বিবরণ:-• Arrival Time: Time at which the process arrives in the ready queue. • Completion Time: Time at which the process completes its execution. • Burst Time: Time required by a process for CPU execution. • Turn Around Time: Time Difference between completion time and arrival time. Turn Around Time = Completion Time – Arrival Time • Waiting Time(W.T): Time Difference between turn around time and burst time. Waiting Time = Turn Around Time – Burst Time Response time is the total amount of time it takes to respond to a request for service.

বিবরণ:-Variable (or dynamic) Partitioning in Operating System Variable partitioning is part of the contiguous allocation technique. It is used to alleviate the problem faced by fixed partitioning. As opposed to fixed partitioning, in variable partitioning, partitions are not created until a process executes. At the time it is read into main memory, the process is given exactly the amount of memory needed. This technique, like the fixed partitioning scheme previously discussed have been replaced by more complex and efficient techniques. Various features associated with variable partitioning. • Initially RAM is empty and partitions are made during the run-time according to process’s need instead of partitioning during system configure. • The size of partition will be equal to incoming process. • The partition size varies according to the need of the process so that the internal fragmentation can be avoided to ensure efficient utilisation of RAM. • Number of partitions in RAM is not fixed and depends on the number of incoming process and Main Memory’s size. Advantages of Variable Partitioning 1. No Internal Fragmentation: In variable Partitioning, space in main memory is allocated strictly according to the need of process, hence there is no case of internal fragmentation. There will be no unused space left in the partition. 2. No restriction on Degree of Multiprogramming: More number of processes can be accommodated due to absence of internal fragmentation. A process can be loaded until the memory is empty. 3. No Limitation on the size of the process: In Fixed partitioning, the process with the size greater than the size of the largest partition could not be loaded and process can not be divided as it is invalid in contiguous allocation technique. Here, In variable partitioning, the process size can’t be restricted since the partition size is decided according to the process size. Disadvantages of Variable Partitioning 1. Difficult Implementation: Implementing variable Partitioning is difficult as compared to Fixed Partitioning as it involves allocation of memory during run-time rather than during system configure. 2. External Fragmentation: There will be external fragmentation inspite of absence of internal fragmentation. For example, suppose in above example- process P1(2MB) and process P3(1MB) completed their execution. Hence two spaces are left i.e. 2MB and 1MB. Let’s suppose process P5 of size 3MB comes. The empty space in memory cannot be allocated as no spanning is allowed in contiguous allocation. The rule says that process must be contiguously present in main memory to get executed. Hence it results in External Fragmentation.

বিবরণ:-• A page fault occurs when a page is not found in the memory and needs to be loaded from the disk. • If a page fault occurs and all memory frames have been already allocated, then the replacement of a page in memory is required on the request of a new page. This is referred to as demand-paging. • The choice of which page to replace is specified by page replacement algorithms. The commonly used page replacement algorithms are FIFO, LRU, optimal page replacement algorithms, etc. • Generally, on increasing the number of frames to a process’ virtual memory, its execution becomes faster as fewer page faults occur. Sometimes the reverse happens, i.e. more page faults occur when more frames are allocated to a process. This most unexpected result is termed Belady’s Anomaly. • This phenomenon is commonly experienced in the following page replacement algorithms: 1. First in first out (FIFO) 2. Second chance algorithm Random page replacement algorithm

বিবরণ:-A process state diagram for pre-emptive scheduling is: New A program which is going to be picked up by the OS into the main memory is called a new process. Ready Whenever a process is created, it directly enters in the ready state, in which, it waits for the CPU to be assigned. The OS picks the new processes from the secondary memory and put all of them in the main memory. Running One of the processes from the ready state will be chosen by the OS depending upon the scheduling algorithm. Hence, if we have only one CPU in our system, the number of running processes for a particular time will always be one. Block or wait From the Running state, a process can make the transition to the block or wait state depending upon the scheduling algorithm or the intrinsic behavior of the process. Completion or termination When a process finishes its execution, it comes in the termination state. All the context of the process (Process Control Block) will also be deleted the process will be terminated by the Operating system. Suspend ready A process in the ready state, which is moved to secondary memory from the main memory due to lack of the resources (mainly primary memory) is called in the suspend ready state. Suspend wait Instead of removing the process from the ready queue, it's better to remove the blocked process which is waiting for some resources in the main memory. Since it is already waiting for some resource to get available hence it is better if it waits in the secondary memory and make room for the higher priority process.

বিবরণ:-There are basically four main sections through which each of the process has to pass through. The universal algorithm is:- Critical section:- Consider a system consist of n processes P0 to Pn every process has a segment of code, called as critical section, in which the process perhaps changing common variables updating a table, writing a file and so on. Therefore critical section is that area of code in which the processors try to access the shared information. Therefore critical section occurs in the area where race condition occurs. Entry section:- Code just previous to the critical section is termed as the entry section. Every processor must request permission to enter into the critical section, the area of code which execute this request is called as the entry section. Exit section:- The code segment just subsequent to the critical section is termed as exit section. Remainder section:- The code remaining subsequent to the exit section is remainder section.

বিবরণ:-Concept: Semaphore is a counting variable used to synchronize the processes. Two atomic operations that can be performed are wait and signal. Explanation: Wait: It controls the entry of processes in the critical section. When semaphore value becomes negative or zero, then process is blocked. It decreases the value of semaphore by 1. Signal: It controls the exit of processes from the critical section. It increases the value of semaphore by 1. So, in order to allow only one process to enter the critical section, binary semaphore is initialized to 1. Because after entering critical section, semaphore values become 0. After this, no other process can enter the critical section.