A page fault occurs when a program attempts to access a block of memory that is not stored in the physical memory, or RAM. The fault notifies the operating system that it must locate the data in virtual memory, then transfer it from the storage device, such as an HDD or SSD, to the system RAM.
Though the term "page fault" sounds like an error, page faults are common and are part of the normal way computers handle virtual memory. In programming terms, a page fault generates an exception, which notifies the operating system that it must retrieve the memory blocks or "pages" from virtual memory in order for the program to continue. Once the data is moved into physical memory, the program continues as normal. This process takes place in the background and usually goes unnoticed by the user.
Most page faults are handled without any problems. However, an invalid page fault may cause a program to hang or crash. This type of page fault may occur when a program tries to access a memory address that does not exist. Some programs can handle these types of errors by finding a new memory address or relocating the data. However, if the program cannot handle the invalid page fault, it will get passed to the operating system, which may terminate the process. This can cause the program to unexpectedly quit.
While page faults are common when working with virtual memory, each page fault requires transferring data from secondary memory to primary memory. This process may only take a few milliseconds, but that can still be several thousand times slower than accessing data directly from memory. Therefore, installing more system memory can increase your computer's performance, since it will need to access virtual memory less often.