Most modern computers have special hardware called a memory management unit (MMU). This unit sits between the CPU and the memory unit. Whenever the CPU wants to access memory (whether it is to load an instruction or load or store data), it sends the desired memory address to the MMU, which translates it to another address before passing it on the memory unit. The address generated by the CPU, after any indexing or other addressing-mode arithmetic, is called a virtual address, and the address it gets translated to by the MMU is called a physical address.

Normally, the translation is done at the granularity of a page. Each page is a power of 2 bytes long, usually between 1024 and 8192 bytes. If virtual address p is mapped to physical address f (where p is a multiple of the page size), then address p+o is mapped to physical address f+o for any offset o less than the page size. In other words, each page is mapped to a contiguous region of physical memory called a page frame.

The MMU allows a contiguous region of virtual memory to be mapped to page frames scattered around physical memory making life much easier for the OS when allocating memory. Much more importantly, however, it allows infrequently-used pages to be stored on disk. Here’s how it works: The tables used by the MMU have a valid bit for each page in the virtual address space. If this bit is set, the translation of virtual addresses on a page proceeds as normal. If it is clear, any attempt by the CPU to access an address on the page generates an interrupt called a page fault trap. The OS has an interrupt handler for page faults, just as it has a handler for any other kind of interrupt. It is the job of this handler to get the requested page into memory.