扩展内存(Expanded Memory)是1985年4月24日，由Lotus、英特尔与微软联合发布的一项DOS内存技术，可以提供MS-DOS程序更多的内存。

八十年代的个人计算机(IBM PC/AT)多半使用实模式，只有1MB的寻址空间，且640KB以上的寻址空间不可被应用进程读写使用。

扩展内存的原理是使用1MB扣除640KB后剩下的384KB,称作上端内存区(upper memory area), 这本来是用来跟外设沟通的空间， 给进程使用。把内存的寻址窗口插入到上端内存区的外设寻址空间，引入了bank switching (存储体转换)的技巧。 同一时间只有一部分的额外内存可以被访问。 使用64KB的memory windows做为bank switching 的单位。

In DOS memory management, expanded memory is a system of bank switching that provided additional memory to DOS programs beyond the limit of conventional memory (640 KB).

Expanded memory is an umbrella term for several incompatible technology variants. The most widely used variant was the Expanded Memory Specification (EMS), which was developed jointly by Lotus Software, Intel, and Microsoft, so that this specification was sometimes referred to as "LIM EMS." LIM EMS had several versions. The first widely implemented version was EMS 3.2, which supported up to 8 MB of expanded memory and uses parts of the address space normally dedicated to communication with peripherals (upper memory) to map portions of the expanded memory. EEMS, an expanded memory management standard competing with LIM EMS 3.x, was developed by AST Research, Quadram and Ashton-Tate; it could map any area of the lower 1 MB. EEMS ultimately was incorporated in LIM EMS 4.0, which supported up to 32 MB of expanded memory and provided some support for DOS multitasking as well. IBM however created its own expanded memory standard called XMA.