OS/2 Version 2.0 provides significantly enhanced DOS application support capability over previous versions of OS/2, using a component known as Multiple Virtual DOS Machines. MVDM offers many significant functions and features, including:
OS/2 Version 2.0 provides DOS 5.0 compatibility within the virtual 8086 mode of the 80386 processor, and allows execution of multiple concurrent DOS applications, each operating in its own 1MB virtual address space. This brings true multiprogramming to the DOS environment under OS/2. A user may run multiple DOS applications in much the same way as they run multiple OS/2 applications. DOS and OS/2 applications can be started in the same ways:
The DOS environment is more DOS-compatible than the DOS Compatibility Box implemented under previous versions of OS/2, since OS/2 Version 2.0 encapsulates the entire DOS environment in a virtual machine. This provides better protection for other processes in the system and for the operating system environment itself. With MVDM, an errant DOS application can only "hang" its own virtual DOS machine, which may then be terminated by the user without affecting other applications in the system.
DOS applications may be run full-screen, windowed, or iconized in the background. Besides being better protected, providing better compatibility and more concurrent DOS applications, the OS/2 Version 2.0 MVDM environment leaves applications with more than 630KB of memory in which to execute.
OS/2 Version 2.0 also provides support for the execution of Microsoft Windows applications (written for Windows 3.0 and/or previous versions of Windows, except version 1.0x) to execute under the control of OS/2 Version 2.0. Each Windows application executes as a separate protected mode task, and is therefore provided with the same pre-emptive multitasking and memory protection as other protected mode tasks under OS/2 Version 2.0.
Support for both the Expanded Memory Specification (EMS) and the Extended Memory Specification (XMS) is provided. DOS asynchronous communications applications can support communication speeds of up to 9600 baud. Since the DOS environments are swappable, starting many DOS sessions will not increase requirements for more physical (real) memory.
The MVDM environment also provides an extendable architecture that allows the environment to be custom tailored to emulate any DOS environment. The virtual device driver architecture supports this flexible environment. All of the low-level DOS support, which in previous versions of OS/2 resided in physical device drivers, has been moved into virtual device drivers. In virtual 8086 mode, all interrupt processing is done in protected mode; bimodal device drivers are no longer needed. The new driver architecture provides physical device drivers for basic device support and virtual device drivers for supporting virtual devices to the DOS environments. DOS settings allow the user to tailor the functioning of DOS applications in the VDM environment.