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An expanded memory specification (EMS) was a technique introduced in about 1984 for expanding the conventional or main memory beyond 1 MB in IBM XT compatible computers. The process was known as bank switching and involved expanding memory beyond that which was directly addressed by the processor. EMS was designed for disk operating system (DOS) software programs requiring the additional memory.
EMS is also known as expanded memory, LIM EMS, LIM 4.0 or EMS 4.0.
The latest version of expanded memory specification was developed in 1987 by Lotus Software, Intel and Microsoft.
The 8088 microprocessor only addressed one MB of memory. Thus, of the 1024 KB, 640 KB were used for Random Access Memory (RAM) for reading and writing, and the remaining 384 Kb were used for system basic input/output system (BIOS), video memory and memory for peripheral expansion boards.
An expanded memory management standard, known as extended EMS (EEMS), competed with LIM EMS. It was developed by AST Research, Quadram and Ashton-Tate, which allowed entire programs to be switched in and out of the extra RAM. The two technologies were later combined into what was later known as LIM EMS 4.0.
Later software switches were developed to determine how much memory could be used as expanded memory and how much could be used as extended memory (memory above 1024 KB). In approximately 1987, hardware solutions were no longer necessary, as expanded memory could be created in software. Still, later software expanded memory managers were developed with additional but closely related functionality to EMS 4.0. They created RAM in unused parts of the 384 Kb known as the upper memory area, which created space for loading small programs known as terminate and stay residents (TSR).
Until 1990, expanded memory was the preferred method used for adding memory to a PC. Windows 3.0 was released and used as an extended memory manager which enabled programs to use expanded memory without interference. Additionally, Windows 3.0 could simulate expanded memory if required by software applications.
EMS was used commonly in games and business programs from the late 1980s to the mid 1990s. Later, its use declined as consumers changed from the DOS operating system (OS) to Microsoft Windows OS.