In order to fit potentially much more memory than the 384 KB of free address space would allow, a bank switching scheme was devised, where only selected portions of the additional memory would be accessible at the same time. Originally, a single 64 KB window of memory was possible; later this was made more flexible. Applications had to be written in a specific way in order to access expanded memory.
This insertion of a memory window into the peripheral address space could originally be accomplished only through specific expansion boards, plugged into the ISA expansion bus of the computer. Famous 1980s expanded memory boards were AST RAMpage, IBM PS/2 80286 Memory Expansion Option, AT&T Expanded Memory Adapter and the Intel Above Board. Given the price of RAM during the period, up to several hundred dollars per megabyte, and the quality and reputation of the above brand names, an expanded memory board was very expensive.
Later, some motherboards of Intel 80286-based computers implemented an expanded memory scheme that did not require add-on boards. Typically, software switches determined how much memory should be used as expanded memory and how much should be used as extended memory.
The first software expanded memory management (emulation) program was probably CEMM, available in November 1987 with Compaq DOS 3.31. A popular and well-featured commercial solution was Quarterdeck's QEMM. A contender was Qualitas' 386MAX. Functionality was later incorporated into MS-DOS 4.01 in 1989 and into DR-DOS 5.0 in 1990, as EMM386.
Software expanded memory managers in general offered additional, but closely related functionality. Notably, they could create ordinary memory areas (Upper Memory Blocks) in unused parts of the high 384 KB of real mode address space and provided tools for loading small programs, typically TSRs inside ("loadhi" or "loadhigh").
Certain emulation programs, colloquially known as LIMulators, did not rely on motherboard or 80386 features at all. Instead, they reserved 64 KB of the base RAM for the expanded memory window, where they copied data to and from either extended memory or the hard disk when application programs requested page switches. This was programmatically easy to implement, but performance was low. This technique was offered by AboveDisk from Above Software and by several shareware programs.
An expanded memory board, being a hardware peripheral, needed a software device driver, which exported its services. Such a device driver was called "expanded memory manager". Its name was variable; the previously mentioned boards used remm.sys (AST), ps2emm.sys (IBM), aemm.sys (AT&T) and emm.sys (Intel) respectively. Later, the expression became associated with software-only solutions requiring the 80386 processor, for example Quarterdeck's QEMM.
Expanded memory was a common term for several incompatible technology variants. The Expanded Memory Specification (EMS) was developed jointly by Lotus, Intel, and Microsoft, so this specification was sometimes referred to as "LIM EMS". EEMS, a competing expanded memory management standard, was developed by AST Research, Quadram and Ashton-Tate. It allowed to also remap some or all of the lower 640 kB of memory, so that entire programs could be switched in and out of the extra RAM. The two standards were eventually combined as LIM EMS 4.0.