OS

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TYPES OF OS

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1. Batch

Users dont interact with pc directly.

To speed up processing., similar jobs are grouped together and executed.

The problems with Batch Systems are as follows −

Lack of interaction between the user and the job.

CPU is often idle, because the speed of the mechanical I/O devices is slower than the CPU.

Difficult to provide the desired priority.

2. Time sharing OS

Multiple people can use same OS.

Every process is given a time quantum and then run for that time period.

Advantages of Timesharing operating systems are as follows −

Provides the advantage of quick response.

Avoids duplication of software.

Reduces CPU idle time.

Disadvantages of Time-sharing operating systems are as follows −

Problem of reliability.

Question of security and integrity of user programs and data.

Problem of data communication.

3. Distributed OS

Multiple processors are used simultaneously.

Communication between processor using Buses

ADvantage -

If 1 one fails, we have other

Reduced load and delay

4. Real time OS

Real Time operating System

A real-time system is defined as a data processing system in which the time interval required to process and respond to inputs is so small that it controls the environment. The time taken by the system to respond to an input and display of required updated information is termed as the response time. So in this method, the response time is very less as compared to online processing.

Real-time systems are used when there are rigid time requirements on the operation of a processor or the flow of data and real-time systems can be used as a control device in a dedicated application. A real-time operating system must have well-defined, fixed time constraints, otherwise the system will fail. For example, Scientific experiments, medical imaging systems, industrial control systems, weapon systems, robots, air traffic control systems, etc.

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Following are a few common services provided by an operating system −

Program execution

I/O operations

File System manipulation

Communication

Error Detection

Resource Allocation

Protection

Program execution

Loads a program into memory.

Executes the program.

Handles program's execution.

Provides a mechanism for process synchronization.

Provides a mechanism for process communication.

Provides a mechanism for deadlock handling.

I/O Operation

I/O operation means read or write operation with any file or any specific I/O device.

Operating system provides the access to the required I/O device when required.

File system manipulation

Program needs to read a file or write a file.

The operating system gives the permission to the program for operation on file.

Permission varies from read-only, read-write, denied and so on.

Operating System provides an interface to the user to create/delete files.

Operating System provides an interface to the user to create/delete directories.

Operating System provides an interface to create the backup of file system.

Communication

Two processes often require data to be transferred between them

Both the processes can be on one computer or on different computers, but are connected through a computer network.

Communication may be implemented by two methods, either by Shared Memory or by Message Passing.

Error handling

The OS constantly checks for possible errors.

The OS takes an appropriate action to ensure correct and consistent computing.

Resource Management

The OS manages all kinds of resources using schedulers.

CPU scheduling algorithms are used for better utilization of CPU.

Protection

The OS ensures that all access to system resources is controlled.

The OS ensures that external I/O devices are protected from invalid access attempts.

The OS provides authentication features for each user by means of passwords

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Process Life Cycle

When a process executes, it passes through different states. These stages may differ in different operating systems, and the names of these states are also not standardized.

In general, a process can have one of the following five states at a time.

S.N.State & Description

1

Start

This is the initial state when a process is first started/created.

2

Ready

The process is waiting to be assigned to a processor. Ready processes are waiting to have the processor allocated to them by the operating system so that they can run. Process may come into this state after Start state or while running it by but interrupted by the scheduler to assign CPU to some other process.

3

Running

Once the process has been assigned to a processor by the OS scheduler, the process state is set to running and the processor executes its instructions.

4

Waiting

Process moves into the waiting state if it needs to wait for a resource, such as waiting for user input, or waiting for a file to become available.

5

Terminated or Exit

Once the process finishes its execution, or it is terminated by the operating system, it is moved to the terminated state where it waits to be removed from main memory.

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The Operating System maintains the following important process scheduling queues −

Job queue − This queue keeps all the processes in the system.

Ready queue − This queue keeps a set of all processes residing in main memory, ready and waiting to execute. A new process is always put in this queue.

Device queues − The processes which are blocked due to unavailability of an I/O device constitute this queue.

Long Term Scheduler

It is also called a job scheduler. A long-term scheduler determines which programs are admitted to the system for processing. It selects processes from the queue and loads them into memory for execution. Process loads into the memory for CPU scheduling.

The primary objective of the job scheduler is to provide a balanced mix of jobs, such as I/O bound and processor bound. It also controls the degree of multiprogramming. If the degree of multiprogramming is stable, then the average rate of process creation must be equal to the average departure rate of processes leaving the system.

On some systems, the long-term scheduler may not be available or minimal. Time-sharing operating systems have no long term scheduler. When a process changes the state from new to ready, then there is use of long-term scheduler.

Short Term Scheduler

It is also called as CPU scheduler. Its main objective is to increase system performance in accordance with the chosen set of criteria. It is the change of ready state to running state of the process. CPU scheduler selects a process among the processes that are ready to execute and allocates CPU to one of them.

Short-term schedulers, also known as dispatchers, make the decision of which process to execute next. Short-term schedulers are faster than long-term schedulers.

Medium Term Scheduler

Medium-term scheduling is a part of swapping. It removes the processes from the memory. It reduces the degree of multiprogramming. The medium-term scheduler is in-charge of handling the swapped out-processes.

A running process may become suspended if it makes an I/O request. A suspended processes cannot make any progress towards completion. In this condition, to remove the process from memory and make space for other processes, the suspended process is moved to the secondary storage. This process is called swapping, and the process is said to be swapped out or rolled out. Swapping may be necessary to improve the process mix.

Context Switch

A context switch is the mechanism to store and restore the state or context of a CPU in Process Control block so that a process execution can be resumed from the same point at a later time. Using this technique, a context switcher enables multiple processes to share a single CPU. Context switching is an essential part of a multitasking operating system features.

When the scheduler switches the CPU from executing one process to execute another, the state from the current running process is stored into the process control block. After this, the state for the process to run next is loaded from its own PCB and used to set the PC, registers, etc. At that point, the second process can start executing.

Context switches are computationally intensive since register and memory state must be saved and restored. To avoid the amount of context switching time, some hardware systems employ two or more sets of processor registers. When the process is switched, the following information is stored for later use.

Program Counter

Scheduling information

Base and limit register value

Currently used register

Changed State

I/O State information

Accounting information

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These algorithms are either non-preemptive or preemptive. Non-preemptive algorithms are designed so that once a process enters the running state, it cannot be preempted until it completes its allotted time, whereas the preemptive scheduling is based on priority where a scheduler may preempt a low priority running process anytime when a high priority process enters into a ready state.

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