Operating System Interview Questions and Answers Web Development

An operating system serves as a conduit between the user and the hardware of the system. The operating system provides a platform for users to easily execute application programs. An operating system's job is to provide services such as memory management.

An important task of an operating system is -

• Security - The operating system gives you authentication protection to protect your privacy. This leads to unauthorized access to your data. 

• System performance - the operating system monitors overall the health of your systems. It records your service request and response time. so that it can have a complete view of the system performance. 

• File Management - These programs create, delete, copy, rename, print, and list and generally, we can manipulate the file directories. 

• Error Detection - Errors can occur in the CPU and memory hardware, in I/O devices, and in the user program. The operating system should take an appropriate approach to resolve these issues. 

• Resource Allocation - when multiple users are logged on the system or multiple jobs are running at the same time, resources such as CPU cycles, main memory, file storage etc. must be allocated to each of them. 

• Communication - one process needs to exchange information with another process. Communication may be implemented via shared memory, or by the technique of message passing, in which packets of information are moved between processes by the O.S. 

A common question in OS interview questions and answers; don't miss this one.

An operating system performs tasks like managing the files and processing and allocating the memory. Here are the types of operating systems that are widely used as follows- 

• Batch operating system - to improve utilization, the concept of a batch operating system was developed. In a batch operating system, the prime concern is CPU efficiency. The batch operating system operates in a strict one-job-at-a-time manner. With a job, it executes the programs one after another. Thus, only one program is under execution at any time. 
• Time Sharing operating system - a time sharing Operating System focuses on facilitating quick response to sub-requests made by all processors, which provides a tangible benefit to users. It is achieved by giving a fair execution opportunity to each process through two means: The OS services all processes by turn, which is called round-robin scheduling. I also prevent a process from using too much CPU time when scheduled to execute, which is called time-slicing. 
• Real-Time operating system - it is used to implement a computer application for controlling or tracking its computational tasks in a timely manner to keep abreast of external events in the activity that it controls. OS permits a user to create several processes within an application program and uses real-time scheduling to interleave the execution of processes such that the application can complete its execution within its time consistency. 
• The distributed Operating system permits a user to access resources located in other computer systems conveniently and reliably. To enhance convenience, it does not expect a user to know the location of resources in the system, which is called transparency. To enhance efficiency, it may execute parts of computation in different computer systems at the same time. It uses distributed control, i.e., it spreads its decision-making actions across different computers in the system. 

Operating systems like Windows and Linux, run on multiprocessors. Multiprocessing refers to the execution of multiple concurrent software processes in a system as opposed to a single process at any one instant. To increase the performance of an OS it uses the multiprocessor on a system. 

Types of multiprocessor systems -  

1. Symmetric multiprocessing system - Symmetric multiprocessing or SMP involves a multiprocessor computer architecture where two or more identical processors can connect to a single shared main memory. 
2. Asymmetric multiprocessing system - Asymmetric hardware systems commonly dedicated individual processors to specific tasks. ASMP application will run on one processor but outsource the smaller tasks to another. 

A variety of disk-organization techniques, collectively called redundant arrays of expensive disks(RAIDs) are commonly used to address performance and reliability issues. RAID composed of small, cheap disks were viewed as a cost-effective alternative to large, expensive disks. RAID concepts have been extended to tape drives, and for broadcasting of data. 

RAID Levels 

• Level 0 - refers to disk arrays with striping at the level of clocks but without any redundancy. 
• Level 1 - RAID level 1 refers to disk mirroring. 

• Level 2 – this level is another level of RIAD 2, which is a very high level of standard data transfer. This level uses bit-level stripping and every bit stored in different hardware. 
• Level 3 – This RAID level, helps you to transfer a very high amount of data and it also provides high efficiency. 
• Level 4 -  this level, contains block-level data stripping. It is also another unpopular level of RAID 4. 
• Level 5 – RAID level 5, ensures you create security and integrity of data stored on the hard disks .
• Level 6 - RAID level 6, is a data storage level, that lets you combine four or more hard drives into single logical hard drives. It also boost the system to work fast. 

The kernel is a medial component of an operating system that manages the operations of the computer system and the hardware. Kernel acts as a bridge between the applications and data processing performed at hardware using the interprocess communication and the system calls. The kernel does tasks like disk management, task management and file management.

Virtual memory is memory allocation in which secondary memory can be addressed in the main memory. Virtual Memory is a space where a large form of data is stored in the form of paging. Virtual memory can be implemented with the help of -  

• Demand Paging - When a process is to be executed, its pages are loaded into any available memory frames. Every address generated by the CPU is divided into two parts: a page number and page offset.  

• Demand Segmentation - A user program can be subdivided using segmentation, in which the program and its associated data are divided into a number of segments.  

Expect to come across this popular operating system interview questions and answers for freshers.

A graphical User Interface: 

• Usually, use the mouse, keyboard, and monitor. 
• Icons represent files, programs, actions, etc. 
• Various mouse buttons over objects in the interface cause various actions (provide information, options, execute function, open directory) 

A pipe acts as a  conduit allowing two processes to communicate. Pipes were one of the first IPC mechanisms in UNIX systems and it provides similar ways for processes to communicate with one another. Two common types of pipes used on both UNIX and Windows systems are ordinary pipes and named pipes. The pipe allows two processes to communicate in a standard Producer consumer fashion; the producer writes to one end and the consumer reads from the other end.

The operating system is provided for the convenience of the programmer, to make the programming task easier. 

Here, is the view of operating system services. 

• PROGRAM EXECUTION - The system must be able to load a program into memory and run the program. The program must be able to end its execution, either normally or forcefully. 

• I/O OPERATION - I/O may involve a file or an I/O device. Special functions may be desired ( such as rewinding a tape drive, or blanking a CRT screen). I/O devices are controlled by O.S 

• FILE SYSTEMS - The file system program reads, writes, creates and deletes files by name. 

• COMMUNICATIONS - Communication may be implemented via shared memory, or by the technique of message passing. 

• RESOURCE ALLOCATION - When multiple users are logged on the system or multiple jobs are running at the same time, resources such as CPU cycles, main memory, file storage etc. must be allocated to each of them. 

• ACCOUNTING - To keep track of which user uses how many and what kinds of computer resources. This record-keeping may be used for accounting. 

• ERROR DETECTION - error may occur in the CPU and memory hardware, in I/O devices, and in the user program. O.S should take appropriate action to resolve these errors. 

• PROTECTION AND SECURITY - Concurrent processes should not conflict with one another because the owners of information stored in a multiuser or networked computer system may want to regulate how that information is used. 

Only one task may be completed at a time by the process, thanks to a single thread of control. For instance, the user cannot utilize the spell checker and input characters at the same time in the same process. Many contemporary operating systems have expanded the idea of a process to let it execute numerous threads at once, allowing it to handle multiple tasks concurrently.

The PCB is enlarged on systems that support threads to provide details for each thread. To support threads, additional adjustments must be made throughout the system.

A common question in operating system interview questions for freshers, don't miss this one.

The features of the real-time operating system are -  

• Concurrency within an application - A programmer can indicate that some parts of an application should be executed concurrently with one another. The OS considers the execution of each such part as a process. 
• Process priorities - A programmer can assign priorities to processes. 
• Scheduling - The OS uses priority-based or deadline-aware scheduling. 
• Domain-Specific events interrupt - A programmer can define special situations within the external system as events, associate interrupts with them, and specify event handling actions for them. 
• Predictability - Policies and overhead of the OS should be predictable. 
• Reliability - The OS ensures that an application can continue to function even when faults occur in the computer 

FCFS stands for FIRST_COME, FIRST-SERVED (FCFS) SCHEDULING 

Requests are scheduled in the order in which they arrive in the system. The list of pending requests is organized as a queue. An example of First Come First Serve scheduling is a batch processing system in which jobs are ordered according to their arrival times and the results of a job are released to the user immediately upon completion of the job. The following example illustrates the operation of an FCFS scheduler. 

Table 7.1 Process for scheduling 

ta= 7.40 seconds 

W = 2.22 

Figure 7.1 illustrates the scheduling decisions made by the FCFS scheduling policy for the processes of Table 7.1. Process P1 is scheduled at time 0. The pending list contains P2 and P3 when P1 completes at 3 seconds, so P2 is scheduled. The Completed column shows the id of the completed process and its turnaround time(ta) and weighted turnaround(w). The mean values of ta and w are shown below the table. The timing chart shows how the processes operated. 

It is a pre-emptive algorithm. It is mostly designed for time-sharing algorithms.  This algorithm is similar to the FCFS algorithm. But, in RR scheduling are added which lets the system enables the system between the process.

Round Robin is mostly used when conducting the basic tests to ensure that new balancers have been configured correctly or not.   It is the hybrid model which works as a clock-driven approach.

A zombie process is a process that has completed its execution but still has an entry in the process table to report to its parent process. Before being removed from the process table, a child process always becomes a zombie. The parent process reads the child process's exit status and removes the child process entry from the process table.

In a multiprogramming environment, several processes may compete for a finite number of resources. A process requests resources; if the resources are not available at that time, the process enters a wait state. It may happen that waiting processes will never again change state because the resources they have requested are held by other waiting processes. 

When all four of the following criteria are met concurrently in a system, a deadlock might happen.

1. Mutual exclusion: At least one resource must be held in a non-sharable mode, which limits its use to a single process at a time. If another process requests that resource, the requesting process must be delayed until the resource has been released. 
2. Hold and wait: A process must hold at least one resource and is waiting to acquire additional resources currently being held by other processes. 
3. No pre-emption: Resources cannot be preempted; i.e., a resource can be found voluntarily by the process holding it; it, after that process, has completed its task. 
4. Circular wait: There must exist a set {P0, P1,......., Pn} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2,....., Pn-1 is waiting for a resource that Pn holds, and Pn is waiting for a resource that is held by P0. 

Thrashing occurs when a computer's performance degrades or collapses. When a system spends more time processing page faults than executing transactions, it is said to be thrashing. While processing page faults is required to reap the benefits of virtual memory, thrashing is detrimental to the system. More transactions require processing from the paging device as the page fault rate increases. The queue at the paging device grows, resulting in a longer response time for a page fault.

Bélády's anomaly occurs when some page replacement policies increase the number of page frames, resulting in an increase in the number of page faults. It happens when you use First in First Out page replacement.

An illustration of Bélády's anomaly. Nine-page faults occur when three-page frames are used. Ten-page defects happen when the number of page frames is increased to four. Page errors are highlighted in red. This can be attributed to "Penny Wise, Pound Foolish"  behavior.

A thread is equipped with its program counter (PC), register set, and stack space. Threads, unlike processes, are not independent of one another. As a result, threads share their code section, data section, and OS resources such as open files and signals with other threads.

• Microsoft Windows: Microsoft Windows is an operating system for personal computers developed by Microsoft that was released in November 1985. 

• MacOS: The Mac OS is a series of Unix-based graphical interface operating systems for Mac computers created and marketed by Apple Inc. and first released on January 24, 1984. 

• Linux: Linux is a Unix-based cross-platform operating system. The software on a computer allows applications and the computer operator to access the computer's devices and perform desired functions. It is compatible with PCs, laptops, netbooks, mobile and tablet devices, video game consoles, servers, supercomputers, and other devices. 

One of the most frequently posed basic operating system interview questions, be ready for it.  

Semaphore is a variable that has an integer value upon which the following three operations are defined: 

1. A semaphore may be initialized to a non–negative value. 
2. The wait operation decrements the semaphore value. If the value becomes negative, then the process executing the wait is blocked. 
3. The signal operation increments the semaphore value. If the value is not positive, then a process blocked by the a-wait operation is unblocked. 

Concept of operation:

Two or more processes can cooperate by means of simple signals, such that the process can be forced to stop at a specified place until it has received a specific signal. For signaling, special variables called semaphores, a process executes the primitive wait(s); if the corresponding signal has not yet been transmitted, the process is suspended until the transmission takes place.

The wait and signal primitives are assumed to be atomic; that is, they cannot be interrupted, and each routine can be treated as an indivisible step.

It is a mechanism that enables the execution of processes even when memory isn't fully utilized. Programs can be larger than physical memory, which is the key advantage that is immediately apparent in this system. User logical memory is divided from physical memory into virtual memory. Because of this split, programmers can be given access to a very large virtual memory, even when just a tiny physical memory is available.

Demand paging is frequently used to implement virtual memory. It can be used in a segmentation system as well. Virtual memory can also be provided using demand segmentation.

The goal of a DOS assault is to prevent computers from being used for any meaningful tasks by clogging them up so poorly that they cannot be accessed or damaged. This exploit manifests itself most obviously in tight loops that repeatedly request system services. DOS assaults can also use social engineering, including Internet chain letters that instruct recipients to "share this instantly to 10 of your friends, and then go to a certain URL," which jams up both the web server that everyone is directed to and the Internet mail system. 

Security systems that lock accounts after a certain number of failed login attempts are subject to DOS attacks which repeatedly attempt logins to all accounts with invalid passwords strictly in order to lock up all accounts.

The send and receive system calls are used to communicate between processes but there are different design options for implementing these calls. Message passing may be either blocking or non-blocking – also known as synchronous and asynchronous. 

1. Blocking send: The sending process is blocked until the message is received by the receiving process or by the mailbox. 
2. Non-blocking send: The sending process sends the message and resumes operation. 
3. Blocking receive: The receiver blocks until a message is an available. 
4. Non-blocking receive: The receiver retrieves either a valid message or a null. 

When you turn on your computer, the operating system is likely to boot (load into RAM) in the following sequence: 

1. When the computer's power is first turned on, the CPU initializes itself. This is accomplished by triggering a series of clock ticks generated by the system clock. 
2. Following that, the CPU searches the system's ROM BIOS for the first instruction in the startup program. This first instruction is stored in the ROM BIOS and instructs the system to run POST (Power On Self-Test) at a predetermined memory address. 
3. POST checks the BIOS chip first, then the CMOS RAM. If POST detects no battery failure, it will continue to initialize the CPU. 
4. POST also checks the hardware devices, secondary storage devices like hard drives, ports, and so on, as well as other hardware devices like the mouse and keyboard. This is done to ensure that they are in good working order. 
5. After POST ensures that all of the components are operational, the BIOS searches for an operating system to load. 
6. The operating system in most computers is loaded from the C drive onto the hard drive. The CMOS chip typically informs the BIOS of the location of the operating system. 
7. The boot sequence refers to the order of the various drives that CMOS examines when looking for the operating system. The CMOS setup can be changed to alter this sequence. 
8. After locating the appropriate boot drive, the BIOS searches for the boot record, which instructs it to locate the beginning of the operating system. 
9. The BIOS copies the files into memory after the operating system has been initialized. The boot process is then managed by the operating system. 
10. Finally, the operating system performs a final memory inventory and loads the device drivers required to control the peripheral devices. 
11. Users can use the system applications to perform a variety of tasks. 

It is an interprocess communication (IPC) mechanism that allows independent but cooperating tasks (that is, active classes) to communicate with one another within a single CPU.

Throughput is the number of units of information a system can process in a given amount of time. It is broadly applied to systems ranging from computer and network systems to organizations.

The functions of Operating System are :

1. File Management: These programs help you to create, delete, copy, rename, print, dump, list, and generally manipulate files and directories. 
2. Status Information: Some programmers only request the date, time, amount of memory or disc space, number of users, or other status data from the system. Others are more complex, providing detailed performance, logging, and debugging information. These programs format and print the output to the terminal or the other output devices or files or display it in a window of the GUI. 
3. File Modification: Several text editors may be available to create and modify the content of files stored on disk or other storage devices. For example – notepad, word and other third party applications. 
4. Programming language support: The Compilers, assemblers, debuggers, and interpreters for common programming language tools are often provided to the user with the help of the operating system. 
5. Communications: These programs provide the mechanism for creating virtual connections among process, users, and computer systems. They allow users to send messages to one another’s screens to browse web pages, to send electronic mail messages, to log in remotely, or to a transfer files from one machine to another. 
6. Application programs: most operating systems are supplied with programs that are useful in solving common problems or performing common operations. Such applications include web browsers, word processors and text formatters, spreadsheets, and database systems. 

A socket is defined as an endpoint for communication. A pair of processes communicating over a network employs identify a pair of sockets-one for each process. A socket is identified by an IP address with a port number. This way we can able to identified the personal computer.  

Sockets typically employ client-server architecture. The server listens to a specific port for incoming client requests. The server accepts a connection from the client socket after receiving a request to complete the connection. Severs implementing specific services (such as telnet, FTP, and I-HTTP) listen to well-known ports (a telnet server listens to port 23; an FT server listens to port 21; and a web, or HTTP, server listens to port 80). All ports below 1024 are considered well-known; we can use them to implement standard services. When a client process initiates a request for a connection, it is assigned a port by its host computer. This port is some arbitrary number greater than 1024.

Expect to come across this popular operating system interview questions and answers.

Starvation: Indefinite blocking or starvation is a phenomenon associated with Priority scheduling algorithms in which a process ready for the CPU (resources) can wait indefinitely due to low priority. A steady stream of higher-priority processes in a heavily loaded computer system can prevent a low-priority process from ever receiving the CPU. According to rumors, Priority Scheduling was used in IBM 7094 at MIT in 1967, and they discovered a low-priority process that had not been submitted until 1973. 

Aging: Aging is a method of gradually raising the priority of processes that have been waiting in the system for a long time. For example, if the priority range is 127 (low) to 0, we could raise the priority of a waiting process by 1 every 15 minutes. Even if a process had a starting priority of 127, it would take no more than 32 hours for the priority 127 processes to age to a priority-0 process. 

Assembly language is a type of low-level language and program it is more commonly known as an assembler, with the inverse program known as a disassembler. 

An assembler converts the assembly codes into binary codes and then it assembles the machine understandable code into the main memory of the computer, making it ready for execution. 

Functions of an assemblers are given below: 

• It allows the programmer to use mnemonics while writing source code programs, which are easier to read and follow. 

• It allows the variables to be represented by symbolic names, not as memory locations. 

• It translates mnemonic operations codes to machine code and corresponding register addresses. 

• It checks the syntax of the assembly program and generates diagnostic messages on syntax errors. 

• It assembles all the instructions in the main memory for execution. 

• In case of large assembly programs, it also provides linking facility among the subroutines. 

• It facilitates the generation of output on required output medium. 

Process can be defined as: 

• A program in execution. 
• An instance of a program running on a computer. 
• The entity that can be assigned to and executed on a processor. 
• A unit of activity characterized by the execution of a sequence of instructions, a current state, and an associated set of system resources.

A process generally also includes the process stack, which contains temporary data, and a data section, which contains global variables. 

As a process executes, it changes the state 

• New: the process is being created 
• Running: instructions are being executed 
• Waiting: the process is waiting for some event to occur 
• Ready: the process is waiting to be assigned to a processor 
• Terminated: the process has finished execution. 

Spooling is the temporary storage of data for use and execution by a device, program, or system. Data is sent to and stored in memory or other volatile storage until it is requested for execution by a program or computer.

SPOOL stands for concurrent peripheral operations online. In most cases, the spool is kept in physical memory, buffers, or I/O device-specific interrupts on the computer. The spool is processed in ascending order, using a first-in, first-out (FIFO) algorithm.

divided into pages, which are identical-sized blocks. The pages of a process are loaded into any free memory frames prior to execution. Each address produced by the CPU has two components: a page number (p) and a page offset (d). A page table's index is created using the page number. Each page in physical memory has a base address, which is contained in the page table. The physical address that is supplied to the memory unit is defined by this base address and the gauge offset. 

The page size is defined by the hardware. The size of a page is typically a power of 2 varying between 512 bytes and 8192 bytes per page, depending on the computer architecture. The selection of a power of 2 as a page size makes the translation of a The high order m-m bits of a logical address identify the page number, while the n-low order bits designate the page offset when the logical address space is 2m and the page size is 2n addressing units. Thus the logical address is as follows: 

Page number page offset 

P d 

m-n n 

Where p is an index into the page table and d is the displacement within the page. 

It is the mechanism provided by the operating system for communicating between processes This communication could include a process informing another process of an event or data being transferred from one process to another.

Pipe: A pipe is a unidirectional data channel. A two-way data channel between two processes can be created using two pipes. This employs conventional input and output methods. Pipes are used in both POSIX and Windows operating systems. 

Socket: The socket is the network endpoint for sending and receiving data. This applies to data sent between processes on the same computer as well as data sent between computers on the same network. For interprocess communication, most operating systems employ sockets. 

There are three different methods for dealing with the deadlock problem: 

• A protocol can be used to guarantee that the system never enters a stalemate state. 
• We can let the system get stuck in deadlock and then fix it. 
• We can completely ignore the issue and act as though system deadlocks never happen. The majority of operating systems, including UNIX, employ this solution.

A common question in OS interview questions, don't miss this one.

There are two types of fragmentation i.e. Internal fragmentation and External Fragmentation. 

• Internal Fragmentation: Internal fragmentation occurs when memory is partitioned into mounted-sized blocks. When a memory method is requested, the mounted-sized block is assigned to the method. When the memory allotted to the method is slightly larger than the memory requested, the difference is referred to as internal fragmentation. 
• External Fragmentation: External fragmentation occurs when there is sufficient space in memory to satisfy a method's memory request. However, the process's memory request cannot be met because the memory provided is not contiguous. External fragmentation will occur whether you use a first-fit or best-fit memory allocation strategy. 

A Process Scheduler assigns different processes to the CPU based on specific scheduling algorithms. There are six popular process scheduling algorithms that we will discuss in this chapter.

First-come, first-served basis (FCFS), Priority of Shortest-Job-Next (SJN), Scheduling Shortest Remaining Time, Round Scheduling Robin(RR), Scheduling of Multiple-Level Queues.

The kernel is the central component of an operating system that manages computer and hardware operations. 

Types of Kernel:  

1. Monolithic Kernel: It is a type of kernel in which all operating system services run in kernel space. It is dependent on the components of the system. 
2. Micro Kernel: Kernel types take a minimalist approach. It has virtual memory and thread scheduling capabilities. 
3. Hybrid Kernel: It is a hybrid of a monolithic kernel and a microkernel. It combines the speed and design of a monolithic kernel with the modularity and stability of a microkernel. 
4. Exo Kernel: It is the type of kernel that adheres to the end-to-end principle. It employs as few hardware abstractions as possible. It allots physical resources to various applications. 
5. Nano Kernel: It is a kernel that provides hardware abstraction but no system services. Because the Micro Kernel lacks system services, the Micro Kernel and Nano Kernel have become analogous. 

Compiler: A compiler is a computer program that reads code written in a high-level language such as FORTRAN, PL/I, COBOL, and so on. It is capable of translating it into the same program in a low-level language such as machine language. The compiler also detects various errors that occur during the compilation of a program. 

Interpreter: An interpreter is a program that directly executes programming code rather than simply translating it into another format. It sequentially translates and executes programming language statements. In comparison to a compiler, an interpreter takes less time to interpret a source program. 

Every machine in a cluster runs a distributed lock manager (DLM), which has an identical copy of a cluster-wide lock database. A DLM, in this way, allows software applications distributed across a cluster on multiple machines to synchronize their access to shared resources.

Compaction is a memory management technique that compacts a running system's free space in order to reduce fragmentation and improve memory allocation efficiency. Many modern operating systems, including Windows, Linux, and Mac OS X, use compaction. As shown in the diagram, we have some used memory (in black) and some unused memory (white color). The memory that has been used is combined. All of the empty spaces are combined. This is known as compaction. This is done to avoid fragmentation, but it consumes an inordinate amount of CPU time.

Static Linking: The process of incorporating the code of a program or library into a program so that it is linked at compile time is known as static linking. Because the library's code is copied into the executable file, it generally results in faster program startup times. It makes debugging programs easier because all symbols are resolved at compile time. 

Dynamic Linking: The library is not copied into the program's code in this case. Instead, the program contains a reference to the location of the library. When you run the program, it will load the library from that location.