A similar mechanism is used for memory-mapped files, which are mapped to virtual memory and loaded to physical memory on demand. When this happens the page needs to be taken from disk and put back into physical memory. In this case the page is paged out to a secondary store located on a medium such as a hard disk drive (this secondary store, or "backing store", is often called a swap partition if it is a disk partition, or a swap file, swapfile or page file if it is a file). This will occur if the requested page has been moved out of physical memory to make room for another page. The lookup may also fail if the page is currently not resident in physical memory.On modern operating systems, it will cause a segmentation fault signal being sent to the offending program. This will typically occur because of a programming error, and the operating system must take some action to deal with the problem. The lookup may fail if there is no translation available for the virtual address, meaning that virtual address is invalid.The page table lookup may fail, triggering a page fault, for two reasons: The subsequent translation will result in a TLB hit, and the memory access will continue. 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. 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 a match is found, which is known as a TLB hit, the physical address is returned and memory access can continue. When a virtual address needs to be translated into a physical address, the TLB is searched first. This is called the translation lookaside buffer (TLB), which is an associative cache. The memory management unit (MMU) inside the CPU stores a cache of recently used mappings from the operating system's page table.
Each translation is restarted if a TLB miss occurs, so that the lookup can occur correctly through hardware.
The translation process Īctions taken upon a virtual to physical address translation. 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). 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. 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). In operating systems that use virtual memory, every process is given the impression that it is working with large, contiguous sections of memory.
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