A paging file is a hidden system file on a computer’s secondary storage device (typically a hard disk drive or SSD) that serves as an extension of the computer’s physical memory (RAM). When the RAM becomes fully utilized, the operating system uses the paging file to offload less frequently accessed data, freeing up RAM for active processes. This process is known as paging or swapping.
The paging file allows a computer to run larger applications or handle more simultaneous tasks than physical memory alone could support. It works by moving blocks of memory, called pages, between the RAM and the paging file. Pages that are not immediately needed are written to the paging file, and when they are needed again, they are read back into RAM—sometimes replacing other pages in the process.
Importantly, the CPU can only fetch and execute instructions and access data that reside in physical RAM. If a required memory page is located in the paging file on disk, the operating system must first load it back into RAM before the CPU can access it. This process—triggered by a page fault—ensures that all executable code and accessible data are physically present in RAM, even though they may appear to reside in a much larger, continuous virtual memory space.
In Windows, the paging file is usually named pagefile.sys and is located on the system drive (usually C drive). You can configure its size or let Windows manage it automatically. Although the paging file enables virtual memory and helps prevent out-of-memory errors, accessing data from a disk is much slower than accessing it from RAM, which can lead to performance degradation if the system relies too heavily on the paging file.