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AMD's Radeon HD 7950 graphics processor


Just a smidge less
— 1:37 AM on January 31, 2012

If you're plugged into the PC hardware and gaming worlds at all, you probably already know about the Radeon HD 7970, AMD's world-beating graphics card that debuted recently. As the world's first GPU built with 28-nm fabrication tech, the 7970 asserted its dominance in our tests by humbling the prior king of the hill, the GeForce GTX 580, through higher performance and lower power consumption. Not only that, but the 7970 is based on a new GPU architecture—oh-so creatively named Graphics Core Next—that establishes rough parity with Nvidia in terms of GPU-computing capabilities, as well. What's not to like about that?

Well, the price, for one thing.

As the most desirable single-GPU graphics card in the world, the 7970 commands a premium heftier than Kloe Kardashian—prices start at 550 bucks and go up from there, depending on the variant. Right now, street prices are hovering somewhere between $550 and $600 at online vendors, although we'd expect things to settle down a bit in the coming weeks. Any way you cut it, that's a tremendous ransom for a graphics card, one that most folks—even most dedicated PC gamers—will be hesitant to fork out.

Happily, AMD is practicing the time-honored tradition of product segmentation, spinning off a new version of the product that's slightly hobbled and somewhat cheaper, in order to service multiple points along the supply and demand curves. (Yes, sexy, I know. Economics has that undeniable allure.) That's where the subject of our attention today, the Radeon HD 7950, comes into the picture. AMD has tied the Radeon HD 7970 to the bed, handed the axe to Kathy Bates, and told her to swing away. The result is a graphics card that may never quite live up to its former potential but is much easier to catch in the wild. And heck, once it's healed up, you might never know about that harrowing experience in a secluded cabin. Its final specs are pretty darned good, after all.

Radeon HD 7970 Radeon HD 7950
GPU core clock 925 MHz 800 MHz
ROP pixels/clock 32 32
Peak pixel fill rate 30 Gpixels/s 26 Gpixels/s
Filtered texels/clock (int8/FP16) 128/64 112/56
Peak bilinear filtering (int8/FP16) 118/59 Gtexels/s 90/45 Gtexels/s
Shader ALUs 2048 1792
Peak shader arithmetic 3.8 TFLOPS 2.9 TFLOPS
Memory spec 3GB GDDR5 3GB GDDR5
Memory bus width 384 bits 384 bits
Memory transfer speed 5500 MT/s 5000 MT/s
Memory bandwidth 264 GB/s 240 GB/s
Max board power 250W 200W

The same GPU silicon, a chip code-named Tahiti, drives both of these graphics cards. In the 7970, it's at the height of its massively parallel powers, with 32 "compute units" (CUs) and clock speeds approaching one gigahertz, quite a lot for a graphics chip. For the 7950, AMD has disabled four of Tahiti's CUs, leaving it with 28 CUs intact and operational. That means a minor reduction in a few key graphics capabilities, including FLOPS and textures filtered, in each clock cycle. AMD's recommended clock speeds are also down, from 925 to 800 MHz, further tempering the 7950's potential throughput. The consequences of these changes may look pretty big on paper. After all, the 7950 gives up nearly a teraflop of computing power and an equal percentage of texture filtering prowess versus the 7970.

But Tahiti arguably has an abundance of those attributes. What hasn't changed so much may be more consequential. All of the chip's ROP units—used for pixel output and antialiasing—remain intact, as are all six of its memory controllers. Furthermore, memory clocks are down less than 10%, so the resulting drop in memory bandwidth is smaller than Rick Santorum's share of a primary vote, from 264 to 240 GB/s. These things, memory bandwidth and ROP throughput, are perhaps more likely to be at a premium in a Tahiti-based graphics card running today's games. In other words, the 7950 may not be giving up much in terms of real-world gaming performance.

The most notable thing it's giving up? About a hundred bucks worth of sticker shock. AMD says prices for 7950 cards will start at $449 (by which it means $450 minus a penny.) As if to underscore the relatively small reductions in this product's real-world performance, AMD expects the 7950 to undercut the GeForce GTX 580 on price while offering superior performance. Since the 7970 isn't that much faster than the GTX 580, well, you do the math.


The 7950 with its spiritual predecessor, the Radeon HD 6950


Thanks to the drops in clock speed and the number of active transistors, the 7950's max power requirement is 200W, or 50W lower than the 7970's. That difference allows the 7950 to get away with only two six-pin auxiliary power inputs, cementing its man-of-the-people status. The 7970's eight-pin power input will likely require a different, more expensive class of PSU.

Beyond that change, AMD's reference version of the 7950 is superficially very similar to the 7970, with the same 10.5" board length and the same glossy plastic cooling shroud. Most of the first wave of cards available will probably look like the reference model, until the various board makers have had time to develop custom variants. Then again, there are already exceptions to that rule.