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80386 The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistorsmicroprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...
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 Hardware virtualization is the virtualization of computers as complete hardware platforms, certain logical abstractions of their componentry, or only the functionality required to run various operating systems. Virtualization hides the physica ...
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 DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems. DOS may also refer to: Computing * Data over signalling (DoS), multiplexing data onto a signalling channel * Denial-of-service attack (DoS), an attack on a communicatio ...
) 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 DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems. DOS may also refer to: Computing * Data over signalling (DoS), multiplexing data onto a signalling channel * Denial-of-service attack (DoS), an attack on a communicatio ...
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 OS/2 (Operating System/2) is a series of computer operating systems, initially created by Microsoft and IBM under the leadership of IBM software designer Ed Iacobucci. As a result of a feud between the two companies over how to position OS/2 ...
2.x (since 1992),
4690 OS 4690 Operating System (sometimes shortened to 4690 OS or 4690) is a specially designed point of sale (POS) operating system, originally sold by IBM. In 2012, IBM sold its retail business, including this product, to Toshiba, which assumed support. ...
(since 1993), REAL/32 (since 1995) running in 386 Enhanced Mode as well as in
Windows 95 Windows 95 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of operating systems. The first operating system in the 9x family, it is the successor to Windows 3.1x, and was released to manufacturi ...
, 98, 98 SE and ME through
virtual DOS machine Virtual DOS machines (VDM) refer to a technology that allows running 16-bit/32-bit DOS and 16-bit Windows programs when there is already another operating system running and controlling the hardware. Overview Virtual DOS machines can operate ei ...
s, in
SCO UNIX Xinuos OpenServer, previously SCO UNIX and SCO Open Desktop (SCO ODT), is a closed source computer operating system developed by Santa Cruz Operation (SCO), later acquired by SCO Group, and now owned by Xinuos. Early versions of OpenServer wer ...
through
Merge Merge, merging, or merger may refer to: Concepts * Merge (traffic), the reduction of the number of lanes on a road * Merge (linguistics), a basic syntactic operation in generative syntax in the Minimalist Program * Merger (politics), the com ...
, and in
Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, whic ...
through DOSEMU. (Other
DOS DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems. DOS may also refer to: Computing * Data over signalling (DoS), multiplexing data onto a signalling channel * Denial-of-service attack (DoS), an attack on a communicatio ...
programs which use protected mode execute using user mode under the emulator.)
NTVDM Virtual DOS machines (VDM) refer to a technology that allows running 16-bit/32-bit DOS and 16-bit Windows programs when there is already another operating system running and controlling the hardware. Overview Virtual DOS machines can operate ei ...
in
x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was int ...
Windows NT-based operating systems also use VM86 mode, but with very limited direct hardware access. Some boot loaders (e.g.
GRUB Grub can refer to Grub (larva), of the beetle superfamily Scarabaeoidea, or as a slang term for food. It can also refer to: Places * Grub, Appenzell Ausserrhoden, Switzerland * Grub, St. Gallen, Switzerland * Grub (Amerang), a hamlet in Bavar ...
) 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 Memory protection is a way to control memory access rights on a computer, and is a part of most modern instruction set architectures and operating systems. The main purpose of memory protection is to prevent a process from accessing memory that ...
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 extensions (VME)

The Pentium architecture added a number of enhancements to the virtual 8086 mode. These were however documented by Intel only starting with the subsequent P6 (microarchitecture); their more recent formal name is Virtual-8086 Mode Extensions, abbreviated VME (older documentation may use "Virtual 8086 mode enhancements" as the VME acronym expansion). Some later Intel 486 chips also support it. The enhancements address mainly the 8086 virtualization overhead, with a particular focus on (virtual) interrupts. Before the extensions were publicly documented in the P6 documentation, the official documentation referred to the famed Appendix H, which was omitted from the public documentation and shared only with selected partners under NDA. Activating VME is done by setting bit number 0 (0x1 in value) of CR4. Because the VME interrupt speed-up enhancements were found useful for non-VM86 protected tasks, they can also be enabled separately by setting only bit number 1 (0x2 in value), which is called PVI (Protected Mode Virtual Interrupts). Detecting whether a processor supports VME (including PVI) is done using the
CPUID In the x86 architecture, the CPUID instruction (identified by a CPUID opcode) is a processor supplementary instruction (its name derived from CPU IDentification) allowing software to discover details of the processor. It was introduced by Intel ...
instruction, with an initial EAX value of 0x1, by testing the value of second bit (bit number 1, 0x2 in value) in EDX register, which is set if VME is supported by the processor. In Linux, this latter bit is reported as the
flag A flag is a piece of fabric (most often rectangular or quadrilateral) with a distinctive design and colours. It is used as a symbol, a signalling device, or for decoration. The term ''flag'' is also used to refer to the graphic design emp ...
in the file, under the "flags" section. In virtual 8086 mode, the basic idea is that when IOPL is less than 3, PUSHF/POPF/STI/CLI/INT/IRET instructions will treat the value of VIF in the real 32-bit EFLAGS register as the value of IF in the simulated 16-bit FLAGS register (32-bit PUSHFD/POPFD continues to GP fault). VIP will cause a GP fault on the setting of simulated IF, directing the OS to process any pending interrupts. PVI is the same idea but only affects CLI/STI instructions. First generation
AMD Ryzen Ryzen ( ) is a brand of multi-core x86-64 microprocessors designed and marketed by AMD for desktop, mobile, server, and embedded platforms based on the Zen microarchitecture. It consists of central processing units (CPUs) marketed for mai ...
CPUs have been found to feature a broken VME implementation. The second generation Ryzen (2000 series) has fixed this issue.


64-bit and VMX support

Virtual 8086 mode is not available in
x86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mo ...
long mode, although it is still present on x86-64 capable processors running in legacy mode.
Intel VT-x x86 virtualization is the use of hardware-assisted virtualization capabilities on an x86/x86-64 CPU. In the late 1990s x86 virtualization was achieved by complex software techniques, necessary to compensate for the processor's lack of hardware-as ...
brings back the ability to run virtual 8086 mode from x86-64 long mode, but it has to be done by transitioning the (physical) processor to VMX root mode and launching a logical (virtual) processor itself running in virtual 8086 mode. Westmere and later Intel processors usually can start the virtual processor directly in real mode using the "unrestricted guest" feature (which itself requires
Extended Page Table Second Level Address Translation (SLAT), also known as nested paging, is a hardware-assisted virtualization technology which makes it possible to avoid the overhead associated with software-managed shadow page tables. AMD has supported SLAT through ...
s); this method removes the need to resort to the nested virtual 8086 mode simply to run the legacy BIOS for booting.
AMD-V x86 virtualization is the use of hardware-assisted virtualization capabilities on an x86/x86-64 CPU. In the late 1990s x86 virtualization was achieved by complex software techniques, necessary to compensate for the processor's lack of hardware-as ...
can do virtual 8086 mode in guests, too, but it can also just run the guest in "paged real mode" using the following steps: you create a SVM (Secure Virtual Machine) mode guest with CR0.PE=0, but CR0.PG=1 (that is, with protected mode disabled but paging enabled), which is ordinarily impossible, but is allowed for SVM guests if the host intercepts page faults.


See also

*
IA-32 IA-32 (short for "Intel Architecture, 32-bit", commonly called i386) is the 32-bit version of the x86 instruction set architecture, designed by Intel and first implemented in the 80386 microprocessor in 1985. IA-32 is the first incarnation of x ...
*
x86 assembly language x86 assembly language is the name for the family of assembly languages which provide some level of backward compatibility with CPUs back to the Intel 8008 microprocessor, which was launched in April 1972. It is used to produce object code for ...


Notes


References

{{DEFAULTSORT:Virtual 8086 Mode X86 operating modes Virtualization Intel products Programming language implementation