What Page Fault Really Means

John Spacey, December 06, 2019

A page fault is an exception that occurs when a program requests virtual memory that isn't ready. This is not an error but is the standard way that operating systems and memory hardware handle virtual memory. By definition, virtual memory isn't in memory all the time but is cached to a data storage device such as a disk. As such, page faults regularly occur on an operating system with virtual memory whenever a page needs to be pulled into physical memory.

Summary: Page Faults
Subject Area	Operating Systems Memory Hardware Memory Management Units
Applies To	Virtual Memory
Type	Exception
Is Not An	Error
Basic Definition	An exception that occurs when a program requests virtual memory that isn't ready.
Technical Definition	An exception that occurs when a memory page is requested that is not currently mapped by the memory management unit into the virtual address space of a process.

Occurs When	Memory is requested by a process that is not in memory or that is not mapped by the memory management unit into the virtual address space for the process.
Types	Minor The memory page is in memory but is not yet mapped by the memory management unit. A mapping is performed and the process can quickly access the memory. Also known as a soft page fault. Major Page Fault The memory page is not in memory. This requires an algorithm to run that frees memory and loads the memory page. Where this occurs regularly an OS may slow down. Invalid A page fault can result from an attempt by the process to access a memory address outside its virtual address space. This requires the operating system to issue an error such as a segmentation fault and potentially terminate the offending process.
Related Concepts	Memory Page A fixed length chunk of data. For the purposes of efficiency, memory management deals with pages as opposed to bytes or words. Virtual Memory A technique that offers a single interface to access physical memory and data that has been temporarily swapped to a data storage device such as a disk. Allows an operating system to offer memory beyond the physical memory of the machine. Segmentation Fault An error that occurs when the memory address requested by a process is invalid.