Virtual8086

Virtual8086, commonly referred to as virtual 8086 mode (VM86), is a processor feature introduced with the Intel 80386 that allows real-mode 8086-compatible software to run inside a protected, multitasking operating environment. It enables an operating system to execute legacy DOS programs and other real-mode code without leaving protected mode, preserving isolation and system stability while providing access to the real-mode software.

In VM86 mode, the CPU executes 8086-compatible instructions but within a controlled environment managed by the host operating system. The processor uses a special virtual 8086 task context stored in a task state segment (TSS). When the running code performs privileged operations, I/O instructions, or otherwise requests access beyond the VM86 environment, the CPU traps to the host’s VM86 monitor. The host then emulates or handles the operation, returning control to the virtualized code. This arrangement provides memory protection and multitasking for real-mode software while leveraging the protections of protected mode.

VM86 mode has been used by early operating systems and compatibility layers, such as Windows running DOS programs and certain DOS extenders. It relies on the host OS to provide device emulation and to manage system resources, which can limit performance and complicate compatibility for some real-mode software. It is also constrained by the architectural rules of VM86, and some modern hardware features are not fully exposed in this mode.

Today, VM86 is largely superseded by full hardware virtualization and modern emulation techniques, such as VT-x/AMD-V-based virtual machines and advanced DOS emulators. Nevertheless, VM86 remains an important historical mechanism that enabled real-mode software to run in protected environments and helped shape early system virtualization concepts.