"Seamless" WIN-OS/2 support is the ability to run Windows applications in a window right on top of the Workplace Shell desktop. This requires the Windows video device driver and the PM video device driver communicate and coordinate their access of the video hardware. Each device driver effectively owns its piece of the screen. Allowing the Windows display device driver to access the video hardware directly avoids the more cumbersome process of a thorough task switch. However this hardware access poses integrity problems in the areas of simultaneous access of hardware, rectangle invalidation handling, window management, and the exchange of window state information between PM and seamless VDMs supported by separate video device drivers.
To address these problems, a high performance, interprocess communication, virtual device driver (VWIN.SYS) was created. It serializes the simultaneous accesses to the hardware, oversees the exchange of window state information between PM and seamless VDMs, and establishes the addressability of PM resources (either directly or indirectly) by the Windows display device driver.