Page fault occurs when the page is not in main memory.

Page fault is a type of interrupt caused by a program accessing a page that is not currently mapped by the memory management unit (MMU) to any physical memory location. It happens when the requested page is not present in the main memory (RAM) and needs to be brought in from secondary storage (usually the hard disk). Let's understand the reasons behind page faults in more detail.

Causes of Page Fault:
Page faults can occur due to various reasons. Some common causes include:

1. Demand Paging: Most modern operating systems use demand paging, where only the required pages are loaded into memory. This technique allows efficient use of memory resources but can lead to page faults when a program accesses a page that is not currently in main memory.

2. Page Replacement: When the memory becomes full, the operating system needs to replace some pages in the main memory with new ones. This process is called page replacement. If a requested page is not present in memory and there are no free frames available, a page fault occurs, triggering the page replacement algorithm.

3. Memory Swapping: In some cases, the operating system may swap out entire processes or parts of processes to secondary storage to free up memory for other processes. When a swapped-out page is accessed, a page fault occurs, and the corresponding page is brought back into memory.

4. Stack and Heap Growth: As programs execute, the stack and heap segments of the process may grow dynamically. If the stack or heap needs to expand, new pages may need to be allocated, leading to page faults.

5. Copy-on-Write: Copy-on-Write (COW) is a memory management technique used to optimize the creation of child processes. Initially, child processes share the same physical memory with the parent process. However, when a write operation is performed on a shared page, a copy of the page is made, and a page fault occurs.

6. Invalid Memory Access: If a program tries to access an invalid memory location, it may result in a page fault.

Conclusion:
In summary, a page fault occurs when the requested page is not present in the main memory and needs to be fetched from secondary storage. This can happen due to demand paging, page replacement, memory swapping, stack/heap growth, copy-on-write, or invalid memory access. Handling page faults efficiently is crucial for maintaining system performance and ensuring smooth execution of processes.