A display driver sets the display mode and registers for the display hardware whenever GDI calls the Enable and Disable functions. When running under 386 enhanced-mode Windows (or other operating systems featuring pre-emptive multitasking), the driver is also responsible for saving and restoring the display mode and registers whenever Windows is switching between Windows applications and non-Windows applications.
A display driver prepares the display hardware for Windows whenever GDI calls the Enable function. To prepare the hardware, the display driver saves the current display mode, then sets the display hardware to graphics mode, initializing hardware registers as needed. Although GDI calls the BitBlt function to clear the screen as Windows starts, many display drivers eliminate the possibility of the user seeing any random data by also clearing the screen as they initialize the display hardware.
A display driver restores the display hardware to its original state whenever GDI calls the Disable function. To restore the display hardware, the driver sets the display hardware to a text mode, and restores the original number of lines. If possible, the driver should use the same text mode as before Windows started.
In a pre-emptive multitasking environment, such as 386 enhanced-mode Windows, the display driver should save and restore the display hardware whenever the environment switches Windows to or from the foreground. 386 enhanced-mode Windows switches Windows to and from the foreground whenever the user switches to or from non-Windows applications. Saving and restor-ing the display hardware ensures that the display driver is not affected by changes non-Windows applications make to the display hardware.
To detect screen switches, the display driver hooks Interrupt 2Fh and checks for the Notify Background Switch and Notify Foreground Switch functions (Interrupt 2Fh Functions 4001h and 4002h). When the driver detects one of these functions, it either saves or restores the display mode and registers accordingly. When the driver detects a switch to the foreground, the driver also calls the RepaintScreen function (USER.275) to direct Windows to restore the entire contents of the screen by repainting it.
The display driver should hook Interrupt 2Fh whenever GDI calls the Enable function. The driver hooks the interrupt by using the MS-DOS functions Get Interrupt Vector (Interrupt 21h Function 35h) and Set Interrupt Vector (Interrupt 21h Function 25h).
A display driver can temporarily disable screen switching by calling the Enter Critical Section function (Interrupt 2Fh Function 4003h). This function prevents 386 enhanced-mode Windows from proceeding with a switch until the display driver calls Exit Critical Section (Interrupt 2Fh Function 4004h).
If a display driver uses portions of video memory that are not used by the current display mode (for example, the driver uses extra video RAM for saving screen bitmaps), the driver should call the Enable VM-Assisted Save/Restore function (Interrupt 2Fh Function 4000h) to pass 386 enhanced-mode Windows the address of save and restore flags. Windows sets or clears these flags depending on whether it used the extra video memory while a non-Windows application was in the foreground. In such cases, the driver must call the Disable VM-Assisted Save/Restore function (Interrupt 2Fh Function 4007h) to disable this feature whenever GDI calls the Disable function.
Although the display driver is responsible for repainting the screen when Windows is switched to the foreground, occasionally the driver must postpone repainting because Windows is in a critical section and is not ready to process the repainting. In such cases, Windows calls the UserRepaintDisable function in the display driver directing the driver to postpone repainting. Windows will call UserRepaintDisable a second time when it completes the critical section.