## Display Device Driver Reference for OS/2

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### Presentation Manager Display Device Driver

To enable a seamless session, the Presentation Manager display device driver must call the special virtual device driver, VWIN, using DOSRequestVDD. The PM driver passes initialization data to enable the seamless windows driver's access to necessary PM data and the shared data and code. After initialization, the PM driver again calls VWIN to define the current video mode and cursor size.

The PM display driver is responsible for initializing a fast safe RAM semaphore. (FSRamSemaphore) for synchronizing access to the video hardware. Both the PM and the seamless windows drivers must use the FSRamSemaphore to coordinate video access. The PM driver can force release of this semaphore when necessary, for example, if the seamless DOS session stops with the Windows driver owning the semaphore. To avoid forcing an inappropriate semaphore release, heartbeat processing has been incorporated into VWIN. The PM driver requests VWIN to initiate heartbeat processing when a timeout occurs on the PM driver's request for the FSRamSemaphore.

The PM Shield component, shown in the diagram in the previous figure, communicates with the PM driver, sending notifications whenever a seamless DOS session is created or destroyed. This enables the PM driver to modify its processing logic to be compatible with the seamless environment.

There are several other calls the PM video driver can issue to VWIN. At PM death and resurrection (that is, on a switch to a full-screen DOS or WIN-OS /2 session), the driver calls VWIN to broadcast suspend/resume drawing messages to the seamless DOS sessions. These messages are transmitted to the DOS session's drivers by way of INT 2Fh.