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An operating system is system software. It is an interface between the user and the hardware. It performs tasks like memory management, resource allocation, file management, process management, security, etc. It also controls the peripheral devices like printers and hard disks.

The objectives of an operating system are:
1. Efficiency
The operating system must improve the efficiency of the system by the efficient allocation of resources.
2. Convenience
The foremost objective of an operating system is to provide an easy-to-use interface for the users to communicate with the device.
3. Evolution ability
An operating system must be capable of allowing new changes to be introduced without interfering with its performance.
4. Resource management
The operating system should be able to keep track of the resources in a system.  It should decide which process has to access the resource when multiple processes request for the same resource (priority assignment)
5. Security 
The operating system should be able to prevent any unauthorized access to the data and programs in the system.

A buffer is an area in the main memory that stores temporary data. It is mainly used when the data is moved from one place to another. Buffering helps in increasing the performance of the system. It makes the CPU and the I/O devices work at the maximum possible speed without allowing them to be idle.

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The drawbacks of batch processing OS are:
1.Job priority cannot be assigned
2.Users cannot directly interact with the computer.
3.The CPU utilization is very low. The CPU is idle most of the time because the speed of the I/O devices is slower than that of the CPU.

In batch processing, the user cannot interact directly with the computer. The jobs (or programs) in punch cards are submitted to an operator. The operator sorts the similar jobs into batches. The CPU can execute only one batch at a time. Here, the CPU is idle most of the time because the speed of the I/O devices is slower than that of the CPU.

Multi-threading
Simultaneous execution of multiple threads of a process (or an application) is called multi-threading. Multi-threading improves the performance of the operating system by making efficient use of the CPUs in a multi-processor system. Multi-threading involves the sub-division of various operations of a single process into separate threads that are executed simultaneously.

Multi-tasking
The simultaneous execution of many processes by the CPU is called multi-tasking. It improves CPU utilization. Multi-tasking is also called time-sharing. In multitasking, different tasks (or processes) are performed at the same time, while in multi-threading, multiple operations of a single process are performed at the same time.

Parallel systems have multiple processors running simultaneously in a computer system. Parallelism can be achieved in such systems. A process is sub-divided into many parts and each part is executed in parallel in separate processors. It decreases the execution time of the process. Thus, the efficiency of the computer can be increased.

The advantages of the distributed systems are:
a.Malfunctioning of a single computer in a distributed system does not affect the other computers. Distributed systems are fault-tolerant.
b.Resource sharing is easy in a distributed system.
c.It provides better performance than the centralized systems.
d.Distributed systems are scalable and flexible.
e.Distributed systems have high computational power.

A system whose response must be guaranteed within the specified timing is called a real-time system. A real-time system must always meet the deadline. It is a time-bound system. It may be either time-sharing or event-driven.
Example: Air traffic control system