Editing Virtual DOS machine (section)

==Overview==
Virtual DOS machines can operate either exclusively through typical [[Emulator|software emulation]] methods (e.g. [[dynamic recompilation]]) or can rely on the [[virtual 8086 mode]] of the [[Intel]] [[80386]] processor, which allows [[real mode]] 8086 software to run in a controlled environment by catching all operations which involve accessing protected hardware and forwarding them to the normal operating system (as [[Exception handling|exception]]s). The operating system can then perform an emulation and resume the execution of the DOS software.

VDMs generally also implement support for running [[16-bit]] and [[32-bit]] [[protected mode]] software ([[DOS extender]]s), which has to conform to the [[DOS Protected Mode Interface]] (DPMI).<ref name="Schulman_1994_Undocumented-DOS"/>

When a DOS program running inside a VDM needs to access a peripheral, [[Microsoft Windows|Windows]] will either allow this directly (rarely), or will present the DOS program with a [[Device driver#Virtual device drivers|virtual device driver]] (VDD) which emulates the hardware using operating system functions. A VDM will systematically have emulations for the Intel [[8259A]] [[interrupt]] controllers, the [[Intel 8254|8254]] timer chips, the 8237 DMA controller, etc.<ref name="Schulman_1994_Undocumented-DOS"/>