Operating System

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.