Expanded memory
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Expanded Memory was a trick invented around 1984 that provided more memory to byte-hungry, business-oriented MS-DOS programs. These were typically spreadsheets and databases running on the original IBM PC, and on its successors like the IBM XT and IBM AT. IBM PC and IBM XT had real mode memory architecture, which only allowed programs to use 1 mebibyte of address space, of which only up to 640 KiB was available for normal ram (the remainder from 640 KiB to 1 mebibyte being reserved for peripherals, most notably the screen memory in the display adapter). The IBM AT, with it Intel 80286 supported protected mode, but this computer also used MS-DOS, a real mode operating system that did not use extended memory directly.
The idea behind expanded memory was to use part of the remaining 384 KiB, normally dedicated to communication with peripherals, for program memory as well. In order to fit potentially much more memory than the 384 KiB of free address space would allow, a banking scheme was devised, where only selected portions of the additional memory would be accessible at the same time. Originally, a single 64 KiB 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.
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[edit] Different implementations
[edit] Expansion boards
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 1980's 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.
[edit] Motherboards
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.
[edit] Software emulation
Beginning in 1987, the built-in memory management features of Intel 80386 processor freely modeled the address space when running legacy real mode software, making hardware solutions unnecessary. Expanded memory could be simulated only by the software.
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 manager 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 KiB of real mode address space and provided tools for loading small programs, typically TSRs inside ("loadhi" or "loadhigh").
Interaction between extended memory, expanded memory emulation and DOS extenders ended up being regulated by the XMS, VCPI and DPMI specifications.
Certain emulation programs, colloquially known as LIMulators, did not rely on motherboard or 80386 features at all. Instead, they reserved 64 KiB 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.
[edit] Details
An expanded memory board being a hardware peripheral, it 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 (see below).
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 KiB 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.
[edit] See also
- Unreal mode
- Conventional memory
- Upper Memory Area
- High Memory Area
- Extended memory
- Extended Memory Specification
[edit] References
- Lotus, Intel, Microsoft (October 1987). Expanded Memory Specification. Version 4.0.
- A complete discussion of EMS and programming examples can be found in ["PC System Programming for developers", 1989, ISBN 1-55755-035-2 (Book only) and ISBN 1-55755-036-0 (Book and diskette)].
- Understanding EMS 4.0 by Lloyd Borrett, Technical Cornucopia, January–February 1989
This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
