Expanded Memory is memory used through EMS. In systems based on Intel 80386 or later processors expanded memory is part of the extended memory that is mapped into the expanded memory page frame by the processor. The mapping is controlled by the Expanded Memory Manager (EMM) software. In earlier systems, a dedicated EMS hardware adaptor is needed to map memory into the page frame. In both cases, an appropriate device driver is needed for the proper communication between hardware and EMM.

Since expanded memory is a bank switching technique it is less efficient than directly accessing extended memory, which is the approach taken by modern protected mode operating systems. The concept of expanded memory is thus only relevant to applications which must operate in the restricted address space provided by real mode operating systems such as MS-DOS.

This article was originally based on material from the Free On-line Dictionary of Computing, used with permission. Update as needed.