AMD Questions Nvidia Kepler's DirectX 11.1 Support

On Friday David Nalasco, Senior Technical Marketing Manager of AMD's Product Group, updated the AMD Game Blog with an argument that AMD offers full DirectX 11.1 support while Nvidia's Kepler GPU does not. Nvidia has since declined to comment on the post, but has previously stated that Kepler supports the key 11.1 features that are useful for gaming.

Up until now, most of what we've heard about Windows 8 has been the new Modern UI, and the seemingly lackluster market response. We've seen the introduction of new form factors that take advantage of the touchy new OS, and we've even seen the launch of Microsoft's own ARM-based tablet as part of its assault on Apple and Google.

What we generally don't hear about is what's behind the new interface and desktop wallpaper as Nalasco points out. "As with every major iteration of Windows, the lead up to its launch was filled with months, even years, of preparation by hardware and software vendors alike," Nalasco wrote. "While many industry followers focused attention on the rejigged user interface, the more important changes, at least from the perspective of a graphics hardware company like AMD, were 'under the hood'."

This is where he jumps into the DirectX 11.1 topic, saying that the company worked closely with Microsoft as DirectX 11.1 was refined and rolled out. AMD "enthusiastically embraced all of the changes" and made them a part of AMD's GCN architecture ahead of Windows 8's commercial release, he said.

"As with previous iterations, DirectX 11.1 is intended to support a wide range of different GPU architectures with different capabilities," Nalasco said. "While this does ensure broader support for the standard, it also leaves the door open for confusion. A good example of this is the recent issue regarding Nvidia’s Kepler-based products and whether they do in fact support DirectX 11.1."

He goes on to point out that by Nvidia's own admission, Kepler does not include all aspects of DirectX 11.1 – it doesn't support 4 out of 10 key technologies comprising Feature Level 11_1 – yet Nvidia previously claimed that the architecture does in fact support the platform. "So, what exactly happened?" he asked.

To defend his case, Nalasco explains what the Feature Level aspect of DirectX 11 is all about. "While every new version of DirectX adds support for new features, it also has to maintain support for older, less capable hardware," he said. "Since the release of DirectX 11, this has been managed using 'feature level' terminology.  A feature level determines a particular set of features from a given DirectX version that a GPU design can support.  If the feature level of a GPU matches the DirectX version it supports, that generally means all of the features of that version are available, while lower feature levels indicate only a subset of the features are exposed."

He goes on to offer an example of how feature levels can add confusion. "DirectX 11.1 comprises Feature Level 11_0 and Feature Level 11_1," he said. "This means that even though a GPU claims to support DirectX 11.1, it may in fact only support Feature Level 11_0, or, as in the case of Nvidia’s Kepler GPUs, only partially support Feature Level 11_1."

Naturally this is where AMD starts promoting its Graphics Core Next (GCN) Architecture which is available in AMD Radeon HD 7700, HD 7800 and HD 7900 Series graphics, along with AMD FirePro W series cards. GCN provides "complete" support for DirectX 11.1 Feature Level 11_1 in Windows 8, he said, including Target Independent Rasterization, UAV improvements and Sum of Absolute Differences, the latter of which exposes new shader instructions on the GPU that can massively accelerate a wide range of image processing tasks.

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