Page table
A page table is the data structure used by a virtual memory system in a computer operating system to store the mapping between virtual addresses and physical addresses. Virtual addresses are used by the program executed by the accessing process, while physical addresses are used by the hardware, or more specifically, by the random-access memory (RAM) subsystem. The page table is a key component of virtual address translation that is necessary to access data in memory. In operating systems that use virtual memory, every process is given the impression that it is working with large, contiguous sections of memory. Physically, the memory of each process may be dispersed across different areas of physical memory, or may have been moved (paged out) to secondary storage, typically to a hard disk drive (HDD) or solid-state drive (SSD). When a process requests access to data in its memory, it is the responsibility of the operating system to map the virtual address provided by the process to the physical address of the actual memory where that data is stored. The page table is where the operating system stores its mappings of virtual addresses to physical addresses, with each mapping also known as a page table entry (PTE). The memory management unit (MMU) inside the CPU stores a cache of recently used mappings from the operating system's page table. This is called the translation lookaside buffer (TLB), which is an associative cache. When a virtual address needs to be translated into a physical address, the TLB is searched first. If a match is found, which is known as a TLB hit, the physical address is returned and memory access can continue. However, if there is no match, which is called a TLB miss, the MMU or the operating system's TLB miss handler will typically look up the address mapping in the page table to see whether a mapping exists, which is called a page walk. If one exists, it is written back to the TLB, which must be done because the hardware accesses memory through the TLB in a virtual memory system, and the faulting instruction is restarted, which may happen in parallel as well.
