
In
DOS memory management, expanded memory is a system of
bank switching that provided additional memory to
DOS programs beyond the limit of
conventional memory (640 KiB).
''Expanded memory'' is an umbrella term for several incompatible technology variants. The most widely used variant was the Expanded Memory Specification (EMS), which was developed jointly by
Lotus Software
Lotus Software (called Lotus Development Corporation before its acquisition by IBM) was an American software company based in Massachusetts; it was sold to India's HCL Technologies in 2018.
Lotus is most commonly known for the Lotus 1-2-3 sprea ...
,
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, and Delaware General Corporation Law, incorporated in Delaware. Intel designs, manufactures, and sells computer compo ...
, and
Microsoft
Microsoft Corporation is an American multinational corporation and technology company, technology conglomerate headquartered in Redmond, Washington. Founded in 1975, the company became influential in the History of personal computers#The ear ...
, so that this specification was sometimes referred to as "LIM EMS". LIM EMS had three versions: 3.0, 3.2, and 4.0. The first widely implemented version was EMS 3.2, which supported up to 8 MiB of expanded memory and uses parts of the address space normally dedicated to communication with peripherals (
upper memory) to map portions of the expanded memory. EEMS, an expanded-memory management standard competing with LIM EMS 3.x, was developed by
AST Research,
Quadram and
Ashton-Tate ("AQA"); it could map any area of the lower 1 MiB. EEMS ultimately was incorporated in LIM EMS 4.0, which supported up to 32 MiB of expanded memory and provided some support for DOS multitasking as well. IBM, however, created its own expanded-memory standard called XMA.
The use of expanded memory became common with games and business programs such as
Lotus 1-2-3
Lotus 1-2-3 is a discontinued spreadsheet program from Lotus Software (later part of IBM). It was the first killer application of the IBM PC, was hugely popular in the 1980s, and significantly contributed to the success of IBM PC-compatibles ...
in the late 1980s through the mid-1990s, but its use declined as users switched from DOS to
protected-mode operating systems such as
Linux
Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...
,
IBM OS/2, and
Microsoft Windows
Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...
.
Background

The
8088 processor of the
IBM PC
The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first microcomputer released in the List of IBM Personal Computer models, IBM PC model line and the basis for the IBM PC compatible ''de facto'' standard. Released on ...
and
IBM PC/XT can address one
megabyte
The megabyte is a multiple of the unit byte for digital information. Its recommended unit symbol is MB. The unit prefix ''mega'' is a multiplier of (106) in the International System of Units (SI). Therefore, one megabyte is one million bytes ...
(MiB, or 2
20 bytes) of memory. It inherited this limit from the 20-bit external address bus (and overall memory addressing architecture) of the
Intel 8086
The 8086 (also called iAPX 86) is a 16-bit computing, 16-bit microprocessor chip designed by Intel between early 1976 and June 8, 1978, when it was released. The Intel 8088, released July 1, 1979, is a slightly modified chip with an external 8-b ...
. The designers of the PC allocated the lower 640
KiB ( bytes) of address space for read-write program memory (RAM), called ''conventional memory'', and the remaining 384 KiB of memory space is reserved for uses such as the system
BIOS
In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is a type of firmware used to provide runtime services for operating systems and programs and to perform hardware initialization d ...
, video memory, and memory on expansion peripheral boards.
Even though the
IBM PC AT, introduced in 1984, uses the
80286 chip that can address up to 16 MiB of RAM as
extended memory, it can only do so in
protected mode
In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units (CPUs). It allows system software to use features such as Memory_segmentation, segmentation, virtual mem ...
. The scarcity of software compatible with protected mode (no standard
DOS applications can run in it) meant that the market was still open for another solution.
To make more memory accessible, a
bank switching scheme was devised, where only selected parts of the additional memory is accessible at any given time. Originally, a single 64 KiB (2
16 bytes) window of memory, called a
page frame, was used; later this was made more flexible. Programs are written in a specific way to access expanded memory. The ''window'' between conventional memory and expanded memory can be adjusted to access different locations within the expanded memory.
A first attempt to use a bank switching technique was made by Tall Tree Systems with its JRAM boards,
but these did not catch on.
(Tall Tree Systems later made EMS-based boards using the same JRAM brand.)
Expanded Memory Specification (EMS)
Lotus Development,
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, and Delaware General Corporation Law, incorporated in Delaware. Intel designs, manufactures, and sells computer compo ...
, and
Microsoft
Microsoft Corporation is an American multinational corporation and technology company, technology conglomerate headquartered in Redmond, Washington. Founded in 1975, the company became influential in the History of personal computers#The ear ...
cooperated to develop the EMS standard (aka LIM EMS). The first publicly available version of EMS, version 3.0 allows access of up to 4 MiB of expanded memory. This was increased to 8 MiB with version 3.2 of the specification. The final version of EMS, version 4.0 increased the maximum amount of expanded memory to 32 MiB and supports additional functionality.
Microsoft thought that bank switching was an inelegant and temporary, but necessary stopgap measure. Slamming his fist on the table during an interview
Bill Gates
William Henry Gates III (born October 28, 1955) is an American businessman and philanthropist. A pioneer of the microcomputer revolution of the 1970s and 1980s, he co-founded the software company Microsoft in 1975 with his childhood friend ...
said of expanded memory, "It's garbage! It's a
kludge! … But we're going to do it". The companies planned to launch the standard at the Spring 1985
COMDEX, with many expansion-card and software companies announcing their support.
AST Research,
STB Systems,
Persyst,
Quadram, and
Tecmar quickly designed EMS-compliant cards to compete with Intel's own Above Board expansion card. By mid-1985 some already called EMS a
de facto standard.
The first public version of the EMS standard, called EMS 3.0 was released in 1985; EMS 3.0, however, saw almost no hardware implementations before being superseded by EMS 3.2. EMS 3.2 uses a 64 KiB region in the upper 384 KiB (
upper memory area) divided into four 16 KiB pages, which can be used to map portions of the expanded memory.
Quadram, AST, and
Ashton-Tate created the Enhanced EMS (EEMS) standard. EEMS, a superset of EMS 3.2, allows any 16 KiB region in lower RAM to be mapped to expanded memory, as long as it was not associated with interrupts or dedicated I/O memory such as network or video cards. Thus, entire programs can be switched in and out of the extra RAM. EEMS also added support for two sets of mapping registers. These features are used by early DOS multitasker software such as
DESQview. 1987's LIM EMS 4.0 specification incorporated practically all features of EEMS.
A new feature added in LIM EMS 4.0 was that EMS boards can have multiple sets of page-mapping registers (up to 64 sets). This allows a primitive form of DOS
multitasking. The caveat is, however, that the standard does not specify how many register sets a board should have, so there is great variability between hardware implementations in this respect.
The Expanded Memory Specification (EMS) is the specification describing the use of expanded memory. EMS functions are accessible through software
interrupt
In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted ...
67h. Programs using EMS must first establish the presence of an installed expanded memory manager (EMM) by checking for a device driver with the device name EMMXXXX0.
Expanded Memory Adapter (XMA)
IBM remained silent as the industry widely adopted the LIM standard. The company developed its own memory standard called Expanded Memory Adapter (XMA); the IBM DOS driver for it is XMAEM.SYS. Unlike EMS, the IBM expansion boards can be addressed both using an expanded memory model and as
extended memory.
The expanded memory hardware interface used by XMA boards is, however, incompatible with EMS,
but a XMA2EMS.SYS driver provides EMS emulation for XMA boards.
XMA boards were first introduced for the 1986 (revamped) models of the
3270 PC.
Implementations
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 1980s expanded memory boards were
AST RAMpage, IBM PS/2 80286 Memory Expansion Option,
AT&T
AT&T Inc., an abbreviation for its predecessor's former name, the American Telephone and Telegraph Company, is an American multinational telecommunications holding company headquartered at Whitacre Tower in Downtown Dallas, Texas. It is the w ...
Expanded Memory Adapter and the
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, and Delaware General Corporation Law, incorporated in Delaware. Intel designs, manufactures, and sells computer compo ...
Above Board. Given the price of RAM during the period, up to several hundred dollars per MiB, and the quality and reputation of the above brand names, an expanded memory board was very expensive.
Motherboard chipsets
Later, some
motherboard
A motherboard, also called a mainboard, a system board, a logic board, and informally a mobo (see #Nomenclature, "Nomenclature" section), is the main printed circuit board (PCB) in general-purpose computers and other expandable systems. It ho ...
chipset
In a computer system, a chipset is a set of electronic components on one or more integrated circuits that manages the data flow between the processor, memory and peripherals. The chipset is usually found on the motherboard of computers. Chips ...
s of
Intel 80286
The Intel 80286 (also marketed as the iAPX 286 and often called Intel 286) is a 16-bit microprocessor that was introduced on February 1, 1982. It was the first 8086-based CPU with separate, non- multiplexed address and data buses and also the f ...
-based computers implemented an expanded memory scheme that did not require add-on boards, notably the
NEAT chipset. Typically, software switches determined how much memory should be used as ''expanded memory'' and how much should be used as ''
extended memory''.
Device drivers
An expanded-memory board, being a hardware peripheral, needed a software
device driver
In the context of an operating system, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabli ...
, 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
Intel 80386
The Intel 386, originally released as the 80386 and later renamed i386, is the third-generation x86 architecture microprocessor from Intel. It was the first 32-bit computing, 32-bit processor in the line, making it a significant evolution in ...
processor, for example
Quarterdeck's
QEMM,
Qualitas'
386MAX or the default
EMM386 in MS-DOS, PC DOS and DR-DOS.
Software emulation
Beginning in 1986, the built-in memory management features of
Intel 80386
The Intel 386, originally released as the 80386 and later renamed i386, is the third-generation x86 architecture microprocessor from Intel. It was the first 32-bit computing, 32-bit processor in the line, making it a significant evolution in ...
processor freely modeled the address space when running legacy real-mode software, making hardware solutions unnecessary. Expanded memory could be simulated in software.
The first software expanded-memory ''management'' (emulation) program was
CEMM, available in September 1986 as a utility for the
Compaq Deskpro 386. 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 allowed using parts of the
upper memory area (UMA) (the upper 384 KiB of real-mode address space) called ''upper memory blocks'' (UMBs) and provided tools for loading small programs, typically
terminate-and-stay-resident program
A terminate-and-stay-resident program (commonly TSR) is a computer program running under DOS that uses a system call to return control to DOS as though it has finished, but remains in computer memory so it can be reactivated later. This techni ...
s inside ("LOADHI" or "LOADHIGH").
Interaction between
extended memory, expanded-memory emulation and DOS extenders ended up being regulated by the XMS,
Virtual Control Program Interface (VCPI),
DOS Protected Mode Interface
In computing, the DOS Protected Mode Interface (DPMI) is a specification introduced in 1989 which allows a DOS program to run in protected mode, giving access to many features of the new PC processors of the time not available in real mode. It w ...
(DPMI) and
DOS Protected Mode Services
DOS Protected Mode Services (DPMS) is a set of extended DOS memory management services to allow DPMS-enabled DOS drivers to load and execute in extended memory and protected mode.
Not being a DOS extender by itself, DPMS is a minimal set of ...
(DPMS) 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
Shareware is a type of proprietary software that is initially shared by the owner for trial use at little or no cost. Often the software has limited functionality or incomplete documentation until the user sends payment to the software developer. ...
programs.
It is also possible to emulate EMS by using XMS memory on 286 CPUs using 3rd party utilities like EMM286 (.SYS driver).
Decline
Expanded Memory usage declined in the 1990s. New operating systems like
Linux
Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...
,
Windows 9x
Windows 9x is a generic term referring to a line of discontinued Microsoft Windows operating systems released from 1995 to 2000 and supported until 2006, which were based on the kernel introduced in Windows 95 and modified in succeeding version ...
,
Windows NT
Windows NT is a Proprietary software, proprietary Graphical user interface, graphical operating system produced by Microsoft as part of its Windows product line, the first version of which, Windows NT 3.1, was released on July 27, 1993. Original ...
,
OS/2
OS/2 is a Proprietary software, proprietary computer operating system for x86 and PowerPC based personal computers. It was created and initially developed jointly by IBM and Microsoft, under the leadership of IBM software designer Ed Iacobucci, ...
, and
BSD/OS supported protected mode "out of the box". These and similar developments rendered Expanded Memory an obsolete concept.
Similar concepts
Other platforms have implemented the same basic concept – additional memory outside of the main address space – but in technically incompatible ways:
*
Expanded storage was a feature on IBM mainframes providing additional memory outside of the main system memory, first introduced with the IBM 3090 high-end mainframe series in 1985. Expanded storage could not be directly addressed by applications; an MVS feature known as "window services" enabled applications to allocate movable windows to expanded storage within their own address space. There was also a "data mover" feature which could be invoked to move data between main memory (central storage) and expanded storage; later, an "Asynchronous Data Mover Facility" (ADMF) was introduced, which enabled applications to request data to be moved between the two in the background, while they performed other processing. By the mid-1990s, expanded storage had ceased to be a physically separate memory, and had become merely a logical division within the system memory enforced by firmware; but it was not until the November 2016 release of z/VM 6.4 that IBM finally removed all support for expanded storage from its mainframe operating systems.
*
Address Windowing Extensions (AWE) is a conceptually similar feature in Microsoft Windows, used to enable 32-bit applications to access more memory than the 2–4GB that can fit in a 32-bit address space. Although still supported by current versions of Windows, its use has been superseded by 64-bit applications, which can access >4GB of memory directly.
*
Virtual memory
In computing, virtual memory, or virtual storage, is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a ver ...
creates the illusion of available memory using, for instance,
disk storage
Disc or disk may refer to:
* Disk (mathematics)
In geometry, a disk (Spelling of disc, also spelled disc) is the region in a plane (geometry), plane bounded by a circle. A disk is said to be ''closed'' if it contains the circle that constitut ...
.
See also
*
Conventional memory
*
DOS memory management
*
Extended memory (XMS)
*
High memory area (HMA)
*
Overlay (programming)
*
Upper memory area (UMA)
*
Global EMM Import Specification (GEMMIS)
*
x86 memory segmentation
x86 memory segmentation is a term for the kind of memory segmentation characteristic of the Intel x86 computer instruction set architecture. The x86 architecture has supported memory segmentation since the original Intel 8086 (1978), but ''x86 ...
*
Address Windowing Extensions (AWE)
*
Physical Address Extension
In computing, Physical Address Extension (PAE), sometimes referred to as Page Address Extension,
is a memory management feature for the x86 architecture. PAE was first introduced by Intel in the Pentium Pro, and later by AMD in the Athlon process ...
(PAE)
*
Sideways address space on the Acorn ''BBC Micro'' home computer
References
Further reading
*
*
* {{cite web , title=Understanding EMS 4.0 , author-first=Lloyd Robert , author-last=Borrett , work=Technical Cornucopia , date=January–February 1989 , url=http://www.borrett.id.au/computing/art-1989-01-02.htm , access-date=2020-02-08 , url-status=live , archive-url=https://web.archive.org/web/20200208213504/http://www.borrett.id.au/computing/art-1989-01-02.htm , archive-date=2020-02-08
X86 memory management
DOS memory management
Memory expansion