Extended memory is memory with addresses in the range 1MB to 8MB, and is used only by the following:
Certain operating systems, such as OS/2Some MS-DOS interfaces, such as WindowsSome RAM disk programs, such as VDISKSome hard disk caching programs, such as SMARTDRVCertain specially-written MS-DOS applications.Use of extended memory requires a memory manager. Most versions of MS-DOS include a standard extended memory manager. If you are using MS-DOS, Windows, and other compatible programs, it is best to use one of the memory managers (such as HIMEM.SYS) that came with your software because these memory managers have been tested and proven reliable.
Most MS-DOS commands and application programs cannot use extended memory directly. They need to use expanded memory, which uses a portion of reserved memory as a window to access RAM beyond 1MB. This type of memory allows some MS-DOS applications to get around the 640KB size limitation. You control expanded memory with a memory manager (such as EMM386.EXE), which enables the computer to use extended memory as expanded memory.