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 tec ...

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, Washin ...

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 Gr ...

's DR-DOS, and Datalight's ROM-DOS 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. D ...

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 software and supporting embedded systems. FreeDOS can ...

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 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 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 running a ...

. 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 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,

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 manufacturi ...

, Windows for Workgroups 3.1x, and Windows 3.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.

References

External DOS commands DOS memory management Expanded memory managers {{DOS-stub

Overview

See also

References