Operating System Questions Long
A page fault is an exception that occurs when a program or process attempts to access a page of memory that is currently not mapped to physical memory. This can happen due to various reasons such as the page being swapped out to disk, not yet allocated, or being accessed for the first time.
When a page fault occurs, the operating system needs to handle it in order to bring the required page into physical memory and resume the execution of the program. The handling of a page fault typically involves the following steps:
1. Interrupt: The page fault triggers an interrupt, which transfers control to the operating system.
2. Page fault handler: The operating system identifies the cause of the page fault and determines the appropriate action to take. It checks if the page is present in physical memory or if it needs to be fetched from secondary storage.
3. Fetching the page: If the page is not present in physical memory, the operating system initiates a page replacement algorithm to select a page to be evicted from memory to make space for the required page. The evicted page is written back to disk if it has been modified.
4. Disk I/O: If the required page is on disk, the operating system initiates a disk I/O operation to read the page into a free frame in physical memory.
5. Updating page tables: Once the required page is in physical memory, the operating system updates the page table entries to reflect the new mapping between the virtual and physical addresses.
6. Resuming execution: Finally, the operating system returns control to the program, which can now access the requested page in physical memory.
It is worth noting that handling a page fault can be a time-consuming process, as it involves disk I/O operations and potentially evicting pages from memory. To optimize performance, operating systems employ various techniques such as demand paging, where pages are fetched into memory only when they are actually needed, and page replacement algorithms, which aim to minimize the number of page faults by selecting the most suitable pages to evict.