Microsoft KB Archive/34178
Extended Versus Expanded Memory PSS ID Number: Q34178 Article last modified on 09-19-1991 PSS database name: PSS
Many applications available today support memory above the 640K limit of DOS. Some of these require extended memory, while others require expanded memory. The information below provides a brief explanation of the major differences between the two typies of memories.
Expanded memory is typically used by spreadsheets, e.g. Microsoft Excel and Lotus 1-2-3, and consists of a “page frame” for extra memory. The page frame is located in high memory and the Memory Manager (a software driver included with memory boards) changes the memory addressed by the page frame as necessary. This feature can be visualized as having all the expanded memory of a book with the memory manager turning the pages; only one page can be accessed at any time.
Extended memory is used by OS/2 and some other applications that are designed specifically for “AT” class machines. Extended memory is a continuation of RAM above the 1-megabyte point. This type of memory is typically used by RAM disks, and disk caching.
Some memory boards use only extended memory while others use only expanded memory; still others can be used as either. The most recent standard, LIM 4.0, can be used for either extended or expanded memory with the appropriate driver (included with the memory board). Certain boards use only extended memory; however, they include drivers to allow expanded memory emulation.
For specific information about expanded and extended memory, contact the memory-board manufacturer.
Copyright Microsoft Corporation 1991.