## Virtual Device Driver Reference for OS/2

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### Virtual Device Driver Initialization

After a virtual device driver is loaded, it performs the following operations, as appropriate:

Verifies the presence of corresponding hardware
Establishes communication with the corresponding physical device driver
Reserves regions of linear memory containing device ROM and RAM
Saves the initial physical device state for initializing the virtual device state upon DOS session creation
Set hooks for various DOS session events (creation, termination, foreground/background switch). See VDHInstallUserHook for a complete list of user hooks.

Virtual device drivers are initialized at Ring 0. For this reason, they cannot make Ring-3 API calls, and they only interact with OS/2 through use of the virtual DevHlp services.