GDI enables the display driver by calling the Enable function and directing the driver to initialize a physical device for subsequent graphics output. A physical device is a PDEVICE structure that represents the display and its current operating state. A display driver uses the physical device information to determine how to carry out specific tasks, such as which display mode to use. The display driver initializes the physical device by copying information to the PDEVICE structure.
In 386 enhanced-mode Windows, GDI calls the Enable function only when the display driver is first loaded. In standard-mode Windows, GDI calls the Enable function when first loaded and whenever the user switches back to Windows from a non-Windows application.
GDI calls Enable twice: Once to retrieve a copy of the driver's GDIINFO structure, and a second time to initialize the PDEVICE structure. After the first call, GDI uses the dpDEVICEsize member in the GDIINFO structure to determine the size of the driver's PDEVICE structure. GDI then allocates memory for the structure and calls Enable for the second time, passing a pointer to structure. At this point, the driver initializes the display hardware and the structure.
When Windows switches back from a non-Windows application, GDI calls Enable once. The driver reinitializes the display hardware and the PDEVICE structure, reinitializing any screen data that may have been discarded when Windows switched to the non-Windows application.
Although only the display driver initializes and uses the PDEVICE structure, it is GDI that allocates memory for the structure, determines when to pass it to the driver's output functions, and deletes the structure when it is no longer needed. Except for the first two bytes (16 bits) of the PDEVICE structure, the content and format of the structure depends entirely on the display driver. Typically, the driver includes all the information that the output functions need to generate appropriate graphics commands. The first two bytes, on the other hand, must be set to a nonzero value. GDI reserves zero to indicate a PBITMAP structure. GDI creates and uses PBITMAP structures in place of PDEVICE structures when an application creates a memory-device context.
GDI disables the display driver by calling the Disable function when Windows quits. GDI expects the driver to free any resources associated with the physical device and to restore the display hardware to the state before Windows started. After the driver returns from the Disable function, GDI frees the memory it allocated for the PDEVICE structure and frees the driver, removing any driver code and data from memory.
In standard-mode Windows, GDI also calls Disable when the user switches to a non-Windows application. In this case, GDI temporarily disables the physical device, expecting the driver to select a text mode for the display hardware so that the non-Windows application has a nongraphics mode in which to start. Although Windows saves the display driver's data segment when it switches, it discards all other segments. Therefore, the display driver should save any data that may be discarded so that the data can be restored when Windows switches back from the non-Windows application.