Important Notice

The pages on this site contain documentation for very old MS-DOS software, purely for historical purposes. If you're looking for up-to-date documentation, particularly for programming, you should not rely on the information found here, as it will be woefully out of date.

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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.
 
Enabling and Disabling the Display Hardware
 
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.
 
Screen Switching
 
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 restoring
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.
 
 
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