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An overview of operating system concepts, focusing on processor and user modes, kernels, system calls, and system programs. It explains the system's view of processes and resources, process abstraction, hierarchy, threads, and threading issues. The document also covers process scheduling, including preemptive and non-preemptive scheduling algorithms. It further discusses system calls, their types, and the services they provide, along with system programs and their role in creating an environment for program development and execution. The document also touches on process states, process control blocks, context switching, and thread management, including thread cancellation and signal handling. It concludes with an explanation of process scheduling queues and various scheduling algorithms, such as preemptive and non-preemptive scheduling, including examples and performance considerations. Useful for students studying operating systems and computer architecture.
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1. Write about User Modes and Kernel Modes? - A processor in a computer running Windows has two different modes: user mode and kernel mode. - The processor switches between the two modes depending on what type of code is running on the processor. Applications run in user mode, and core operating system components run in kernel mode. USER Mode If CPU is executing the user applications then CPU will be in the user mode. The User applications are games, media players, text editor, MS Office etc. Executing code in user mode has no ability to directly access some resources like Hard Disk, memory, Printer and other I/O devices. Because, to access these resources we have to use kernel mode through System Call. Note: User mode is also known as safe mode and Restricted mode. Kernel Mode Kernel mode contains OS which has full functionality to access and maintain all the hardware components. Hardware components are RAM, HARD, CPU, printer and other I/O devices. Whenever the system call generate, CPU switch to kernel mode from user mode. System call is executed in kernel mode. After execution of system call it returns back to user mode. Note: kernel mode also known as system, private or supervisory mode, protected mode because user can’t access this area directly.
Watching Movie in windows OS If we want to play movie then we have to double click on that particular video. As, that video exist in main memory and main memory is hardware. So, CPU cannot access the hardware in user mode. System have to request through system call to open the file in main memory. So, OPEN () system call is generated and CPU switch to Kernel mode. Now OS in kernel, open that particular file from main memory. After opening the file CPU switch back to user mode. Now we are watching the movie in user mode. When user want to close the file then CLOSE system call again generated. Real Life example If we go for back to withdraw money. Then the staff of bank is kernel and our command in user mode is withdraw the money. This request is executed by bank staff to withdraw the money. So, without kernel, user can’t proceed. System Call System calls are the calls (functions) that are required to invoke the services provided by the operating system. These calls provide an interface between the user program and the operating system. We call this interface as Application Program Interface. Explanation of System Call As in above diagram, User Applications run in user mode, and operating system run in kernel mode. As OS control all the hardware’s so User cannot access hardware without kernel mode. To access kernel mode user application generate a system call because System call is a source to go to kernel mode. CPU switch in two modes, while working
Services Provided by System Calls:
3. What is Process abstraction? - Abstraction means hiding the working complexity of the system. The operating system provides a layer of abstraction with the help of OSAL. - OSAL stands for Operating System Abstraction Layer, a set of APIs that a developer can use to quickly develop an application without considering the type of operating system, hardware, and background complexity of the operating system. - The main purpose of abstraction provided by the operating system is to hide the working complexity or technical details of the system. With the help of abstraction, the Operating System provides a separation between hardware and software. - Operating system provides hardware abstraction which allows the system users or developers to develop system-independent applications. - Using a hardware abstraction layer the user processes or system processes can simply use the hardware devices with the help of specific system calls. 4. Explain process Hierarchy? Write about the process and the process state and Process Control Block (PCB) When a process creates another process, then the parent and the child processes tend to associate with each other in certain ways and further. The child process can
also create other processes if required. This parent-child like structure of processes form a hierarchy, called Process Hierarchy. Process: An active program which running now on the Operating System is known as the process. For example, when you want to search something on web then you start a browser. So, this can be process.
1. Process ID: When a new process is created by the user, the operating system assigns a unique ID So, process-ID will get the values from 0 to N-1. - First process will be given ID0. - Second process will be given ID 1. - It continues till N-1. 2. Process State: A process, from its creation to completion goes through different states. Generally, a process may be present in one of the 5 states during its execution:
3. Process Priority: Process priority is a numeric value that represents the priority of each process. 4. Process Accounting Information: This attribute gives the information of the resources used by that process in its lifetime. For Example: CPU time connection time, etc. 5. Program Counter: The program counter is a pointer that points to the next instruction in the program to be executed. 6. CPU registers: A CPU register is a quickly accessible small-sized location available to the CPU. These registers are stored in virtual memory (RAM). 7. Context Switching: A context switching is a process that involves switching the CPU from one process or task to another. 8. PCB pointer: This field contains the address of the next PCB, which is in ready state. This helps the operating system to hierarchically maintain an easy control flow between parent processes and child processes. 9. List of open files: As the name suggests, It contains information on all the files that are used by that process. 10. Process I/O information: In this field, the list of all the input/output devices which are required by that process during its execution is mentioned.
Kernel level thread The kernel-level thread is implemented by the operating system. The kernel knows about all the threads and manages them. The kernel-level thread offers a system call to create and manage the threads from user-space. Components of Thread A thread has the following three components:
Types of Threaded process
6. Write about Thread Issues? Threading Issues Following threading issues are: - The fork() and exec() system call - Signal handling - Thread cancelation - Thread Pool 1. fork( ) and exec( ) system calls: The fork() call is used to create a duplicate child process. During a fork( ) call the issue that arises is whether the whole process should be duplicated or just the thread which made the fork( ) call should be duplicated. The exec( ) call replaces the whole process that called it including all the threads in the process with a new program. 2. Thread cancellation: The termination of a thread before its completion is called thread cancellation and the terminated thread is termed as target thread. Thread cancellation is of two types: 1. Asynchronous Cancellation: In asynchronous cancellation, one thread immediately terminates the target thread. 2. Deferred Cancellation: In deferred cancellation, the target thread periodically checks if it should be terminated.
The main thread libraries which are used are given below −
2. preemptive priority scheduling 1. It is a method of scheduling processes that is based on priority 2. selects the tasks to work as per the priority. 3. Lowest integer value is highest priority. 4. Highest integer value is low priority. EXAMPLE: PID ( PROCESS ID), BT (BURST TIME), AT (ARRIVAL TIME), CT (PROCESS COMPLETION TIME), TAT (TURN AROUND TIME), **WT (WAITING TIME), RT (RESPONSE TIME) PID PRIORITY AT BT CT TAT=CT- AT WT=TAT- BT RT=QUEUEFIRSTVALUE
3. Shortest Remaining Time First (SRTF) Scheduling Algorithm The SRTF scheduling algorithm is the preemptive version of the SJF scheduling algorithm in OS. This calls for the job with the shortest burst time remaining to be executed first, and it keeps preempting jobs on the basis of burst time remaining in ascending order.
