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Hub AI
Virtual 8086 mode AI simulator
(@Virtual 8086 mode_simulator)
Hub AI
Virtual 8086 mode AI simulator
(@Virtual 8086 mode_simulator)
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 protected mode operating system. It is a hardware virtualization technique that allowed multiple 8086 processors to be emulated by the 386 chip. It emerged from the painful experiences with the 80286 protected mode, which by itself was not suitable to run concurrent real-mode applications well. John Crawford developed the Virtual Mode bit at the register set, paving the way to this environment.
VM86 mode uses a segmentation scheme identical to that of real mode (for compatibility reasons), which creates 20-bit linear addresses in the same manner as 20-bit physical addresses are created in real mode, but are subject to protected mode's memory paging mechanism.
The virtual 8086 mode is a mode for a 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. So V86 mode provides a way for real-mode programs designed for a single-tasking environment (like DOS) to run concurrently in a multitasking environment.
It is used to execute certain DOS programs in FlexOS 386 (since 1987), Concurrent DOS 386 (since 1987), Windows/386 2.10 (since 1987), DESQview 386 (since 1988), Windows 3.x (since 1990), Multiuser DOS (since 1991), Windows for Workgroups 3.1x (since 1992), OS/2 2.x (since 1992), 4690 OS (since 1993), REAL/32 (since 1995) running in 386 Enhanced Mode as well as in Windows 95, 98, 98 SE and ME through virtual DOS machines, in SCO UNIX through Merge, and in Linux through DOSEMU. (Other DOS programs which use protected mode execute using user mode under the emulator.) NTVDM in x86 Windows NT-based operating systems also use VM86 mode, but with very limited direct hardware access. Some boot loaders (e.g. GRUB) use the protected mode, and execute the BIOS interrupt calls in Virtual 8086 mode.
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 is reconfigured 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.
When interrupts (hardware, software and int instruction) occur, the processor switches off the VM86 mode and returns to work in full protected mode to handle the interrupt. Also, before servicing the interrupt, the DS, ES, FS, and GS registers are pushed on the new stack and zeroed.
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 protected mode operating system. It is a hardware virtualization technique that allowed multiple 8086 processors to be emulated by the 386 chip. It emerged from the painful experiences with the 80286 protected mode, which by itself was not suitable to run concurrent real-mode applications well. John Crawford developed the Virtual Mode bit at the register set, paving the way to this environment.
VM86 mode uses a segmentation scheme identical to that of real mode (for compatibility reasons), which creates 20-bit linear addresses in the same manner as 20-bit physical addresses are created in real mode, but are subject to protected mode's memory paging mechanism.
The virtual 8086 mode is a mode for a 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. So V86 mode provides a way for real-mode programs designed for a single-tasking environment (like DOS) to run concurrently in a multitasking environment.
It is used to execute certain DOS programs in FlexOS 386 (since 1987), Concurrent DOS 386 (since 1987), Windows/386 2.10 (since 1987), DESQview 386 (since 1988), Windows 3.x (since 1990), Multiuser DOS (since 1991), Windows for Workgroups 3.1x (since 1992), OS/2 2.x (since 1992), 4690 OS (since 1993), REAL/32 (since 1995) running in 386 Enhanced Mode as well as in Windows 95, 98, 98 SE and ME through virtual DOS machines, in SCO UNIX through Merge, and in Linux through DOSEMU. (Other DOS programs which use protected mode execute using user mode under the emulator.) NTVDM in x86 Windows NT-based operating systems also use VM86 mode, but with very limited direct hardware access. Some boot loaders (e.g. GRUB) use the protected mode, and execute the BIOS interrupt calls in Virtual 8086 mode.
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 is reconfigured 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.
When interrupts (hardware, software and int instruction) occur, the processor switches off the VM86 mode and returns to work in full protected mode to handle the interrupt. Also, before servicing the interrupt, the DS, ES, FS, and GS registers are pushed on the new stack and zeroed.