AMD Radeon HD 6990 £550
8th Mar 2011 | 05:02
The fastest card around, just not a particularly desirable one.
AMD Radeon HD 6990: Overview
You need a fair amount of engineering muscle to jam two of the fastest GPUs you've ever manufactured onto one slab of PCB and still get it running happily, and that's exactly what AMD has done with this, the AMD Radeon HD 6990.
Since the colossal catastrophe that was the initial R600 series of cards from then ATI, the graphics strategy has changed for the boys from Texas.
Starting with the second generation of R600 cards, the HD 3000s, AMD realised that it could no longer keep trying to compete with Nvidia creating the very high-end of graphics chips.
Instead its plan was to create more modular designs that could be at once cheaper and less power hungry than its green-tinted rivals.
The lower-end segment of the graphics card market has always been the area of highest volume. The high-end pixel-pushers though have always been the tech-demos which convince the buying public the rest of the company's lineup has some of their magic in them too.
So whoever had the best top-end GPU could call the shots lower down the pecking order too.
What AMD decided to do to still maintain a presence at the top-table though was to take it's lower-powered GPUs and double them up on a single PCB. That way it could take advantage of multi-GPU CrossFire tech and still sell it as a single graphics card.
And thus the Radeon HD 3870 x2 was born.
AMD wasn't the first to come to market with a single unit, multi-GPU solution, Nvidia had previously released the GeForce 7950GX2. That wasn't quite a single card as it had twin PCBs bound together with an SLI bridge between them, though did plug into a solitary PCIe slot.
Unfortunately it wasn't very good, and Nvidia dropped support soon after launch.
To be fair the Radeon HD 3870x2 wasn't much cop either, but it set a precedent for AMD which has seen its multi-GPU CrossFire performance improve exponentially over the last few years and has laid the foundations for some great cards.
The AMD Radeon HD 6990 follows in that tradition taking the top GPU from this current generation, playing around with it a little before fusing a couple onto a single slice of PCB with one hell of a chunky cooler on top to boot.
AMD's focus on multi-GPU as its top end has meant that it's had to drastically improve its CrossFire technology, and now we're at the point where, on a vast percentage of new titles, we're seeing 2x performance increases from that second card.
But are you better off picking up a single card solution such as this AMD Radeon HD 6990 or picking up two of the cards it's based on, the AMD Radeon HD 6970, and building a proper CrossFire rig yourself?
Only one way to find out. Dive in…
AMD Radeon HD 6990: Architecture
As I've already mentioned the AMD Radeon HD 6990 is based on the latest and greatest DirectX 11 GPUs AMD is currently producing, the Cayman XT. That GPU is the one powering the AMD Radeon HD 6970, and with a little software modification in the AMD Radeon HD 6950 too.
There has been a certain amount of modification done to those Cayman GPUs though to squeeze them into the multi-GPU format which we'll come to later. This slightly modified GPU then is codenamed Antilles.
GPU-aside though you're looking at almost exactly double the componentry used on a single Radeon HD 6970. Therefore we've got a full 4GB of GDDR5 graphics memory, the same 256-bit memory bus and one hell of a power draw to boot.
From scratch the Radeon HD 6990 has been devised and designed as a 450w graphics card.
Now before you start getting all defensive over those PCIe power limitations it's time to start explaining exactly what AMD has done to those Cayman GPUs to magically transform them into Antilles chips.
On one hand it's done nothing, but on the other BIOS setting things have changed.
Yup, the Radeon HD 6990 has the dual-BIOS functionality that came with both the Radeon HD 6970 and HD 6950. This time around though it's being used to keep to the power restrictions set by the PCIe bus.
The primary positioning, the default setting you'll see retail cards set at out of the box, restricts the Antilles GPUs to 830MHz core speeds.
That's only slightly quicker that the cores of the Radeon HD 6950 at 800MHz.
As well as limiting the core speed that restriction also means the power draw is limited to 375w, the theoretical maximum power draw possible with a single card. Each of the two 8-pin PCIe power connectors the Radeon HD 6990 needs to run delivers 150w per plug and the PCIe slot itself is capable of delivering another 75w itself.
This adds up to the 375w total.
That primary BIOS setting is read-only, but the second chip is able to be flashed with a new ROM should you wish. At launch the switch will be covered with a warning sticker and you'll have to accept all responsibility for anything that occurs under the second BIOS setting.
Why so serious?
Well, this is what we mean by the card being designed for 450w. Having the GPUs running at the 880MHz core speed the Radeon HD 6970 has its chips running at means the HD 6990 draws a significant amount of power outside the standard PCIe limits.
And you're going to need to make sure you've got a hefty PSU, and one that delivers a solid, stable level of juice to the motherboard and card to enable it to run happily at that overcooked speed.
AMD is likening this dual-BIOS, Antilles Unlocking Switch to its Overdrive software in the Catalyst Control Centre, calling it a 'hardware Overdrive option.'
Previous dual-GPU cards from AMD have used slightly clocked-down GPUs to maintain stable performance, and this is the first time we've seen the full-fat cores enabled on such a beast.
But the HD 6990 still has the same mammoth footprint of the HD 5970, though the layout is markedly different. Instead of having the fan at the end of the card it is now sat directly between the two GPUs.
This ensures both chips get equal amounts of cooling, unlike the previous generations where one was cooled more than the other. This symmetrical layout also reportedly allows for more efficient power flow as the voltage regulators are situated equidistant between the GPUs.
Each chip also has its own vapor-chamber heatsink attached to it via a phase-change thermal interface material. So take care dismantling it…
Big, hot and heavy then, so how does it perform?
AMD Radeon HD 6990: Benchmarks
Despite being the fastest graphics card around the CrossFire setup of two Radeon HD 6970s puts it into second place in our benchmarks.
With the Antilles Unlocking Switch enabled you can hit the same speeds as the HD 6970s but there's still no guarantee your PSU will be able to cope with the extra power draw required of it.
It's obvious that the single-GPU power of the Nvidia GTX 580 cannot compete with the current cream of the AMD crop, though it should give a fair indicator of how good the GeForce GTX 590 could be housing two of the GTX 580's GPUs.
In recent tests SLI was hitting close to the 2x performance boost from a second card too...
DirectX 11 Tessellation performance
DirectX 11 Gaming performance
DirectX 10 gaming performance
AMD Radeon HD 6990: Performance
This is it isn't it? This is what you want to know, how does this frankly enormous graphics card perform in-game.
The short, obvious, answer is very well indeed.
It's obvious because we've already tested how quick two of these GPUs paired up can be by CrossFiring both the AMD Radeon HD 6950 and AMD Radeon HD 6970.
So it will come as no surprise to learn that there really isn't any single graphics card yet capable of coming close to giving the Radeon HD 6990 a real run for its money. As you can see from the benchmarks Nvidia's current top-end card, the GeForce GTX 580, is quite a way off the pace of this twin-GPU setup.
That's not a great surprise considering the single GPU Fermi card has less than half the graphics memory and half as many chips flinging the pixels around. But then it's also £150 less than this pricey ol' card.
The key thing to take away from the Radeon HD 6990's performance at it's stock settings is that, while it consistently outperforms the Radeon HD 6950 in CrossFire, it comes up short against the higher clocks of the standard HD 6970s when paired up.
This is why AMD has dropped in the added extra of the Antilles Unlocking Switch.
That puts its clock speeds to match the HD 6970. When this 'hardware Overdrive' BIOS is used the performance numbers jump up that little bit extra to sit almost exactly where a CrossFire HD 6970 setup does.
The overclocked speed of this Radeon HD 6990 then puts it that little bit ahead of the previous generations of AMD multi-GPU cards. Those cards couldn't get by running their GPUs at the same clock speeds as the fastest chips of their respective generations, whereas this can.
Granted with all this graphical prowess, and four of AMD's tessellation engines working in unison, we still can't beat 30fps in our Metro 2033 benchmark at the top resolution, but we are getting close.
And it's at that 2560x1600, 30-inch panel, resolution that the HD 6990 needs to be tested. After all there's little point pairing such a graphics processing behemoth with a weedy 1080P monitor, you're just not getting the most from your £550-odd outlay.
AMD Radeon HD 6990: Verdict
Fastest graphics card money can buy.
Says it all doesn't it? Therefore it must also be the bestest too. Everyone loves it, AMD will sell out of all its stock and we can all go home knowing that graphics cards have reached their zenith.
Would that it were, dear reader, would that it were…
Unfortunately a number of things have conspired to make the AMD Radeon HD 6990 not quite the graphical giant AMD hoped it would be.
When AMD first started down the road of multi-GPU, single cards, its CrossFire technology wasn't widely trusted, and also didn't give the sort of nigh-on 2x performance increase we felt the extra outlay deserved.
Back then a single card that took much of the driver-related hassle out of a traditional CrossFire setup was almost inspirational. Now a CrossFire setup isn't beyond the reach of most of us, and paying the same sort of price for a generally faster HD 6970 setup makes more sense.
This is where it gets tricky for the HD 6990 you see.
A pair of HD 6970s beats it in performance terms unless you invoke the mystical Antilles Unlocking Switch. But there is no guarantee that will work with your power supply, no matter how many PCIe plugs you've got trailing out of it like some many-tentacled Cthulu.
In order to power the HD 6990 in full-fat Cayman XT mode your PSU needs to have a clean delivery of juice to the motherboard and card and be happy to fire 450w down its pipes without slipping up.
AMD wont guarantee the functionality of this second BIOS, so unless you know for sure your PSU can hack it then you really need to look at it in terms of the stock performance.
And, as we say, there the HD 6970 in CrossFire has it beat.
This is all before we even think about the BIOS tweakery you can do with a reference HD 6950 and its own dual-BIOS switch. A pair of reference-design cards will set you back just £440-odd and even running at stock speeds will deliver performance very close to the HD 6990.
Take the minimal risk of flashing the second BIOS chip with a HD 6970 ROM and you've got it beat for over £100 less cash.
So is there any reason to bag yourself a Radeon HD 6990? To be fair, no. At the moment AMD can say that they have, without doubt, the fastest graphics card available. That's a nice thing to say, but is almost irrelevant given the price and performance of its CrossFire cards.
If the HD 6990 was cheaper than a pair of HD 6970s then it would be worth a look – it's not as fast and so a little saving would go a long way. As it is it's an awful lot of pennies for a setup you can create yourself for a lot less.
And then there's Nvidia.
It has stayed very quiet about the forthcoming GeForce GTX 590, leading us top believe it was waiting to see what price the HD 6990 tipped up at, and how it performed.
If reports are to be believed and the GTX 590 is packing two of the GF110 from its GeForce GTX 580, then it's going to absolutely hose the poor HD 6990. But then it could also be the most expensive graphics card anybody has ever seen…
So the Radeon HD 6990 may be the fastest graphics card around bar none, but that situation may not even last out the month.
The fact AMD has created a single card housing this much graphical horse-power is truly impressive and it does offer some serious performance chops.
The Antilles Unlocking Switch also allows it to behave just like a standard top-end AMD CrossFire setup, something the previous generations of multi-GPU cards couldn't offer.
Unfortunately AMD has priced its Radeon HD 6990 out of the market with a price-tag that's at best the same as a pair of HD 6970s, for generally less performance.
It's also far too expensive considering the relative cost of a pair of reference-design HD 6950s, and all the BIOS tweakery those brilliant cards offer.
I also have some reservations about the cooling setup. I understand the need to have the centralised fan blowing air equally across the twin GPUs but it is expelling an awful lot of heat directly into your PC's chassis.
It's got one hell of a footprint too, making it far too long for a majority of gaming cases.
Despite taking the fastest graphics card in the world title, it's just not a particularly desirable card.