The Microsoft Windows operating system, in 386 enhanced mode, uses the virtual 8086 mode of the Intel 80386 and compatible microprocessors to support single-threaded multitasking for non-Windows applications. Windows provides a virtual machine manager (VMM) which, in conjunction with virtual devices, processes interrupts and carries out I/O operations for a given application without disrupting the execution of other applications. This chapter describes the virtual machine manager, virtual devices, and other components of the operating environment for 386 enhanced-mode Windows.