"One difference between developing SLI for Windows XP was drastically different than it was for Windows Vista is that the NVIDIA hardware and driver could basically work together to make SLI function without letting the OS know what was going on. This put the entire software stack in NVIDIA?s hand, making it easier to find patches and loop holes to get SLI performance to scale well. Vista?s dramatically changed graphics driver model will no longer turn a blind eye though, and because of it, the amount of driver development time has increased."
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For Windows XP, NVIDIA simply needed to create two main driver components; really two separate drivers. One for DirectX rendering and one for OpenGL rendering. With Vista though, things have changed, and NVIDIA now needs to develop six separate drivers. One for DX9 single card, one for DX9 SLI, one for DX10 single card, one for DX10 SLI, one for OpenGL single card and one for OpenGL SLI modes. While you might just think the move from Windows XP?s DX9 driver to Windows Vista?s DX9 driver should be an easy port, the move to the new driver model changed all of that. Microsoft moved the driver stack into the user space in the operating system, effectively making the graphics driver a part of the external OS instead of the OS kernel. This keeps the kernel much more stable in the long run, but adds another layer of abstraction for NVIDIA?s software to get through before directly accessing the hardware."
"The DirectX 10 driver development has been delayed mainly because of the difficulties in being the first to develop such a piece of software (ATI has no DX10 parts available yet). When software developers like Crytek use new DX10 features in their code and something doesn?t work right, it can start a long and difficult process to find the culprit. Is it the developer?s code and implementation of the new DX10 features? Or is it the NVIDIA driver that is handling the code and executing it incorrectly? Or is it the NVIDIA DX10 driver interacting with the new driver layer of Vista? For each issue that arises, the NVIDIA team needs to look at all three options often doing much of the work themselves as part of their ?The Way It?s Meant to Be Played? initiative."
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