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AMD Radeon HD 7970 & CrossFireX review: 28nm and GCN
by Damien Triolet
Published on February 21, 2012

Just before Christmas, AMD decided to lift the veil on the Radeon HD 7970, which was subsequently put on sale in stores as of January 9th. On the menu: a new architecture, support for the most recent technologies and of course the promise of higher performance to finally take top spot from the NVIDIA GeForce GTX 580. We're going to see if this promise has been fulfilled in a close examination of how the Radeon HD 7970 handles, alone and in CrossFire X!

2012 technologies
AMD has got into the good habit of adopting the latest manufacturing technologies and standards for its GPUs, standards that are generally rolled out across the rest of the market later. Once again the company has followed this path for its Radeon HD 7000s… or at least for some of them. The Radeon HD 7900s, 7800s and 7700s are slated to replace the Radeon HD 6900s, 6800s and 6700s with the new Southern Islands GPUs: Tahiti, Pitcairn and Cape Verde. The entry level cards will however largely, or even entirely, consist of a series of renamed current models. While it's probably pertinent for AMD to focus its resources on developing mid and high-end GPUs and APUs, this doesn’t justify an across-the-board renaming calculated to fool certain consumers and gain a sales advantage. If we say ‘certain consumers’, this is because the renamed cards may well only concern the OEM segment, fooling only the newbies who buy complete PCs. This is particularly problematic as the Southern Islands family ushers in a lot of innovations.

These new GPUs, including Tahiti which equips the Radeon HD 7900s, are made by TSMC and engraved at 28 nanometres. Transistor density has been doubled in comparison to the previous fabrication process at 40nm, which makes it possible to add more processing units and new features for a same-sized chip. Note however that energy consumption hasn’t unfortunately come down in proportion with area used. Consumption is more than ever the main parameter to be considered when designing a chip.

Tahiti also introduces Direct3D 11.1 support, which will be rolled out across the board in 2012. Designed for Windows 8, but probably with Windows 7 support too, this new API is a minor development which has above all been designed to integrate certain requests from developers and to facilitate its usage across a wide range of GPUs, something that is important with the opening of Windows to the ARM universe. Compatibility with DirectX 11, 10.1, 10 and 9 is maintained. There’s full Direct3D 11.1 support which facilitates integration with the DXVA video API. The resources of the compute shaders (UAV) can be used with all types of shaders (only the pixel shaders can share them in Direct3D 11), rasterisation is more flexible, logical operations on rendering buffers can be applied, shaders debugged and there’s support for a standard version of 3D stereo.

There’s also full OpenCL 1.2 support, which includes integration with DirectX 11 and video streams and support for multitasking at GPU level. The PCI Express 3.0 standard is also supported and means the practical bandwidth between the GPU and CPU can be doubled, as long as you’re on an X79 platform, the only platform to support it as things stand.

Finally Tahiti also opens the door onto a new architecture: GCN or Graphics Core Next.

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Tahiti: 2048 processing units and 384-bit  

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