Virtual DOS machine
technology that allows running 16-bit/32-bit DOS and 16-bit Windows programs on Intel 80386 or higher computers when there is already another operating system running and controlling the hardware
Virtual DOS machines (VDM) are 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 either exclusively through typical 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 exceptions). 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 extenders), which has to conform to the DOS Protected Mode Interface (DPMI).
When a DOS program running inside a VDM needs to access a peripheral, Windows will either allow this directly (rarely), or will present the DOS program with a 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 8254 timer chips, the 8237 DMA controller, etc.
Concurrent DOS 8086 emulation mode
In January 1985 Digital Research together with Intel previewed Concurrent DOS 286 1.0, a version of Concurrent DOS capable of running real mode DOS programs in the 80286's protected mode. The method devised on B-1 stepping processor chips, however, in May 1985 stopped working on the C-1 and subsequent processor steppings shortly before Digital Research was about to release the product. Although with the E-1 stepping Intel started to address the issues in August 1985, so that Digital Research's "8086 emulation mode" worked again utilizing the undocumented LOADALL processor instruction, it was too slow to be practical.
“Virtual DOS machine” enters the record as technology that allows running 16-bit/32-bit DOS and 16-bit Windows programs on Intel 80386 or higher computers when there is already another operating system running and controlling the hardware. Crown Archives preserves that source wording while asking what Virtual, machine and technology can confirm, complicate or overturn.
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