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"When the CPU executes an instruction that is not available in the memory, a Page Fault occurs. This means a page is being referenced, but the same is not present in the memory. The Operating System is now responsible for bringing the appropriate page into memory from the disk. The total turnaround time of a process is divided into CPU time and IO time. Disk I/O takes a longer time than CPU and the process must wait until the page has been fetched from the disk. A module of the Operating System called the Page Fault Handler is given the control. It gets the Page Table address and the page number of the faulting page along with the information needed to bring the required page into the main memory from the backing store. The information also includes the disk address of the page in the backing store. A longer IO is detrimental to the performance of the system as a whole because the CPU would remain idle for a considerable amount of time which is not feasible for a good Operating System design. So, the CPU would schedule another process that could execute at the same point of time when the Disk IO is being performed so that the CPU idle time can be avoided. "
"The Operating System searches for the page in the File Map Table that contains the page number and the corresponding disk address of the page. After acquiring the disk address, the OS schedules a disk read operation to get the page from the disk/physical storage. The Memory Map Table is then searched to locate the first free frame for that page to be loaded. After getting the free frame number, the OS updates the Page Map Table with the frame number and the reference bit. Now the presence bit is set to indicate the presence of the page in memory. The CPU is now instructed to restart the execution. If the probability of the page fault is ""p"" and the memory access time is ""ma,"" then the total time required is "
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