Editing Bank switching (section)

== The IBM PC ==
[[File:Expanded memory.svg|thumb|right|Expanded memory in the IBM PC]]
In 1985, the companies [[Lotus Software|Lotus]] and [[Intel]] introduced [[Expanded Memory]] Specification (EMS) 3.0 for use in [[IBM PC compatible]] computers running [[MS-DOS]]. [[Microsoft]] joined for versions 3.2 in 1986 and 4.0 in 1987 and the specification became known as Lotus-Intel-Microsoft EMS or LIM EMS.<ref name="Mueller_1992"/><ref name="Lotus_1985"/><ref name="EMS_1987"/> It is a form of bank switching technique that allows more than the 640&nbsp;KB of RAM defined by the original IBM PC architecture, by letting it appear piecewise in a 64&nbsp;KB "window" located in the [[Upper Memory Area]].<ref name="Ross_1995"/> The 64&nbsp;KB is divided into four 16&nbsp;KB "pages" which can each be independently switched. Some [[computer game]]s made use of this, and though EMS is obsolete, the feature is nowadays [[emulator|emulated]] by later [[Microsoft Windows]] [[operating system]]s to provide backwards compatibility with those programs.

The later [[eXtended Memory Specification]] (XMS), also now obsolete, is a standard for, in principle, simulating bank switching for memory above 1&nbsp;MB (called "[[extended memory]]"), which is not directly addressable in the [[Real Mode]] of [[x86]] processors in which MS-DOS runs. XMS allows extended memory to be copied anywhere in conventional memory, so the boundaries of the "banks" are not fixed, but in every other way it works like the bank switching of [[Expanded Memory Specification|EMS]], from the perspective of a program that uses it. Later versions of MS-DOS (starting circa version 5.0) included the EMM386 driver, which simulates EMS memory using XMS, allowing programs to use extended memory even if they were written for EMS. [[Microsoft Windows]] emulates XMS also, for those programs that require it.