Asus’ new EAH6950 graphics card is based on the new AMD Radeon HD 6950 graphics chipset, married to 2GB of GDDR5 memory along with a sprinkling of factory overclocking. That’s AMD Radeon, rather than ATi Radeon, as the time has finally come for AMD to tidy up its corporate branding after its takeover of graphics specialist ATi, bringing everything under the parent company’s branding.
Based on past form, we’d have expected the high-end HD 6970 and HD 6950 chips to have come from the same family as AMD’s mid-range HD 6870 and HD 6850. The budget HD 5770 and 5750 chips clearly hail from the previous generation of silicon. In fact, the ‘Barts’ chip used in HD 6850 and 6870 is a development of the Cypress HD 5000, which has been tidied up and refined to reduce the size of the chip and also to reduce the amount of power it draws.
By contrast, the HD 6950 and 6970 chips, code-named ‘Cayman’, were originally designed to marke the transition from a 40nm fabrication process to 32nm, which would have made the new chips considerably smaller and more efficient. However, this cunning plan was thrown into confusion when the fabrication plant TSMC scrapped its 32nm process and forced AMD to continue using the tried-and-tested 40nm process.
Thankfully, even given this set-back, AMD’s engineers were able to go to town with the design of Cayman, which boasts an increase in the chipsets’ transistor count – 2.64 billion in Radeon HD 6950/6970, compared to 2.1 billion in the earlier, mid-range Radeon HD 5850/5870 chips. These changes start with the addition of a second graphics engine or ‘front-end unit’ to the chip.
The most dramatic change to be seen in the new HD 6950 and 6970 chips is a switch from VLIW5 (‘Very Long Instruction Word’) to VLIW4. This may sound like gobbledegook, but it’s actually quite an important change. With VLIW5, the unified shaders in the graphics core of earlier chipsets were arranged in bunches of five that could handle four simple instructions or one complex instruction.
In practice, these shaders weren’t used very efficiently – so AMD has switched to VLIW4, which uses clusters of four identical shaders. In the past, people have dwelt on the number of shaders in a graphics chip, when it actually makes better sense to concentrate on the number of clusters of shaders. HD 5850 has 1440 shaders, which sounds like a larger number than the 1408 shaders in this HD 6950; however, the HD 5850 uses VLIW5 and has 288 clusters. By using VLIW4, HD 6950 is able to pack in 352 clusters.
AMD has also added a second BIOS chip to HD6950/6970, with a flick-switch located next to its CrossFireX connectors. This allows overclockers to update one BIOS and to indulge in other craziness in the sure knowledge that they will always be able to revert to the second, backup BIOS if it all goes wrong.
Other changes include a system called PowerTune, which is built around a stack of power sensors that are arrayed across the chip. These sensors do not attempt to directly measure power consumption and heat production, but instead keep tabs on the amount of work that is being performed throughout the chip. The control system knows how much power is required for a given workload and is able to anticipate how hot the graphics card will get. If necessary, it can then throttle the speed of the chip just enough to keep it under control. Previously, the thermal controls in ATi graphics chips were more akin to an emergency shut-down that could lead to a severe drop in frame rates during intensive gaming sessions.
Asus has used the HD 6950 chip in a graphics card that follows the reference design almost to the letter. This ‘double-slot; card measures 11 inches in length and has an aluminium casing to help shift the heat, rather than the plastic casing we saw on previous models. The ‘dust buster’ style blows air along the length of the card and out through the slotted bracket which also carries two DVI-I ports, one HDMI and two mini DisplayPort connectors. There are no adapters in the Asus package apart from a single power adapter and one CrossFire connector. On the top of the card, next to the dual BIOS switch, there are two six-pin PCI Express power connectors and two CrossFireX connectors.
Asus boasts that this model is a ‘Factory Overclock’ Edition; however, the extra speed it offers is pretty measly, as the core runs at 810MHz rather than the reference speed of 800MHz. That’s right, a whole extra 10MHz – with the 2GB of GDDR5 memory running at the reference speed of 1250MHz, which is an effective speed of 5000MHz.
We tested the Asus HD 6950 against an HD 6850 with 1GB of memory, as well as an older Radeon HD 5850 with 1GB of memory and the results were surprisingly mixed.
In 3DMark 11 the HD 5850 scored 3,327. The HD 6850′s results came in very similar, with 3,268 marks. The HD 6950 proved considerably faster with 3,981 marks. It’s a similar story in 3DMark Vantage. In Battle Forge, the HD 5850 achieved a frame rate of 41.6fps; the HD 6850 was slower at 36.6fps, and the HD 6950 was faster at 47.2fps.
Finally, in our Far Cry 2 test, the HD 5850 scored 55.3fps, with image quality set at maximum and Anti Aliasing at 8x. The HD 6850 offered similar performance, managing 53.2fps. The HD 6950, on the other hand, pushed that frame rate to 62.7fps.
Our testing brought us to three major conclusions about the HD 6950. The first is that the power management of the HD 6000 series is marginally better than HD 5000 which was already pretty damned good. Secondly the HD 6850 doesn’t perform as well as HD 5850 because it has a smaller number of shaders.
Finally – and surprisingly – the HD 6950 performs pretty much the same as HD 5850 at our chosen screen resolution of 1,920×1,080.
It is perfectly possible that new games in 2011 will be coded to take advantage of VLIW4, and as a result we may see a sudden leap in performance. It may also be the case that driver updates will provide a greater benefit for HD 6900 than they do for the previous generation of graphics architecture.
So what, you may be wondering, has Asus actually added to AMD’s off-the-shelf HD 6950 design?
There are two Asus utilities in the package. We ignored Asus GameOSD, as we consider it to be a gimmick – but the Asus SmartDoctor overclocking utility is generally quite useful.
Not in this case. The stock core voltage for HD 6950 is 1.1V, and you can use the Catalyst drivers to adjust the Power Control settings to +20percent. We used the SmartDoctor utility to bump up the voltage to 1.2V and then investigated the overclocking options. The maximum setting on the core speed slider is 840MHz, which is only 30MHz faster than the original speed and 40MHz faster than the reference speed. Under load, we found that the PC was drawing an extra 30W at the mains socket, yet the handful of extra megahertz made no measurable difference to performance.
The upshot is that AMD appears to be shipping HD 6950 running at the limit of its performance, while using the minimum of core voltage and keeping heat levels as low as possible.
It's perfectly possible that in time there will be future driver updates that help HD 6950, but right now we are just not won over and would stick with the cheaper HD 5850.