Virtual memory

In computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage (such as random-access memory and disk storage), allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which behaves like directly addressable read/write memory (RAM).
Most modern operating systems that support virtual memory also run each process in its own dedicated address space. Each program thus appears to have sole access to the virtual memory. However, some older operating systems (such as OS/VS1 and OS/VS2 SVS) and even modern ones (such as IBM i) are single address space operating systems that run all processes in a single address space composed of virtualized memory.
Virtual memory makes application programming easier by hiding fragmentation of physical memory; by delegating to the kernel the burden of managing the memory hierarchy (eliminating the need for the program to handle overlays explicitly); and, when each process is run in its own dedicated address space, by obviating the need to relocate program code or to access memory with relative addressing.
Memory virtualization is a generalization of the concept of virtual memory.
Virtual memory is an integral part of a computer architecture; all implementations (excluding[dubious – discuss] emulators and virtual machines) require hardware support, typically in the form of a memory management unit built into the CPU. Consequently, older operating systems, such as those for the mainframes of the 1960s, and those for personal computers of the early to mid 1980s (e.g. DOS), generally have no virtual memory functionality,[dubious – discuss] though notable exceptions for mainframes of the 1960s include:
the Atlas Supervisor for the Atlas
MCP for the Burroughs B5000
TSS/360 and CP/CMS for the IBM System/360 Model 67
Multics for the GE 645
the Time Sharing Operating System for the RCA Spectra 70/46
The Apple Lisa is an example of a personal computer of the 1980s that features virtual memory.
Embedded systems and other special-purpose computer systems that require very fast and/or very consistent response times may opt not to use virtual memory due to decreased determinism; virtual memory systems trigger unpredictable interrupts that may produce unwanted "jitter" during I/O operations. This is because embedded hardware costs are often kept low by implementing all such operations with software (a technique called bit-banging) rather than with dedicated hardware.