Microsoft KB Archive/811792

= Frame buffer is mapped with write combining disabled until after you restart and the new display driver is installed =

Article ID: 811792

Article Last Modified on 7/14/2005

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APPLIES TO


 * Microsoft Windows Server 2003 Service Pack 1, when used with:
 * Microsoft Windows Server 2003, Standard Edition (32-bit x86)
 * Microsoft Windows XP Service Pack 2
 * Microsoft Windows XP Service Pack 1, when used with:
 * Microsoft Windows XP Professional

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SYMPTOMS
When you install a new display driver that tries to map the frame buffer with the write combining attribute enabled, write combining remains disabled for the frame buffer until you restart the computer.



CAUSE
The new display driver shares the frame buffer with the Video Graphics Adapter (VGA) display driver. When the VGA display driver is loaded first, the VGA display driver maps the frame buffer with the write combining attribute disabled. However, the operating system does not allow the new display driver to map the frame buffer while write combining is enabled.



RESOLUTION
To resolve this problem, restart the computer to enable write combining for the frame buffer mapping.



STATUS
This behavior is by design.



MORE INFORMATION
This problem may occur when one driver maps the memory resource with write combining enabled, and another driver (or the same driver) maps that same memory resource with write combining disabled.

The video port driver tracks all requests to map the frame buffer into system space and allows only one mapping to exist at any time. If a driver requests a mapping when one already exists, the video port driver returns the virtual address range of the existing mapping. This does not change the write combining behavior of the mapping.

When you start the computer and the display driver is still unknown, the operating system loads the VGA driver as the default (first) driver. However, the VGA display driver maps the frame buffer with write combining disabled.

During installation of a new display driver, the new display driver requests a mapping of the frame buffer. The new display driver receives the same mapping that has write combining disabled that is being used by the VGA display driver. Therefore, even if the new driver requests a mapping with write combining enabled, the new driver receives a mapping with write combining disabled.

If you restart the computer, the new display driver is loaded before the VGA display driver. Therefore, the VGA display driver does not interfere with the request by the new display driver for a mapping that has write combining enabled.

Keywords: kbwdm kbvirtualmem kbprb kbdriver kbvideotech kbdisplay KB811792

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