## Virtual Device Driver Reference for OS/2

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### Discussion

Because the DOS session Window Manager is a Ring 3 component, virtual device drivers run at Ring 0, and DOS sessions have no LDT (and therefore cannot attach to DLLs), a Remote Procedure Call (RPC) mechanism must be used to communicate between the DOS session Window Manager and a virtual device driver. The DOS session Window Manager creates a thread for this purpose, which calls the virtual video device driver to wait for a video event from any of the currently windowed DOS sessions.

Virtual video device drivers are base device drivers. "DEVICE=" statements in the CONFIG.SYS files are used to specify one or more virtual video device drivers to load. The DOS session Manager and DOS session Window Manager identify a virtual video device driver by its registered name (for example, a virtual video device driver for the primary display device would register as VVIDEO1$, one for the secondary display device would register as VVIDEO2$, and so forth). After virtual video device driver loading and initialization is complete, if no virtual device driver has registered for the primary display device, multiple DOS session support is disabled.

IBM-compatible CGA, EGA, VGA, XGA, and 8514/A adapters are supported as primary display devices, and MONO adapters are supported as secondary display devices. For these devices, all the standard configurations (that is, amount of video memory, type of monitor, and so forth) are supported. However, there are a a few exceptions. For example, support for other dual- adapter combinations, such as EGA/monochrome with CGA/color, or VGA/ monochrome with CGA/color, are not specifically supported. In addition, all EGA monitor combinations (RGB, ECD, and monochrome), EGA memory combinations (64KB, 128KB, 192KB, and 256KB), VGA and 8514/A monitor combinations (8512, 8513, and 8514), and VGA and XGA monitor combinations ( 8512, 8513, 8514, and 8515) are supported. The type of monitor used with a monochrome or CGA adapter does not affect the behavior of the adapter.