science

Virtual 8086 mode

Virtual 8086 mode is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Virtual 8086 mode rather than just read about it. In short: 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.

Key takeaways

Virtual 8086 mode belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Virtual 8086 mode to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Virtual 8086 mode from memory before moving on to harder problems.

Reference excerpt

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.

Overview 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.

Usage 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.

Memory addressing and interrupts 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.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Virtual 8086 mode

Start with the simplest possible case. Write down what Virtual 8086 mode claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Virtual 8086 mode before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Virtual 8086 mode ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Virtual 8086 mode

In research: Virtual 8086 mode appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Virtual 8086 mode in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Virtual 8086 mode is common in secondary-school and first-year university syllabi. It links to neighbouring topics Intel products, Programming language implementation, Virtualization, so understanding it makes those chapters shorter.
In everyday life: Look for Virtual 8086 mode outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Virtual 8086 mode in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Virtual 8086 mode means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Virtual 8086 mode out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Virtual 8086 mode in simple terms?

Why does Virtual 8086 mode matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Virtual 8086 mode?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Virtual 8086 mode.