Introduction to Operating system
Definition: An operating system refers to the computer software that provides the interface between the hardware, the user and the application software.
The functions of the operating system are;
a) Job scheduling – includes preparing, scheduling and monitoring jobs for continuous processing by the computer.
b) Resource control – includes controlling the use of computer resources by other system software and application programs being executed.
c) Input/output handling – it controls the allocation of i/o devices and resolves any conflicts that may occur if more than one application program or users request the same device at the same time.
d) Memory management –it determines how much memory is allocated to user programs.
e) Error handling – it deals with errors which are produced during program execution and keep the computer running when errors do occur.
Example of operating system:Ms DOS, PC DOS, UNIX, Linux, Windows etc.

Types of Operating Systems
Following are some of the most widely used types of Operating system.
1. Simple Batch System
2. Multiprogramming Batch System
3. Multiprocessor System
4. Distributed Operating System
5. Realtime Operating System

 In this type of system, there is no direct interaction between user and the computer.
 The user has to submit a job (written on cards or tape) to a computer operator.
 Then computer operator places a batch of several jobs on an input device.
 Jobs are batched together by type of languages and requirement.
 Then a special program, the monitor, manages the execution of each program in the batch.
 The monitor is always in the main memory and available for execution.
Following are some disadvantages of this type of system :
1. Zero interaction between user and computer.
2. No mechanism to prioritize processes.

 In this the operating system, picks and begins to execute one job from memory.
 Once this job needs an I/O operation operating system switches to another job (CPU and OS always busy).
 Jobs in the memory are always less than the number of jobs on disk(Job Pool).
 If several jobs are ready to run at the same time, then system chooses which one to run (CPU Scheduling).
 In Non-multiprogrammed system, there are moments when CPU sits idle and does not do any work.
 In Multiprogramming system, CPU will never be idle and keeps on processing.
Time-Sharing Systems are very similar to Multiprogramming batch systems. In fact time sharing systems are an extension of multiprogramming systems.
In time sharing systems the prime focus is on minimizing the response time, while in multiprogramming the prime focus is to maximize the CPU usage.

A multiprocessor system consists of several processors that share a common physical memory. Multiprocessor system provides higher computing power and speed. In
multiprocessor system all processors operate under single operating system. Multiplicity of the processors and how they do act together are transparent to the others.
Following are some advantages of this type of system.
1. Enhanced performance
2. Execution of several tasks by different processors concurrently, increases the system’s throughput without speeding up the execution of a single task.
3. If possible, system divides task into many subtasks and then these subtasks can be executed in parallel in different processors. Thereby speeding up the execution of single tasks.

The motivation behind developing distributed operating systems is the availability of powerful and inexpensive microprocessors and advances in communication technology.
These advancements in technology have made it possible to design and develop distributed systems comprising of many computers that are inter connected by communication networks.
The main benefit of distributed systems is its low price/performance ratio.
Following are some advantages of this type of system.
1. As there are multiple systems involved, user at one site can utilize the resources of systems at other sites for resource-intensive tasks.
2. Fast processing.
3. Less load on the Host Machine.

It is defined as an operating system known to give maximum time for each of the critical operations that it performs, like OS calls and interrupt handling.

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