Operating Systems Exam Study Notes

An operating system (OS) is a software program that manages the hardware and software resources of a computer system. It provides a platform for other software to run, and it provides a user interface for interacting with the computer. The operating system manages tasks such as memory allocation, process scheduling, file management, and input/output operations.

Schedulers are responsible for managing the execution of processes or threads on a computer system. They determine which process or thread gets to run on the CPU at any given time. This is a crucial aspect of operating system management, as it determines the overall performance and efficiency of the system.

Memory management is vital in operating systems because it ensures that the computer's limited memory resources are allocated and utilized efficiently. It prevents different processes from interfering with each other's memory space and helps optimize the overall performance of the system. Memory management handles allocation, deallocation, and protection of memory space.

<p>The goal of disk scheduling is to optimize the way data is accessed from the hard disk drive. By managing the order in which requests to read or write data are processed, disk scheduling algorithms aim to minimize the time it takes to complete disk operations. This can significantly improve the system's overall performance by reducing the amount of time spent waiting for disk access.</p> Signup and view all the answers

<p>A deadlock occurs when two or more processes are stuck in a state where they are waiting for each other to release resources that they need to continue their execution. This creates a circular dependency, preventing any of the processes from making progress. Deadlocks can arise in operating systems due to issues like resource allocation, improper synchronization, or problems in the design of the application. This can lead to severe performance degradation and even system crashes.</p> Signup and view all the answers

<p>Rotational latency refers to the time it takes for the desired sector on a hard disk drive to rotate under the read/write head. It's one of the key delays involved in disk access. Since disks rotate, the head doesn't immediately land on the correct location. This delay depends on the disk's rotation speed and the position of the desired sector.</p> Signup and view all the answers

<p>A scheduling queue in an operating system is a data structure that holds a list of processes or threads that are waiting to be executed by the CPU. Processes are added to the queue based on their priorities, the type of process, or other scheduling policies. The scheduler then selects processes from this queue to run on the CPU, based on its scheduling algorithm.</p> Signup and view all the answers

<p>Time-sharing is a technique in operating systems where multiple processes or users share the CPU's time. The OS divides the CPU's time into small intervals (time slices), and each process gets to execute for a short period before the CPU is given to another process. This creates an illusion that multiple processes are running simultaneously, even though they are sharing a single CPU.</p> Signup and view all the answers

<p>Process states in operating systems represent the different stages a process goes through during its lifecycle. The common states include: * <strong>New:</strong> The process is being created. * <strong>Ready:</strong> The process is ready to execute and is waiting for the CPU. * <strong>Running:</strong> The process is currently executing on the CPU. * <strong>Waiting:</strong> The process is waiting for some event to occur (e.g., I/O completion). * <strong>Terminated:</strong> The process has completed execution.</p> Signup and view all the answers

<p>Swapping is a memory management technique used in operating systems where processes are moved between main memory (RAM) and secondary storage (hard disk) to optimize memory utilization. When a process is not currently in use, it can be swapped out to secondary storage to free up memory for other processes. When the process is needed again, it is swapped back into main memory.</p> Signup and view all the answers

<p>File access methods refer to different ways in which data can be accessed from files stored on a storage device. Common file access methods include: * <strong>Sequential access:</strong> Data is read or written in a linear order, starting from the beginning of the file. * <strong>Direct access:</strong> Data can be accessed directly at any specific location in the file, without reading the data before it. * <strong>Indexed sequential access:</strong> Data is accessed sequentially but also supports direct access through an index.</p> Signup and view all the answers

<p>Mutual exclusion is a key factor that can contribute to deadlocks in operating systems. It means that only one process can access a shared resource at a time. If multiple processes require the same resource and access it in a mutually exclusive manner, it can lead to a situation where each process is waiting for the other to release the resource, resulting in a deadlock.</p> Signup and view all the answers

<p>Threads in an operating system are lightweight processes that share the same address space and resources. They are a way of creating multiple concurrent execution paths within a single process. By dividing a process into threads, you can improve efficiency and parallelize tasks within the process. Each thread has its own stack, registers, and program counter, allowing them to execute independently within the process.</p> Signup and view all the answers

<p>A distributed operating system manages a set of interconnected computers (nodes) as a single system. These nodes can communicate and share resources over a network. It allows users to access resources available across multiple machines and manage the resources as if they were part of a single computer. Examples include cloud computing platforms and distributed file systems.</p> Signup and view all the answers

<p>Average Waiting Time: 3.25, Average Turnaround Time: 6.25 (A)</p> Signup and view all the answers

<ol> <li>First Fit: 357KB in 400KB, 210KB in 250KB, 468KB in 600KB, 491KB not allocated.</li> <li>Best Fit: 357KB in 400KB, 210KB in 300KB, 468KB in 500KB, 491KB in 600KB (B)</li> </ol> Signup and view all the answers

<p>A directory structure (or file system) is a hierarchical organization of files on a storage device. It's like a tree, where folders (directories) contain other folders and files. This structure makes it easier to organize, locate, and access files. It organizes files into logical groups and establishes relationships between them. Types of directory structures:</p> <ul> <li> <strong>Tree-structured:</strong> A hierarchical structure with a root directory and subdirectories branching out. * <strong>Linear:</strong> Files are arranged in a linear sequence, similar to a list. * <strong>Network:</strong> More complex structures with connections between different subdirectories.</li> </ul> <p>The directory structure plays a crucial role in file management, enabling users to efficiently organize and maintain their files on the computer.</p> Signup and view all the answers

<p>Deadlock prevention and deadlock avoidance are two strategies for addressing the issue of deadlocks in operating systems:</p> <ul> <li> <p><strong>Deadlock prevention:</strong> This approach aims to prevent deadlocks from occurring by designing the system in a way that avoids the conditions necessary for a deadlock to form. This typically involves measures like preventing mutual exclusion, ensuring that processes request resources in a specific order, or preventing a process from holding resources while waiting for other resources.</p> </li> <li> <p><strong>Deadlock avoidance:</strong> This approach involves analyzing the current state of the system and the resource requests of processes to determine whether granting a request could lead to a deadlock. It uses algorithms that allocate resources only if they can be granted without creating a deadlock situation. The system must have information about the resources needed by each process to determine if a deadlock can potentially occur.</p> </li> </ul> <p>Deadlock prevention focuses on preventing the conditions that cause deadlocks, while deadlock avoidance involves actively avoiding the deadlocks by making decisions based on the current system state.</p> Signup and view all the answers

<p>An operating system (OS) is a software program that manages the hardware and software resources of a computer system. It acts as an intermediary between the user and the computer's hardware. It provides a user interface to interact with the computer, manages the execution of applications, allocates resources, and ensures the smooth operation of the system.</p> Signup and view all the answers

<p>A system call is a request from a running application to the operating system to perform a service. This could involve accessing hardware resources, managing files, or controlling processes. System calls provide a way for applications to interact with the operating system's core functions.</p> Signup and view all the answers

<p>A deadlock occurs when two or more processes are blocked indefinitely, each waiting for a resource that is held by another process in the cycle. It's a situation where a circular dependency exists, preventing any progress. Deadlocks can lead to a system hang or freeze.</p> Signup and view all the answers

<p>For a deadlock to occur, four necessary conditions must hold:</p> <ol> <li> <p><strong>Mutual Exclusion:</strong> Resources are non-sharable. Only one process can access a resource at a time.</p> </li> <li> <p><strong>Hold and Wait:</strong> A process holding at least one resource is waiting for another resource held by another process.</p> </li> <li> <p><strong>No preemption:</strong> A resource allocated to a process cannot be forcibly taken from it, the process must release it voluntarily.</p> </li> <li> <p><strong>Circular Wait:</strong> A circular chain of processes exists, where each process is waiting for a resource held by the next process in the chain.</p> </li> </ol> <p>Breaking one of these conditions can help prevent deadlocks.</p> Signup and view all the answers

<p>The operating system structure refers to the way its components (processes, modules, functions) are organized. There are two main approaches:</p> <ul> <li> <strong>Simple Structure:</strong> It comprises a single monolithic kernel that contains all the operating system components and functions. This approach is straightforward but can be less flexible. * <strong>Layered Structure:</strong> It divides the operating system into different layers, with each layer providing a specific set of services. Lower layers provide more fundamental services, while higher layers build on top of them.</li> </ul> <p>The layered structure offers modularity, increased security, and easier debugging.</p> Signup and view all the answers

<p>Deadlock detection refers to techniques used in operating systems to identify if a deadlock has occurred within the system. This involves monitoring the resource allocation and process states to detect a circular dependency among the processes that would indicate a deadlock.</p> <p>Deadlock detection involves algorithms that analyze the resource allocation graph and determine whether a cycle exists in the graph, indicating a deadlock situation.</p> Signup and view all the answers

<p>Deadlock and starvation are two distinct but related concepts in operating systems:</p> <ul> <li> <p><strong>Deadlock:</strong> A deadlock is a situation where two or more processes are blocked indefinitely, each waiting for a resource that is currently held by another process in the cycle. This creates a circular dependency and prevents any process from making progress.</p> </li> <li> <p><strong>Starvation:</strong> Starvation refers to a situation where a process is repeatedly denied access to resources even when they are available, while other processes are getting their fair share. It occurs when the scheduler or resource allocation mechanism consistently favors other processes, preventing the starving process from making progress.</p> </li> </ul> <p>Both deadlocks and starvation can significantly affect system performance, but the underlying causes and mechanisms differ.</p> Signup and view all the answers

<p>A batch operating system processes jobs in a batch mode where the user does not interact directly with the computer during execution. Jobs are collected into batches and run sequentially, without immediate user interaction.</p> <p><strong>Advantages:</strong></p> <ul> <li>Efficient for repetitive tasks</li> <li>Suitable for large processing jobs</li> <li>Reduces CPU idle time.</li> </ul> <p><strong>Disadvantages:</strong></p> <ul> <li>Limited interactivity</li> <li>Slow processing for interactive tasks</li> <li>Difficult to debug and troubleshoot errors.</li> </ul> Signup and view all the answers

<p>System calls are specific instructions used by applications to request services from the operating system. Different types of system calls include:</p> <ul> <li> <strong>Process Management:</strong> Creating, terminating, suspending, and resuming processes.</li> <li> <strong>Memory Management:</strong> Allocating and freeing memory, setting memory protections.</li> <li> <strong>File Management:</strong> Creating, deleting, reading, writing, and manipulating files.</li> <li> <strong>Device Management:</strong> Controlling access to devices like printers, disks, and network interfaces.</li> <li> <strong>Communication:</strong> Sending and receiving messages between processes, establishing network connections.</li> <li> <strong>Security:</strong> Enforcing access controls, managing user authentication.</li> </ul> <p>Specific system calls vary depending on the operating system and the services provided.</p> Signup and view all the answers

<p>In operating systems, burst time and response time are two important metrics used to analyze process scheduling and performance:</p> <ul> <li> <p><strong>Burst Time:</strong> The burst time of a process is the amount of CPU time it needs to execute its tasks. It can vary depending on the process's complexity and the nature of the tasks it performs.</p> </li> <li> <p><strong>Response Time:</strong> The response time of a process refers to the time it takes for the OS to first respond to the process's request or to acknowledge the process after it is submitted. It is the time from when a process is ready to execute to the time it starts executing.</p> </li> </ul> Signup and view all the answers

<p>Static loading and dynamic loading are two methods of loading programs into memory:</p> <ul> <li> <p><strong>Static Loading:</strong> The entire program is loaded into memory before execution begins. The operating system allocates a fixed amount of memory for the program at the time of loading. Suitable for programs with known memory requirements.</p> </li> <li> <p><strong>Dynamic Loading:</strong> Only the essential parts of the program are loaded at startup, and other parts are loaded later as needed. This allows more efficient use of memory, especially for programs that need large memory regions.</p> </li> </ul> <p>Dynamic loading often involves loading libraries or modules when they are first needed, which improves resource utilization and flexibility, especially for applications with modular designs.</p> Signup and view all the answers

<p>In operating systems, files are associated with metadata called file attributes. These attributes provide information about the file to the operating system and users. Some common file attributes include:</p> <ul> <li> <strong>Name:</strong> The unique identifier that the user can use to refer to the file.</li> <li> <strong>Size:</strong> The total number of bytes occupied by the file.</li> <li> <strong>Type:</strong> The type of file (e.g., text, image, executable, etc.).</li> <li> <strong>Location:</strong> The location of the file within the file system (e.g., directory path).</li> <li> <strong>Creation/Modification Dates:</strong> Dates and times when the file was created and last modified</li> <li> <strong>Protection:</strong> Permissions that specify access rights for different users or groups (e.g., read, write, execute).</li> <li> <strong>Links:</strong> References to other files, allowing the sharing of data between multiple files.</li> </ul> Signup and view all the answers

<p>Process and program are distinct concepts in operating systems:</p> <ul> <li> <p><strong>Program:</strong> A program is a set of instructions written in a programming language. It is a passive entity that exists as a file on storage, but not currently running.</p> </li> <li> <p><strong>Process:</strong> A process is an instance of a program in execution. It includes the program code, the data the program is working on, and the resources (CPU, memory, files, etc.) assigned to it. A process is an active entity that is running and executing on the CPU.</p> </li> </ul> Signup and view all the answers

<p>Paging and page fault are related concepts in memory management in operating systems:</p> <ul> <li> <p><strong>Paging:</strong> Paging is a memory management technique that divides the virtual memory of a process into fixed-size units called pages. The physical memory (RAM) is also divided into units called page frames. The OS manages the movement of pages between main memory and secondary storage.</p> </li> <li> <p><strong>Page Fault:</strong> A page fault occurs when a process tries to access a page that is not currently present in the main memory (RAM). The OS then has to retrieve the missing page from secondary storage and load it into memory before the process can continue execution.</p> </li> </ul> Signup and view all the answers

<p>File access methods refer to different ways in which data can be accessed from files stored on a storage device. The choice of access method depends on the type of data, the frequency of access, and performance requirements.</p> <p>Some common file access methods are:</p> <ul> <li> <strong>Sequential Access:</strong> Data is accessed in linear order, starting from the beginning of the file.</li> <li> <strong>Direct Access:</strong> Data can be accessed directly at any specific location in the file, without needing to read the data before it.</li> <li> <strong>Indexed Sequential Access:</strong> A mix of sequential and direct access, using an index to access records directly while maintaining the ability to access data sequentially.</li> </ul> Signup and view all the answers

<p>Average Waiting Time: 2.8, Average Turnaround Time: 6.8 (C)</p> Signup and view all the answers

<p>LRU (Least Recently Used) is a page replacement algorithm used in operating systems to manage the movement of pages between main memory and secondary storage. It aims to optimize memory utilization by replacing the page that was least recently used, with the goal of reducing page faults.</p> <p><strong>Algorithm:</strong> A list tracks the pages in memory and their order of use (most recently used to least recently used). When a page fault occurs, the page at the end of the list (least recently used) is replaced. The new page is added to the beginning of the list as the most recently used.</p> <p><strong>Example Table:</strong></p> <p>Assume a page frame size of 3, a reference string 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2</p> <table> <thead> <tr> <th align="left">Reference String</th> <th align="left">Page Frame 1</th> <th align="left">Page Frame 2</th> <th align="left">Page Frame 3</th> <th align="left">Page Fault</th> </tr> </thead> <tbody> <tr> <td align="left">7</td> <td align="left">7</td> <td align="left"></td> <td align="left"></td> <td align="left">Y</td> </tr> <tr> <td align="left">0</td> <td align="left">7</td> <td align="left">0</td> <td align="left"></td> <td align="left">Y</td> </tr> <tr> <td align="left">1</td> <td align="left">7</td> <td align="left">0</td> <td align="left">1</td> <td align="left">Y</td> </tr> <tr> <td align="left">2</td> <td align="left">7</td> <td align="left">0</td> <td align="left">2</td> <td align="left">Y</td> </tr> <tr> <td align="left">0</td> <td align="left">7</td> <td align="left">0</td> <td align="left">2</td> <td align="left">N</td> </tr> <tr> <td align="left">3</td> <td align="left">7</td> <td align="left">0</td> <td align="left">3</td> <td align="left">Y</td> </tr> <tr> <td align="left">0</td> <td align="left">7</td> <td align="left">3</td> <td align="left">0</td> <td align="left">N</td> </tr> <tr> <td align="left">4</td> <td align="left">7</td> <td align="left">3</td> <td align="left">4</td> <td align="left">Y</td> </tr> <tr> <td align="left">2</td> <td align="left">7</td> <td align="left">3</td> <td align="left">2</td> <td align="left">N</td> </tr> <tr> <td align="left">3</td> <td align="left">2</td> <td align="left">3</td> <td align="left">7</td> <td align="left">N</td> </tr> <tr> <td align="left">0</td> <td align="left">2</td> <td align="left">3</td> <td align="left">0</td> <td align="left">N</td> </tr> <tr> <td align="left">3</td> <td align="left">2</td> <td align="left">3</td> <td align="left">0</td> <td align="left">N</td> </tr> <tr> <td align="left">2</td> <td align="left">2</td> <td align="left">3</td> <td align="left">0</td> <td align="left">N</td> </tr> <tr> <td align="left">1</td> <td align="left">1</td> <td align="left">3</td> <td align="left">0</td> <td align="left">Y</td> </tr> <tr> <td align="left">2</td> <td align="left">1</td> <td align="left">3</td> <td align="left">2</td> <td align="left">N</td> </tr> </tbody> </table> <p>Total Page Faults: 9</p> <p>LRU is a common algorithm used in operating systems to manage page replacement, as it often performs well in practice by keeping frequently used pages in memory.</p> Signup and view all the answers

<p>The SCAN disk scheduling algorithm, also known as the elevator algorithm, is a disk scheduling algorithm that aims to optimize the head movement of a disk drive by moving the head in one direction (up or down) until it reaches the outermost or innermost cylinder, then reverses direction to serve the next request. It serves requests in order as it scans the cylinders.</p> <p><strong>Algorithm:</strong></p> <ol> <li>The disk head starts at the current position and scans in one direction (e.g., from outermost to innermost cylinder).</li> <li>It serves all requests in the queue that are in the same direction of the head movement.</li> <li>When the head reaches the end of the scan range, it changes direction and continues serving requests in the other direction.</li> <li>This process repeats, ensuring the head always continuously moves in one direction.</li> </ol> <p><strong>Example:</strong> Assume the following request queue: 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 The disk head is currently at cylinder 20.</p> <p>The SCAN algorithm would first go towards the outermost cylinder, serving requests 25, 30, 35, 40, 45, 50. Then, it would change direction and serve requests 10, 15, 5. The head would continue scanning in this pattern, moving back and forth between the outermost and innermost cylinders.</p> <p><strong>Advantages:</strong></p> <ul> <li>Reduces average seek time and reduces the time taken to complete requests.</li> <li>Prevents the head from moving back and forth between the same cylinders.</li> </ul> <p><strong>Disadvantages:</strong></p> <ul> <li>Requests on the opposite end of the scan direction may be delayed significantly.</li> </ul> Signup and view all the answers