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Virtual 8086 mode
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==Overview== The virtual 8086 mode is a mode for a [[task state segment|protected-mode task]]. Consequently, the processor can switch between VM86 and non-VM86 tasks, enabling multitasking legacy ([[DOS]]) applications. To use virtual 8086 mode, an operating system sets up a virtual 8086 mode monitor, which is a program that manages the real-mode program and emulates or filters access to system hardware and software resources. The monitor must run at [[privilege level]] 0 and in protected mode. Only the 8086 program runs in VM86 mode and at privilege level 3. When the real-mode program attempts to do things like access certain I/O ports to use hardware devices or access certain regions in its memory space, the CPU traps these events and calls the V86 monitor, which examines what the real mode program is trying to do and either acts as a proxy to interface with the hardware, emulates the intended function the real-mode program was trying to access, or terminates the real-mode program if it is trying to do something that cannot either be allowed or be adequately supported (such as reboot the machine, set a video display into a mode that is not supported by the hardware and is not emulated, or write over operating system code). The V86 monitor can also deny permission gently by emulating the failure of a requested operation—for example, it can make a disk drive always appear not ready when in fact it has not even checked the drive but simply will not permit the real-mode program to access it. Also, the V86 monitor can do things like map memory pages, intercept calls and interrupts, and preempt the real-mode program, allowing real-mode programs to be multitasked like protected-mode programs. By intercepting the hardware and software I/O of the real-mode program and tracking the state that the V86 program expects, it can allow multiple programs to share the same hardware without interfering with each other.{{Efn|For example, if one program writes to a display, then another program gets control and writes to the same display, and then the first program gets control back, it will try to use the display as if the second program had not changed it. The V86 monitor can intercept the display writes, keep track of the display state for each program, and switch the real display between them according to which program the user has selected to interact with presently. The V86 monitor emulates independent displays for each program using only one real display.}} So V86 mode provides a way for real-mode programs designed for a single-tasking environment (like DOS{{Efn|[[DOS]] is mentioned because it was especially the extensive library of existing DOS programs that Intel had in mind when they designed V86 mode.}}) to run concurrently in a multitasking environment.
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