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Information about Virtual 8086 Mode

x86 processor modes
ModeFirst supported
Intel 8086
Intel 80286
  • Virtual 8086 mode
 Intel 80386
Intel 80386
Intel 386SL
AMD Opteron


In the 80386 microprocessor and later, Virtual 8086 mode, also called virtual real mode or VM86, allows the execution of real mode applications that are incapable of running under the control of a protected mode operating system.

VM86 mode uses a segmentation scheme identical to real mode, for compatibility reasons, but used the 20-bit (actually 21-bit) address resulting from it as a linear address, so it is subject to paging.

Usage

It is used to execute DOS programs in Microsoft Windows/386, Windows 3.x, Windows 9x/Me, and OS/2 2.x and later through Virtual DOS machines, in SCO UNIX through Merge, and in Linux through dosemu.

Protected mode DOS programs, either 16 or 32-bit, do not execute in virtual 8086 mode, but rather in user mode (as long as they are DPMI compatible), so the above emulators actually do more than just supporting the virtual 8086 mode.

Memory addressing

The most common problem by running 8086 code from protected mode is memory addressing which is totally different between protected mode and real mode. As mentioned, by working under VM86 mode the segmentation mechanism returns to work just like under real mode, but the paging mechanism is still active, and it is transparent to the real mode code, thus memory protection is still applicable, and so is the isolation of the address space.

Interrupts

When interrupts (both hardware, software and iret instruction) occur, the processor switches off the VM86 mode and returns to work in full protected mode to handle the interrupt. And before servicing the interrupt, the DS, ES, FS, and GS registers are pushed on the new stack and zeroed.

See also

The generic term x86 refers to the "CISC" type instruction set of the most commercially successful CPU architecture[1] in the history of personal computing, used in processors from Intel, AMD, VIA, and others.
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Microprocessor

Die of an Intel 80486DX2 microprocessor (actual size: 12×6.75 mm) in its packaging
Date Invented: Late 1960s/Early 1970s (see article for explanation)

Connects to:
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Real mode, also called real address mode or compatibility mode, is an operating mode of 80286 and later x86-compatible CPUs. Real mode is characterized by a 20 bit segmented memory address space (meaning that a maximum of 1 MB of memory can be addressed),
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Intel Corporation

Public (NASDAQ:  INTC , SEHK: 4335 )
Founded 1968 1
Headquarters Santa Clara, California
 United States

Key people Paul S.
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Intel 8086
Central processing unit

Produced: From 1978-79 to 1982
Manufacturer: Intel
CPU Speeds: 4.
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In computing, protected mode, also called protected virtual address mode,<ref name="'Protected virtual address mode' usage"> Memory access control method and system for realizing the same - US Patent 5483646 (Patent)
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286
Central processing unit

An Intel 80286 Microprocessor
Produced: From 1982 to 1986
Common Manufacturers:
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386
Central processing unit

Intel 80386 DX, 33MHz, foreground
Produced: From 1986 to Sept. 2007
Common Manufacturers:
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Unreal mode, also big real mode, huge real mode, or flat real mode, is a variant of real mode (PE=0), in which one or more data segment registers have been loaded with 32-bit addresses and limits.
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386
Central processing unit

Intel 80386 DX, 33MHz, foreground
Produced: From 1986 to Sept. 2007
Common Manufacturers:
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System Management Mode (SMM) is an operating mode first released with the Intel 386SL and available in later microprocessors in the x86 architecture, in which all normal execution (including the operating system) is suspended, and special separate software (usually firmware or a
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In the x86-64 computer architecture, long mode is the mode where a 64-bit application (or operating system) can access the 64-bit instructions and registers, while 32-bit and 16-bit programs are executed in a compatibility sub-mode.
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Advanced Micro Devices, Inc.

Public (NYSE: AMD )
Founded 1969
Headquarters One AMD Place
Sunnyvale, California
USA

Key people Héctor Ruiz, CEO
Industry Semiconductors
Products Microprocessors
Motherboard chipsets
DTV decoder chips
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The Opteron is AMD's x86 server processor line, and was the first processor to implement the AMD64 instruction set architecture (known generically as x86-64). It was released on April 22, 2003 with the SledgeHammer
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386
Central processing unit

Intel 80386 DX, 33MHz, foreground
Produced: From 1986 to Sept. 2007
Common Manufacturers:
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Microprocessor

Die of an Intel 80486DX2 microprocessor (actual size: 12×6.75 mm) in its packaging
Date Invented: Late 1960s/Early 1970s (see article for explanation)

Connects to:
..... Click the link for more information.
Real mode, also called real address mode or compatibility mode, is an operating mode of 80286 and later x86-compatible CPUs. Real mode is characterized by a 20 bit segmented memory address space (meaning that a maximum of 1 MB of memory can be addressed),
..... Click the link for more information.
In computing, protected mode, also called protected virtual address mode,<ref name="'Protected virtual address mode' usage"> Memory access control method and system for realizing the same - US Patent 5483646 (Patent)
..... Click the link for more information.
An operating system (OS) is the software that manages the sharing of the resources of a computer. An operating system processes system data and user input, and responds by allocating and managing tasks and internal system resources as a service to users and programs of the
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Segmentation is one of the most common ways to achieve memory protection; another common one is paging. Segmentation means that a part or parts of the memory will be sealed off from the currently running process, through the use of hardware registers.
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BIT is an acronym for:
  • Bannari amman Institute of Technology
  • Bangalore Institute of Technology
  • Beijing Institute of Technology
  • Benzisothiazolinone
  • Bilateral Investment Treaty
  • Bhilai Institute of Technology - Durg

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paging, sometimes called swapping, is a transfer of pages between main memory and an auxiliary store, such as hard disk drive.[1] Paging is an important part of virtual memory implemention in most contemporary general-purpose operating systems, allowing to easily
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DOS (from Disk Operating System) commonly refers to the family of closely related operating systems which dominated the IBM PC compatible market between 1981 and 1995 (or until about 2000, if Windows 9x systems are included): DR-DOS, FreeDOS, MS-DOS, Novell-DOS, OpenDOS, PC-DOS,
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Windows 2.0
(Part of the Microsoft Windows family)
Screenshot

Windows 2.0 was the first version to support free-moving overlapping windows
Developer
Microsoft

Release information
Release date:
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Windows 3.x can refer to either an individual or all of the following versions of Microsoft Windows:
  • Windows 3.0
  • Windows 3.1x
These should not be confused with the first three releases of Windows NT, which were also given "3.x" version numbers, i.e.
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Windows 9x is a term used to describe the DOS-based Microsoft Windows operating systems, Windows 95, Windows 98, and often also Windows Me,[1] which were produced in the 1990s and 2000. All these Windows releases have internal version numbers in the 4.x series.
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Windows Me
(Part of the Microsoft Windows family)
Screenshot

Screenshot of Windows Me
Developer
Microsoft

Release information
Release date: September 14 2000 [ [1] info]

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OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as the preferred operating system for IBM's "Personal System/2 (PS/2)" line of
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Virtual DOS machine (VDM) is the name of Microsoft's technology allowing to run MS-DOS programs on Intel 80386 or higher computers when there is already another operating system running and controlling the hardware.
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SCO OpenServer, previously SCO UNIX and SCO Open Desktop (SCO ODT), is a closed source version of the Unix computer operating system developed by Santa Cruz Operation (SCO) and now maintained by the SCO Group.
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