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Encyclopedia > Virtual 8086 mode

In the 80386 and later, Virtual 8086 mode, also called virtual real mode (or VM86), allows the execution of real mode applications that violate the rules under the control of a protected mode operating system. The Intel 80386 is a microprocessor which was used as the central processing unit (CPU) of many personal computers from 1986 until 1994 and later. ... Real mode (also called real address mode in Intels manuals) is an operating mode of 80286 and later x86-compatible CPUs. ... Protected mode is an operational mode of x86-compatible CPUs of the 80286 series or later. ... Protected mode is an operational mode of x86-compatible CPUs of the 80286 series or later. ... An operating system (OS) is a computer program that manages the hardware and software resources of a computer. ...


It used the real mode way for segmentation, but used the 20-bit (actually 21-bit) address resulting from it as a linear address, so it is subject to paging. In computer operating systems, paging memory allocation (also called memory address translation) algorithms divide computer memory into small partitions, and allocate memory using a page as the smallest building block. ...


It is used to execute DOS programs in Microsoft Windows/386 and Windows 3.x and Windows 9x and Windows Me and OS/2 2.x and later through Virtual DOS machines, in SCO UNIX through Merge and in Linux through dosemu. ‹ The template below has been proposed for deletion. ... ... The Windows 3. ... Windows 9x is a term used to describe the DOS-based operating systems Windows 95 and Windows 98, similar versions of Microsoft Windows which were produced in the 1990s. ... Windows Millennium Edition, or Windows Me is a hybrid 16-bit/32-bit graphical operating system released on September 14, 2000 by Microsoft. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Virtual DOS machine (VDM) is the name of Microsofts technology allowing the ability to run MS-DOS programs on Intel 80386 or higher computers when there is already another operating system running and controlling the hardware. ... Tarantella, Inc. ... Merge is a software system which allows a user to run DOS/Windows 3. ... Linux (IPA pronunciation: ) is a Unix-like computer operating system family that uses the Linux kernel. ... DOSEMU, alternatively rendered dosemu, is a compatibility layer software package that enables MS-DOS software to run under Linux on x86-based PCs (IBM PC compatible computers). ...


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 do actually more than just supporting the virtual 8086 mode. DPMI is the method which Microsoft prescribes for a DOS program to run in protected mode and to access extended memory under a multitasking operating system like Microsoft Windows. ...


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. The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... Protected mode is an operational mode of x86-compatible CPUs of the 80286 series or later. ... Real mode (also called real address mode in Intels manuals) is an operating mode of 80286 and later x86-compatible CPUs. ... Segmentation is one of the most common ways to achieve memory protection; another common one is paging. ... In computer operating systems, paging memory allocation (also called memory address translation) algorithms divide computer memory into small partitions, and allocate memory using a page as the smallest building block. ... Memory protection is a system that prevents one process from corrupting the memory of another process running on the same computer at the same time. ...


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


 
 

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