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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documentation, particularly for programming, you should not rely on the information found here, as it will be woefully out of date.

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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.
 
 
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