Virtual Memory

VIRTUAL MEMORY Many of us use computers on a daily basis. Although we use it for many different purposes in many different ways, we share one common reason of using them; to make our job more efficient and easier. However, there are times when computers cannot run as fast as you want it to or just cannot handle certain processes effectively, due to the shortage of system resources. When the limitations of system resources become a major barrier to acheiving your maximum productivity, we often consider the apparent ways of upgrading the system, such as switching to a faster CPU, addding more physical memory (RAM), installing utility programs, and so on. As a computer user, you want to make the most of the resources available; the process of preparing plans to coordinate the total system to operate in the most efficient manner. This is called a system optimization. When it comes to system optimization, there is one great invention of modern computing called virtual memory. It is an imaginary memory area supported by some operating system (for example, Windows but not DOS) in conjunction with the hardware. You can think of virtual memory as an alternate set of memory addresses. Programs use these virtual addresses rather than real addresses to store instructions and data. When the program is actually executed, the virtual addresses are converted into real memory addresses. The purpose of virtual memory is to enlarge the address space, the set of addresses a program can utilize. For example, virtual memory might contain twice as many addresses as main memory. A program using all of virtual memory, therefore, would not be able to fit in main memory all at once. Nevertheless, the computer could execute such a program by copying into main memory those portions of the program needed at any given point during execution. To facilitate copying virtual memory into real memory, the operating system divides virtual memory into pages, each of which contains a fixed number of addresses. Each page is stroed on a disk until it is needed. When the page is needed, the operating system copies it from disk to main memory, translating the virtual addresses into real addresses. The process of translating virtual addresses into real addresses is called mapping. The copying of virtual pages from disk to main memory is known as paging or swapping. Some physical memory is used to keep a list of references to the most recently accessed information on an I/O (input/output) device, such as the hard disk. The optimization it provides, is that it is faster to read the information from physical memory, than use the relevant I/O channel to get that information. This is called caching. It is implemented inside the OS.

Virtual memory is a computer system technique developed at the University of Manchester, which gives an application program the impression that it has contiguous working memory (an address space), while in fact it may be physically fragmented and may even overflow on to disk storage. Developed for multitasking kernels, virtual memory provides two primary functions: 1. Each process has its own address space, thereby not required to be relocated nor required to use relative addressing mode. 2. Each process sees one contiguous block of free memory upon launch. Fragmentation is hidden.

All implementations (excluding emulators) require hardware support. This is typically in the form of a memory management unit built into the CPU. Systems that use this technique make programming of large applications easier and use real physical memory (e.g. RAM) more efficiently than those without virtual memory. Virtual memory differs significantly from memory virtualization in that virtual memory allows resources to be virtualized as memory for a specific system, as opposed to a large pool of memory being virtualized as smaller pools for many different systems. Note that "virtual memory" is more than just "using disk space to extend physical memory size" which is merely the extension of the memory hierarchy to include hard disk drives. Extending memory to disk is a normal consequence of using virtual memory techniques, but could be done by other means such as overlays or swapping programs and their data completely out to disk while they are inactive. The definition of "virtual memory" is based on redefining the address space with a contiguous virtual memory addresses to "trick" programs into thinking they are using large blocks of contiguous addresses. Modern general-purpose computer operating systems generally use virtual memory techniques for ordinary applications, such as word processors, spreadsheets, multimedia players, accounting, etc., except where the required hardware support (a memory management unit) is unavailable. Older operating systems, such as DOS[1] of the 1980s, or those for the mainframes of the 1960s, generally had no virtual memory functionality - notable exceptions being the Atlas, B5000 and Apple Computer's Lisa. Embedded systems and other special-purpose computer systems which require very fast and/or very consistent response times may opt not to use virtual memory due to decreased determinism. This is based on the idea that unpredictable processor exceptions produce unwanted jitter on CPU operated I/O, which the smaller embedded processors often perform directly to keep cost and power consumption low, and the associated simple application has little use for multitasking features