延伸内存（eXtended Memory），指的是超过1MB以上的内存空间，在配备Intel 80286或以上的CPU的IBM PC上。它是在1988年，由微软、莲花研究、英特尔和AST研究共同开会制定出来的规格，他们共同制定了延伸内存的定义、应用程序取用延伸内存的中断调用，这个标准在MS-DOS的个人电脑时代，成为解决个人电脑内存不足的一种方法。

一般来说，在Intel 80286以上的个人电脑，超过640KB部分的RAM，通常都会映射到超过1MB以上的部分，使得在保謢模式下的程序可以访问它们。但即使没有这种映射，还是有可能可以访问1MB以上的RAM。

In DOS memory management, extended memory refers to memory above the first megabyte (2 bytes) of address space in an IBM PC or compatible with an 80286 or later processor. The term is mainly used under the DOS and Windows operating systems. DOS programs, running in real mode or virtual x86 mode, cannot directly access this memory, but are able to do so through an application programming interface called the eXtended Memory Specification (XMS). This API is implemented by a driver (such as HIMEM.SYS) or the operating system, which takes care of memory management and copying memory between conventional and extended memory, by temporarily switching the processor into protected mode. In this context the term "extended memory" may refer to either the whole of the extended memory or only the portion available through this API.