EMM386 is the
expanded memory
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 techn ...
manager of
Microsoft
Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washing ...
's
MS-DOS
MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few ope ...
,
IBM's
PC DOS
PC or pc may refer to:
Arts and entertainment
* Player character or playable character, a fictional character controlled by a human player, usually in role-playing games or computer games
* ''Port Charles'', an American daytime TV soap opera
* ...
,
Digital Research
Digital Research, Inc. (DR or DRI) was a company created by Gary Kildall to market and develop his CP/M operating system and related 8-bit, 16-bit and 32-bit systems like MP/M, Concurrent DOS, FlexOS, Multiuser DOS, DOS Plus, DR DOS and ...
's
DR-DOS
DR-DOS (written as DR DOS, without a hyphen, in versions up to and including 6.0) is a disk operating system for IBM PC compatibles. Upon its introduction in 1988, it was the first DOS attempting to be compatible with IBM PC DOS and MS-D ...
, and
Datalight
Datalight was a privately held software company specializing in power failsafe and high performance software for preserving data integrity in embedded systems. The company was founded in 1983 by Roy Sherrill, and is headquartered in Bothell, Washi ...
's
ROM-DOS
Datalight was a privately held software company specializing in power failsafe and high performance software for preserving data integrity in embedded systems. The company was founded in 1983 by Roy Sherrill, and is headquartered in Bothell, Washi ...
which is used to create expanded memory using
extended memory
In DOS memory management, extended memory refers to memory above the first megabyte (220 bytes) of address space in an IBM PC or compatible with an 80286 or later processor. The term is mainly used under the DOS and Windows operating systems. DOS ...
on
Intel 80386
The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistors[FreeDOS
FreeDOS (formerly Free-DOS and PD-DOS) is a free software operating system for IBM PC compatible computers. It intends to provide a complete MS-DOS-compatible environment for running Legacy system, legacy software and supporting embedded system ...]
.
Overview
EMM386.EXE can map memory into unused blocks in the
upper memory area
In DOS memory management, the upper memory area (UMA) refers to memory between the addresses of 640 KB and 1024 KB ( 0xA0000–0xFFFFF) in an IBM PC or compatible. IBM reserved the uppermost 384 KB of the 8088 CPU's 1024 KB ...
(UMA), allowing device drivers and
TSRs to be "loaded high", preserving
conventional memory
In DOS memory management, conventional memory, also called base memory, is the first 640 kilobytes of the memory on IBM PC or compatible systems. It is the read-write memory directly addressable by the processor for use by the operating system ...
.
The technique probably first appeared with the development of
CEMM
CEMM, for Compaq Expanded Memory Manager was the first so-called PC " memory manager" for Intel 80386 CPUs, able to provide expanded memory (EMS) emulation by using the virtual memory features and the virtual 8086 mode of the CPU.
It was present ...
, included with
Compaq MS-DOS 3.31
MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few oper ...
in 1987. Microsoft's version first appeared, built-in, with
Windows/386
Windows 2.0 is a major release of Microsoft Windows, a family of Graphical user interface, graphical operating system, operating systems for Personal computer, personal computers developed by Microsoft. It was Released-to-manufacturing, release ...
2.1 in 1988 and as standalone EMM386.SYS with
MS-DOS 4.01 in 1989; the more flexible EMM386.EXE version appeared in
MS-DOS 5.0
MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few ope ...
in 1991.
Just as the other expanded memory managers, EMM386 uses the processor's
virtual 8086 mode
In the 80386 microprocessor and later, virtual 8086 mode (also called virtual real mode, V86-mode, or VM86) allows the execution of real mode applications that are incapable of running directly in protected mode while the processor is runnin ...
. This forces memory accesses made by DOS applications to go through the processor's
MMU (introduced in the 386), and the page table entries used by the MMU are configured by EMM386 to map certain regions in upper memory to areas of extended memory (obtained by EMM386 through the extended memory manager
HIMEM.SYS
HIMEM.SYS is a DOS device driver which allows DOS programs to store data in extended memory according to the Extended Memory Specification (XMS). The memory beyond the first 1 MB of address space is required by Windows 9x/ Me in order to load ...
). This technique enabled both EMS (expanded memory) as well as
UMBs - both of which appear to DOS applications to be memory in the upper area but are in fact mapped to physical memory locations beyond 1MB.
It temporarily shuts down during a Windows session in ''386 Enhanced'' mode, with Windows'
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 virtual memory, paging and safe multi-tasking d ...
kernel taking over its role.
Windows uses the GEMMIS API to take over memory management from EMM386.EXE. Global EMM Import Specification (GEMMIS) is supported via a document available to a select number of memory-manager vendors ("Windows/386 Paging Import Specification").
Only a few memory managers implemented the GEMMIS API, some of the ones that include it are: EMM386.EXE, Quarterdeck
QEMM
Quarterdeck Expanded Memory Manager (QEMM) is a memory manager produced by Quarterdeck Office Systems in the late 1980s through the late 1990s. It was the most popular third-party memory manager for the MS-DOS and other DOS operating systems.
...
, Qualitas
386MAX
386MAX (originally 386 to the Max, later Qualitas MAX) is a computer memory manager for DOS-based personal computers. It competed with Quarterdeck's QEMM memory manager. It was manufactured by Qualitas.
BlueMax was a special version designed fo ...
,
Helix Netroom NETROOM is a DOS memory manager produced by Helix Software Company in the 1990s. Introduced in August 1990, NETROOM was originally titled "LAN Memory Manager." Version 1.0 of NETROOM was a re-packaged version of Helix's HeadRoom with the HeadRoo ...
and
DOSBox builtin DOS
DOSBox is a free and open-source emulator which runs software for MS-DOS compatible disk operating systems—primarily video games. It was first released in 2002, when DOS technology was becoming obsolete. Its adoption for running DOS games i ...
. Notably missing are FreeDOS's memory managers.
All of FreeDOS memory managers (HIMEMX.EXE, JEMM386.EXE, JEMMEX.EXE) do not implement the GEMMIS API and Windows fails to start when running in conjunction with JEMMxxx since Windows fails to take over the memory management role.
Windows ME
Windows Millennium Edition, or Windows Me (marketed with the pronunciation of the pronoun "me"), is an operating system developed by Microsoft as part of its Windows 9x family of Microsoft Windows operating systems. It is the successor to Windo ...
,
Windows 98
Windows 98 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of Microsoft Windows operating systems. The second operating system in the 9x line, it is the successor to Windows 95, and was released to ...
,
Windows 95
Windows 95 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of operating systems. The first operating system in the 9x family, it is the successor to Windows 3.1x, and was released to manufacturin ...
,
Windows for Workgroups 3.1
Windows 3.1 is a major release of Microsoft Windows. It was Released-to-manufacturing, released to manufacturing on April 6, 1992, as a successor to Windows 3.0.
Like its predecessors, the Windows 3.1 series ran as a Shell (computing), shell ...
x, and
Windows 3 Windows 3.x means either of, or all of the following versions of Microsoft Windows:
* Windows 3.0
* Windows 3.1x
Windows 3.1 is a major release of Microsoft Windows. It was released to manufacturing on April 6, 1992, as a successor to Windo ...
.xx, all will fail with JEMMxxx displaying:
Cannot run Windows while the currently installed protected-mode software is
running.
Quit the protected-mode software, and then try again.
You may need to restart your computer.
With JEMMxx, it is possible to run Windows 3.x and Windows for Workgroups 3.1x in limited capabilities by forcing Windows to use Standard Mode; i.e. using 80286 Protected Mode, not 80386 Enhanced Mode. Three conditions are required:
# limit total XMS to 64MB
# EMS must be enabled, can't use NOEMS option. For example JEMMEX X2MAX=65422
# Windows operate in Standard Mode only, WIN /S or WIN /2
Note that Windows in standard mode is limited in functionality, it lacks virtual memory, it skips the
86Enhsection in SYSTEM.INI and any device drivers in
86Enhare not loaded.
See also
*
Upper memory area
In DOS memory management, the upper memory area (UMA) refers to memory between the addresses of 640 KB and 1024 KB ( 0xA0000–0xFFFFF) in an IBM PC or compatible. IBM reserved the uppermost 384 KB of the 8088 CPU's 1024 KB ...
(UMA)
*
Extended memory
In DOS memory management, extended memory refers to memory above the first megabyte (220 bytes) of address space in an IBM PC or compatible with an 80286 or later processor. The term is mainly used under the DOS and Windows operating systems. DOS ...
(XMS)
*
Expanded memory
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 techn ...
(EMS)
*
List of DOS commands
This article presents a list of commands used by DOS operating systems, especially as used on x86-based IBM PC compatibles (PCs). Other DOS operating systems are not part of the scope of this list.
In DOS, many standard system commands were pro ...
References
External DOS commands
DOS memory management
Expanded memory managers
{{DOS-stub