The virtual DOS machine environment normally runs a specialized version of DOS known as the DOS Emulation kernel, in which most DOS services are actually provided by components of the OS/2 operating system, transparently to the application and outside of the real mode address space in which the DOS application executes. This kernel is described in MVDM DOS Emulation.
For this reason, many DOS control structures are not accessible to DOS applications running in VDMs. Applications which access these control structures cannot be run in a "normal" VDM due to the lack of these structures. The DOS Emulation kernel does not support the use of block device drivers, and applications which require such device drivers are unable to use the DOS Emulation kernel.
In order to allow such applications to be run in VDMs under OS/2 Version 2.0, the Virtual Machine Boot facility is provided. This facility allows a "real" version of DOS to be loaded into a VDM, either from a DOS bootable diskette or from a diskette image stored on fixed disk. Since the real version of DOS is therefore running in the VDM, all features, characteristics and internal control structures of that version are available to an application running in the VDM.
An example of an application that needs to be run in this way is PC Support/400.
The Virtual Machine Boot facility is described in detail in Virtual Machine Boot.