, B can use arithmetic and logic unit and if A is getting input through keyboard, B is using printer, C can use arithmetic part by using ALU but once a program goes to ALU or control unit it comes out only after complete processing and meanwhile others if waiting for the same have to wait. When a job is submitted for processing means you have given the command it takes some time to give you the result. This time is known as Turn around time.
                 This can be viewed as multiprogramming if when you were in qurqe. So two processes at a time. This type of facility increases the efficiency of the CPU especially when a number of input/output devices are attached.
2. Time sharing:- A time-sharing system allows a number of users to have access to one central computer via terminals which are attached by physical lines or looked with communication lines like telephone lines or satellite. In such type of operating system each user is given a period of time during the use of CPU.
              This defined period is known as TIME SLICE and each working user receives its time slice in rotation. The CPU can still execute one program at a time but only difference is that no program or user has to wait for unlimited time of first program's completion. Here after fixed time slice CPU gets second program no matter whether first program is complete or not. The main idea behind time sharing is to allow all programs to have a brief share of central processor in turn.
Multi processing:- Multi processing means processing with more than one CPU. This term refers to a computer hardware configuration with two or more central processing units linked together for coordinated operation in executing instructions simultaneously. Each CPU process instructions from totally different programs or different instructions from the same program.
              Multiprocessing system usaually fall into one of the three general categories a front-end processor configuration, a back-end configuration, or a system with two or more CPU's.
3. Front-end processor configuration:- In front-end processor configuration a small CPU acts as an interface between a larger CPU and on-line terminals and other peripheral devices. The small CPU coordinates the activities of the larger one by handling all work coming into system from on-line or remote terminals and other peripheral devices. The large CPU can concentrate on performing logical and mathematical operations while the small CPU handles all input/output interupts.
Back-end processor:- In ghis configuration the small CPU is responsible for maintaining a large on-line database including data access and specific data field updates.
              When processing requirements are extremely large and complex two or more CPU's can be used together to form a large multiprocessing system. This type of system may also use a small CPU to perform administrative tasks and to control communications with peripheral devices. The CPU's can be configured in many ways.
1. Each CPU can be assigned to perform a dedicated task such as logical and mathematical operations or handling input and output activities.
2. Both CPU's can be assigned to process the different parts of the same job to speed up response time for on-line and real-time applications.
3. On line processing may be handled by one CPU while batch processing is handled by the other.
4. In some extremely time-sensative systems where down-time is costly the second CPU is simply used to provide backup in case the primary CPU functions.
                 The operating system that support multi-processing capabilities are sophisticated and require careful planning to ensure that workload scheduling makes the most efficient use of computer resources. However, the computer system can have substantially more processing power than much larger single CPU system, Multi-CPU configured microcomputers began to apear in the market during1982.
Real-time processing:- The real time processing is a system which produces an almost immediate response as a result of inputting data. In these systems almost every result/output of a process is given input for next process. These realtime processing system were very much used between USA and Iraq for missile control. 
Multitasking:- Multitasking allows a group of processors to cooperate in accomplishing an activity that can be parcelled into smaller concurrent activities. Multitasking is similar to multi programming except that tasks ( the name for processes in a multitasking system) know about the presence of each other.
Parallel processing:- Parallel processing is an efficient form of information processing which emphasizes the exploitation of concurrent events in the computing process. Concurrency implies parallelism, simultaneity and pipeling. Parallel events may occur in multiple resources during the same time interval. Simultaneous events may occur at the same time instant and pipelined events may occur in overlapped time spans. Parallel processing demands concurrent execution of many program in the computer. It is a cost effective means to improve system performance through concurrent activities in the computer.