pagefault

A page fault is an event that occurs when a program tries to access a page of memory that is not currently loaded into main memory. This typically happens in operating systems that use virtual memory. Virtual memory allows programs to use more memory than is physically available by storing parts of the program on secondary storage, such as a hard drive, and loading them into main memory only when needed.

When a page fault occurs, the processor detects that the requested page is not in RAM. It then generates an interrupt, which signals the operating system to handle the situation. The operating system's page fault handler takes over. It first checks if the page the program is trying to access is valid but simply not in memory. If it is, the operating system locates the page on the secondary storage, finds a free frame in main memory (or evicts an existing page to make space), reads the required page from storage into the memory frame, and updates the page table to reflect the page's new location. Finally, the operating system restarts the instruction that caused the page fault.

If the accessed page is invalid (e.g., it's outside the program's allocated memory space), the operating system usually terminates the program, indicating an error. Page faults are a normal part of virtual memory operation, but frequent page faults, known as "thrashing," can significantly degrade system performance because the system spends too much time moving pages between memory and disk.