The IBM PC
In 1985 the companies 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. It is a form of bank switching technique that allows more than the 640 KB of RAM defined by the original IBM PC architecture, by letting it appear piecewise in a 64 KB "window" located in the Upper Memory Area. The 64 KB is divided into four 16 KB "pages" which can each be independently switched. Some computer games made use of this, and though EMS is obsolete, the feature is nowadays emulated by later Microsoft Windows operating systems 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 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 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.
Read more about this topic: Bank Switching