If the OS or a program does not find what it needs in RAM, then the MMU responds to the missing memory reference with a page fault exception to get the OS to move the page back to memory when it is needed. Entries in the page table indicate whether the page is in RAM. The virtual memory paging process uses page tables, which translate the virtual addresses that the OS and applications use into the physical addresses that the MMU uses. The maximum size of the page file can be 1 ½ to four times the physical memory of the computer. This system ensures the computer's OS and applications do not run out of real memory. When the swap file is needed, it is sent back to RAM using a process called page swapping. A swap file is a space set aside on the hard drive to be used as the virtual memory extension for the computer's RAM. When a computer uses up its available RAM, pages not in use are transferred to the hard drive using a swap file. Paging divides memory into sections or paging files. In general, virtual memory is either paged or segmented. The CPU also generates the virtual address space. In most computers, the MMU hardware is integrated into the central processing unit (CPU). Because of swapping, computers with more RAM are considered to have better performance.Ī computer's MMU manages virtual memory operations. ![]() This means using virtual memory generally causes a noticeable reduction in performance. However, the process of swapping virtual memory to physical is rather slow. Each page is stored on a disk, and when the page is needed, the OS copies it from the disk to main memory and translates the virtual addresses into real addresses. While copying virtual memory into physical memory, the OS divides memory with a fixed number of addresses into either pagefiles or swap files. If that data is needed again, the computer's MMU will use a context switch to resume execution. The computer's memory manager is in charge of keeping track of the shifts between physical and virtual memory. If, at any point, the RAM space is needed for something more urgent, data can be swapped out of RAM and into virtual memory. The MMU can, for example, map a logical address space to a corresponding physical address. A memory management unit ( MMU) maps the address to RAM and automatically translates addresses. When an application is in use, data from that program is stored in a physical address using RAM. Virtual memory uses both hardware and software to operate. ![]() With virtual memory, a system can load larger or multiple programs running at the same time, enabling each one to operate as if it has more space, without having to purchase more RAM. A system using virtual memory uses a section of the hard drive to emulate RAM. Computers have a finite amount of RAM, so memory will eventually run out when multiple programs run at the same time. Virtual memory was developed at a time when physical memory - also referenced as RAM - was expensive. If the OS has to swap data between virtual memory and RAM too often, the computer will begin to slow down - this is called thrashing. However, users should not overly rely on virtual memory, since it is considerably slower than RAM. Virtual memory is important for improving system performance, multitasking and using large programs. Virtual memory frees up RAM by swapping data that has not been used recently over to a storage device, such as a hard drive or solid-state drive ( SSD). But, sometimes, this is not enough to run several programs at one time. Today, most personal computers (PCs) come with at least 8 GB (gigabytes) of RAM. Mapping chunks of memory to disk files enables a computer to treat secondary memory as though it were main memory. Virtual memory uses both hardware and software to enable a computer to compensate for physical memory shortages, temporarily transferring data from random access memory ( RAM) to disk storage. Virtual memory is a common technique used in a computer's operating system (OS). Virtual memory is a memory management technique where secondary memory can be used as if it were a part of the main memory. Stacey Peterson, Senior Managing Editor.
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