The main role of an Operating System is to perform user programs and easily make users understand and interact with computers and also run their applications. It is designed to make sure that computer performance improves by managing all computational activities. It overall manages computer memory, operation, function, and process of all hardware and software.

Types of OS:

• Batched OS (like Transactions Process, Payroll System, etc.)
• Multi-Programmed OS (like Windows O/S, UNIX O/S, etc.)
• Timesharing OS (like, Multics, etc.)
• Distributed OS (like, LOCUS, etc.)
• Real-Time OS (like PSOS, VRTX, etc.)

It is a system that has two or more CPUs. It allows you to run the processing of different computer programs at the same time mostly by a computer system that has two or more CPUs that are using a single memory.

Benefits:

• Nowadays, these systems are used widely to improve performance
• By using multiprocessors you can complete more tasks in a unit of time.
• It increases the throughput and is cost-effective.
• It enhances the reliability of the computer system.

RAID is called Redundant Arrays of Independent Disks helps you in storing data on multiple hard disks this is why it is considered as data storage virtualization technology that combines multiple hard disks and makes it a big storage device. It makes balance by managing data protection, system performance, storage space, etc. It improves overall performance and increases the reliability of data storage. By using multiple hard disks increases the storage capacity of the system and achieves data redundancy to prevent data loss.

Different levels of RAID Configuration

RAID 0 - Non-redundant striping: It increases the performance of the server.

RAID 1 - Mirroring and duplexing: It is also called disk mirroring and it implements fault tolerance.

RAID 2 - Memory-style error-correcting codes: It uses dedicated hamming code parity I.e., a liner form of error correction code.

RAID 3 - Bit-interleaved Parity: It uses a dedicated parity drive to store parity information.

RAID 4 - Block-interleaved Parity: It confines all parity data to a single drive.

RAID 5 - Block-interleaved distributed Parity: It gives better performance than disk mirroring and fault tolerance.

RAID 6 - P+Q Redundancy: It provides fault tolerance for two drive failures.

1. Wait()
2. Signal()

• Attempt to access the page.
• If the page is valid (in memory) it proceeds with normal instructions.
• If a page is not-valid then it tells a page-fault trap occurrence.
• Check if the memory reference is valid to a location on secondary memory. If not, the process is dismissed (illegal memory access).
• Schedule disk operation where it can read the specific pages into the main memory.
• If the instructions are interrupted by the operating system trap then restart it.

RTOS stands for Real-Time Operating System (RTOS) is an OS that is used to run real-time applications i.e., those types of applications where data processing should be done instantly within a fixed and small measure of time. It is great in doing tasks that are needed to be executed within a short duration. Real-Time Operating System also takes care of execution, monitoring, and all-controlling processes, etc. all at once. It consumes less memory and fewer resources. It is used in Air traffic control systems, Anti-lock Brake Systems, and Heart pacemakers.

Types of RTOS:

• Hard Real-Time
• Firm Real-Time
• Soft Real-Time

1. Independent Process
2. Cooperative Process

IPC (Inter-process Communication) is a mechanism that uses memory that is shared between processes or threads to make a communication. With IPC, Operating System allows different processes to communicate with each other. Its major purpose is to exchange data between multiple threads in more than one program or process.

Different IPC Mechanisms:

• Pipes
• Message Queuing
• Semaphores
• Socket
• Shared Memory
• Signals

Main memory: The Main memory of a computer is RAM. It is the primary memory or read-write memory or internal memory of a system. The programs and data requires to run during the execution of a program are stored in this memory.

Secondary memory: Secondary memories are the additional memory in a computer, in the form of storage devices that can store data and programs. They are also known as external memory or backup memory or auxiliary memory. These storage devices are capable of saving high-volume data. Some Storage devices are hard drives, USB flash drives, CDs, etc.W

• MS-Windows
• Ubuntu
• Mac OS
• Fedora
• Solaris
• Free BSD
• Chrome OS
• CentOS
• Debian
• Android

The process is a program that is currently running or executing. The main role of an OS is to coordinate and handle all of these processes. There are four divisions when a program is loaded into the memory and becomes a process, those four different states of the process are ─ stack, heap, text, and data. There are two types of processes:

1. Operating System Processes
2. User Processes

The different States of Process:

• New State: a process is just created.
• Running: CPU starts to run the process’s instructions.
• Waiting: Now the process is just waiting for some events to occur and cannot run the command
• Ready: All the processes with all resources are ready and wait to get allocated to a processor because CPUs are currently not working on as per the instructions passed by the process.
• Terminate: At last, the process is completed as the process has completed execution.

• Program code cannot be changed or edited by itself.
• Local data for every client process is to be stored separately on different disks.

Paging: It is a technique of managing memory that permits the Operating system to retrieve processes from secondary to main memory. It is a non-contiguous allocation that divides each process in the form of pages.

Segmentation: It is a technique that divides processes into modules and parts of different sizes. The divided parts or modules are called segments that can be allocated to the process.

Multitasking: It is a system that works more efficiently in the use of computer hardware. This helps the system to work on more than one task at once by quickly switching between various processes or tasks. It is also called a time-sharing system.

Multiprocessing: It is a system where multiple processors are used in a computer to operate two or more different portions of the same program at the same time. It completes more work in a short time.

OS Interview Questions and Answers

The socket in Operating System is the endpoint for IPC (Inter-process Communication). The endpoint is the combination of an IP address and port number. Sockets help software developers easily create network-enabled programs. It allows the exchange of information and communication between two different processes on the same or different machines. It is used in client-server-based systems.

Types of Sockets

The four types of sockets are:

• Stream Sockets
• Datagram Sockets
• Sequenced Packet Sockets
• Raw Sockets

Starvation: It is an issue that occurs when a process has not had sufficient resources it requires to proceed with its execution for a long period. In this case, low priority processes get stuck and blocked and process that has high priority proceed further towards completion and this is all because of lack of resources.

Aging: It is a solution or technique which helps in overcoming the problem of starvation. It prioritizes the processes that are in queue for resources for a long duration. It is the best technique to resolve the process issues of starvation and adds an aging factor to the priority of each request by various processes for resources. It also makes sure that low-level queue processes will complete their execution.

A semaphore is a mechanism that gives a signal. It only holds one positive integer value which is used to solve the problem or issue of important sections in the synchronization process by using two atomic operations i.e., wait() and signal().

Types of Semaphore

• Binary Semaphore
• Counting Semaphore

• Monolithic Kernel
• MicroKernel
• Hybrid Kernel
• Nano Kernel
• Exo Kernel

MicroKernel: It is a minimal OS that only runs the important functions of Operating systems. It has a minimum number of features and functions that are required to be executed by OS. Some Examples are QNX, Mac OS X, K42, etc.

Monolithic Kernel: It is an OS structure that supports all basic features of a computer such as resource management, memory, file, etc. Some examples are Solaris, DOS, OpenVMS, Linux, etc.

Kernel: It is a system program that manages and controls all programs running on the computer or devices. It works as a bridge between the software and hardware of the computer system.

Operating System: It is a system program that performs or runs on the computer to give it an advanced user interface so it can easily operate the device.

Process: A program that is currently running by one or more than one thread. It is an important part of the modern-day Operating system.

Thread: Thread is an execution path that is composed of a program counter, thread id, stack, and set of registers within the process.

• Stack: It is used as local variables and returns addresses.
• Heap: For dynamic memory allocation it is used.
• Data: Global and static variables are stored here.
• Code: It comprises compiled program code.

It is a situation where several processes are blocked as they are holding resources and waiting to consume more resources held by another process for smooth functioning. In this case, two or more processes try to run simultaneously and also wait for each other to finish their execution as they are dependent on each other. We can see the system starts hanging whenever a deadlock problem occurs in a program.

Necessary Conditions for Deadlock

The four necessary conditions for deadlock are:

• Mutual Exclusion
• Hold and Wait
• No Pre-emption
• Circular Wait or Resource Wait

1) Mutual Exclusion Condition: It tells the resources used are non-sharable.

2) Hold and Wait Condition: It tells that there is a process running which is holding a resource and waiting for an additional resource that is currently being used or held by other processes.

3) No-Preemptive Condition: Resources cannot be shared or taken away while they are being used by processes in the execution.

4) Circular Wait Condition: It tells that the system processes make a circular list or a chain where every process in the chain is waiting for a resource that was held by the next process in the chain.

• Internal fragmentation: It happens when we deal with systems that have fixed-size allocation units.
• External fragmentation: It happens when we deal with systems that have variable-size allocation units.

• It manages all computer resources such as CPU, memory, files, processes, etc.
• It manages and enhances the interaction between components of hardware and software.
• It manages RAM to make the programs and systems work effectively and efficiently.
• It manages and controls all primary tasks of the operating system and access and use different peripherals connected to the device.
• It schedules the work of the CPU so each process is executed as efficiently as possible.