expanded memory

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expanded memory

Memory used through EMS. In systems based on Intel 80386 or later processor 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 EMM. 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.


(Expanded Memory Manager) Starting with 386-based PCs, an EMM is software that converts extended memory (beyond one megabyte) into EMS memory, the first technique used to increase memory in the PC. In the very first PCs (XTs and ATs), EMS was provided by plugging in EMS boards. See EMS.


(1) (Enhanced Message Service) An extension to the original cellphone text messaging service (see SMS) that added formatted text, icons, animations and ringtones. Introduced in 2001 by Alcatel, Ericsson, Motorola and Siemens, EMS was superseded by full multimedia support (see MMS).

(2) (Electronic Message Service) The part of the radio spectrum assigned to electronic messaging over digital satellite circuits.

(3) (Electronics Manufacturing Services) A company that makes electronic devices for other companies. See contract manufacturer.

(4) (Enterprise Messaging Server) The original name for Microsoft's Exchange Server. See Microsoft Exchange.

(5) The plural of "em" spacing. See em.

(6) (Expanded Memory Specification) A technique that allowed DOS to reach beyond 1MB of RAM. Introduced in 1985, EMS provided access to 4MB, 8MB and later 32MB of expanded memory by bank switching through a 64KB page frame in the upper memory area between 640K and 1M (see UMA). Users had to specify how much EMS was needed, and applications had to be written for EMS (Lotus 1-2-3, AutoCAD, etc.), or they were run with EMS-compliant software such as DESQview. In IBM PC XTs and ATs, EMS required a board and driver, but 386 PCs could create EMS from extended memory. When Windows came out, it allocated EMS automatically. See DESQview and extended memory.

expanded storage

Additional memory in IBM mainframes that is not normally addressable by applications. Introduced for the 3090 series, the data are usually transferred in 4K pages from expanded storage to central storage (main memory). See hiperspace.