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Radeon HD 6800-Series Architechture

AMD Radeon HD 6870 And 6850: Is Barts A Step Forward?
By , Chris Angelini

In the past, we've seen a refresh to the Radeon lineup on a yearly basis, accompanied by a process improvement. But the Radeon HD 6800-series is being delivered using the same TSMC-based 40 nm node first used in April of 2009 to manufacture the Radeon HD 4770. Forty nanometer technology was also deployed across the Radeon HD 5000-series. And now it's being recycled for the Radeon HD 6800s. According to Eric Demers, CTO of AMD's GPU division, sticking with the established process made more fiscal sense this time around. Based on yields, the company essentially had to decide between two 32 nm processors or three 40 nm GPUs at a similar cost. From here, though, we're expecting AMD to skip 32 nm and transition straight to 28 nm manufacturing.

With no process improvement of which to speak, making Barts a worthwhile step forward becomes a more challenging task for the GPU architects.

We expect that you'll have a tough time telling the difference between AMD's Radeon 5800- and 6800-series GPUs on this schematic. In fact, if you squint and count the SIMD engines, the new Barts GPU looks notably weaker than the Radeon HD 5870's Cypress. Of course, performance is all in the numbers:


Radeon HD 5850
Radeon HD 5830
Radeon HD 6870
Radeon HD 6850
Shader Processors:
1,440
1,120
1,120
960
Texture Units:
72
56
56
56
Color ROPs:
32
16
32
16
Core Clock:
725 MHz
800 MHz
900 MHz
775 MHz
GDDR5 Memory Clock:
1,000 MHz
1,000 MHz
1,050 MHz
1,000 MHz
Memory Bus:
256-bit
256-bit
256-bit
256-bit
Memory Bandwidth (GB/s):
128
128
134.4
128
Die Size:
334 mm2
334 mm2255 mm2
255 mm2
Transistors (billion):
2.15
2.15
1.7
1.7
Maximum Power:
151W
175W
151W
?W
Idle Power:
27W
25W
19W
19W


The Radeon HD 6870 employs 14 SIMD engines, each hosting four texture units and 16 stream processors. Each stream processor is armed with five ALUs (AMD calls these stream cores). As a result, this GPU has a total 1120 stream cores and 56 texture units. The GPU channels output through four render back-ends, each containing eight color ROP units, resulting in an effective total of 32 ROPs. Four 64-bit memory controllers yield a 256-bit aggregate memory interface.

If any of this sounds familiar, it’s probably because, from a raw specifications standpoint, the new Radeon HD 6870 is essentially a Radeon HD 5830 with twice the render back-ends. Or, viewed differently, the Radeon HD 6870 has the same back-end as the 5870, just fewer shader cores. When the Radeon HD 5830 was released, we lamented the merciless halving of ROPs, and in a way, the Radeon HD 6870 shows us what the 5830 could have been.

But there’s one more critical component to performance, and that’s clock rate. Despite the re-use of 40 nm manufacturing, at 900 MHz the less complex Barts GPU is clocked much higher than the 5830 or even the 5850. This means geometry throughput is almost 25% higher than the 725 MHz Radeon HD 5850 because it’s limited to one primitive and one vertex per clock cycle. At the same time, the 6870’s texture unit and ALU deficit (compared to the 5850) is offset by a higher core clock, resulting in roughly the same overall performance.

At the end of the day, this means the Radeon HD 6870 performs a little better than the Radeon HD 5850, but does it with about 25% less silicon, purportedly reducing idle power and overall cost--two definite benefits for the end user. The Radeon HD 6870's memory runs at 1050 MHz, slightly faster than the 5850's 1000 MHz. The MSRP of the Radeon HD 6870 is $239.99, a bit less than the Radeon HD 5850, which still sells online for $260 and up (though rebates take it lower).

The second product based on AMD's RV870 GPU is the Radeon HD 6850. The only difference here is that two of the GPU’s 14 SIMDs are disabled, resulting in 12 SIMDs and a total 960 stream cores. The core clock drops to 775 MHz, but all of the render back-ends are fortunately left intact. The resulting performance should be notably faster than Radeon HD 5830. AMD’s suggested retail price of the Radeon HD 6850 is $179.99, and while Radeon HD 5830 cards can sometimes be found for that price, the majority of them are in the $200 range. More threatening to the Radeon HD 6850 will be an onslaught from Nvidia, which we'll also dissect in more depth.

Tessellation

On the surface, both Barts-based SKUs appear to be (at least architecturally) derived wholly from Cypress. But AMD's engineers assure us that notable improvements have been made, and features added. Perhaps the highest-profile performance-adder is an enhanced tessellation unit. According to the company, improved thread management and buffering yields between 1.5 and 2x performance in what it considers the most important (real-world) range, below 14 tessellation factors. We can see the result of this in the Unigine Heaven benchmark:

The first chart shows the benchmark results with no tessellation applied, and the second demonstrates results with the normal tessellation level enabled. When tessellation is used, we can see the Radeon HD 6850 reach Radeon HD 5850-class performance, and the Radeon HD 6870 leaves the other Radeons behind.

This is a particularly sensitive subject for AMD. When Nvidia launched GF100, it went straight for AMD's throat, claiming the single fixed-function tessellation unit would not scale well. Instead, Nvidia advocated its Polymorph Engine, present on each of its architecture's Shader Multiprocessors. Parallelizing geometry, it claimed, was the key to adding the next level of realism. AMD counters that geometry is actually better-addressed through its single unit is actually much more efficient.

At the end of the day, Nvidia still outperforms AMD in synthetic measures of tessellation. But we still haven't seen a game capable of coming anywhere close to giving Nvidia an advantage due to its geometric processing potential. From what we hear, Nvidia took Ubisoft to bed, and the result, HAWX 2, employs what amounts to a worst-case geometry scenario for AMD. That might turn out to be the first example of Nvidia's advantage, even if it was sponsored. It remains to be seen whether AMD can get the developer to add a slider for geometry detail. At least, that's the current plan, according to company representatives.

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  • 0 Hide
    jamie_macdonald , 22 October 2010 16:40
    Will wait to see them in a real PC first ... sound half OK though, see what the high end ones perform like ^^
  • 0 Hide
    tranzz , 22 October 2010 18:11
    would be nice to see dynamic tessalation from the software. So visuals scale to maintain frame rates across a variety of cards.
  • 0 Hide
    nesters , 22 October 2010 18:31
    As much as I have seen, those cards scale pretty good in CrossFire, in some games HD6870CF outperforming HD5870CF.
  • 1 Hide
    mi1ez , 22 October 2010 19:52
    WHATWHATWAHT!
    Quote:
    But this sure would be a good time to introduce a card with a fully-equipped GF104 and 384 CUDA cores enabled (Ed.: I can’t comment, but I know something that you don’t, Don).

    Naming scheme aside, these look like pretty competent cards and I for one am looking forward to the high end and indeed 7-series cards these are supposedly leading up to.
  • 0 Hide
    the_krell , 22 October 2010 20:02
    Just to correct your Shakespeare...
    Wherefore art thou, Radeon 6700? Should read Wherefore art thou, Radeon 6800?

    Wherefore means Why? For what reason?

    :) 
  • 1 Hide
    aje21 , 22 October 2010 20:16
    Quote:
    Meet AMD Accelerated Parralel Processing (APP)

    At least the image had Parallel spelt correctly ;-)
  • 0 Hide
    dizzycriminal , 22 October 2010 20:59
    All the other review sites have found the 6850 performs better than the 460 1Gb. So im not sure what to believe. Apart from now is looking like a good time to get a new GPU. A 6850 is looking like the way to go.
  • 0 Hide
    lemonadesoda , 22 October 2010 21:45
    Read each review "test setup" carefully. You will see that tom has used CURRENT and not LAST MONTHS drivers. Read the intro. There is a slight OC on this GTX460 BECAUSE all 68xx cards are OC'ed. Tom used the same average OC.
  • 0 Hide
    aln135 , 22 October 2010 22:08
    These two new mid range cards seem pretty good and considering they are £150 for HD6850 and £200 for HD6870 a very good buy indeed. cant wait for the reviews of the HD6970 and HD6990 though
  • 0 Hide
    Anonymous , 22 October 2010 23:28
    Why are you benching with NoAA. At least use 2xAA. Unless your tested show that the HD68xx can't hack AA.
  • 0 Hide
    sam_p_lay , 23 October 2010 04:58
    Mi1ez - we already had the Radeon 7000 series. That's where it started, before moving onto the 8500 and then the 9700 and 9500. ATi/AMD have gone right around the clock now - they may need to come up with a new name. Or, just follow nVIDIA's example and knock off a '0' and start using 3-digit model numbering.

    Did anyone else notice a few charts with the size of the bars not matching the numbers? There was a 5850 score of 23fps that I'm pretty sure was meant to read 33fps based on the fact that the 27fps GTX460 score below it had a longer bar.

    Also surprised THG didn't make a bigger deal of the 94% FPS gain from Crossfiring these new Radeons... that's even better than the average gain from two-way GF104/GF106 SLI! And if morphological AA can deliver supersampling level smoothing with negligible FPS and definition loss at decent res, that's some very attractive value add!
  • 2 Hide
    LePhuronn , 25 October 2010 02:38
    sam_p_layMi1ez - we already had the Radeon 7000 series.


    OK, Radeon HD 7000 series, Mr Pedantic ;-)
  • -2 Hide
    Anonymous , 25 October 2010 15:21
    looking at the local prices here the 5850 is the same price as the gtx460 and the 5870 is more expensive than the gtx470

    Looking at that and the quality of drivers it's easy to say that NVIDIA is the pick of the moment, this might change if prices start shifting around
  • 0 Hide
    makrish , 25 October 2010 20:59
    putsomethingherelooking at the local prices here the 5850 is the same price as the gtx460 and the 5870 is more expensive than the gtx470Looking at that and the quality of drivers it's easy to say that NVIDIA is the pick of the moment, this might change if prices start shifting around


    Not the 5870 or the 5850, the 6870. The next gen. The 6870 is about 200 pounds on ebuyer.com (UK prices), and the 5870 is 280, and the 470 is about 250. This puts the 6870 at 50 pounds cheaper, and it scales better than 5870's in CF. Realistically, this is the best card out there for its price range. I'd prefer two of these to a 5870 or even a 5970.
  • 1 Hide
    LePhuronn , 25 October 2010 21:27
    Roll on HD 6990 vs GTX 485

    Full-fat Southern Islands vs full-fat Fermi

    XD
  • 0 Hide
    ben BOys , 26 October 2010 00:11
    goes to show price/proformance is outrigh winner against proforma=nce on it's own. Nvidia need to step up there game if they want catch there falling crown
  • 0 Hide
    williehmmm , 26 October 2010 01:37
    makrishNot the 5870 or the 5850, the 6870. The next gen. The 6870 is about 200 pounds on ebuyer.com (UK prices), and the 5870 is 280, and the 470 is about 250. This puts the 6870 at 50 pounds cheaper, and it scales better than 5870's in CF. Realistically, this is the best card out there for its price range. I'd prefer two of these to a 5870 or even a 5970.


    Ebuyer shows the 6870 starting at £193, but the GTX 470 starting at £186.

    Hopefuly this cut throat competitve pricing does remain, then we all benefit.

    I paid over £300 each for my pair of GTX 470s, 7 months ago. Depreciation of £230, but by golly, I've had a lot of 3D stereoscopic fun in that time!
  • 1 Hide
    Silmarunya , 26 October 2010 03:32
    While their naming scheme is pretty absurd, the cards themselves deliver. Nvidia's GTX 460 enjoyed a brief moment of utter dominance at the €200 price point, while the new 6850 offers same performance, same price and lower power consumption. As such, Nvidia's Fermi series now fails to dominate a single segment of the market. Sad for a company that wore the performance crown for years...

    The 6870 isn't a clear winner in its market segment, but it's certainly worth buying. As such, I think AMD just made another winning move.

    Bring on the 6970, with some luck it's enough to hand the single GPU performance crown back to AMD. And then Nvidia has... erm... CUDA and PhysX. The first isn't useful for gamers, the second is nothing more than a gimmick. Life's good for AMD fans atm :) 
  • -1 Hide
    williehmmm , 26 October 2010 17:59
    And then Nvidia has... erm... CUDA and PhysX. The first isn't useful for gamers, the second is nothing more than a gimmick. Life's good for AMD fans atm


    Nvidia has 3D vision and a backbone of support for 3D sterescopic compliance in significantly more titles. AMD is going to be catching up for years and even then relying on 3rd party software, drivers and peripherals, so they have very little influence in getting it to work right.

    Not to mention that if you've spent several hundred pounds/euros on a high end 5xxx series card, you now have to ditch that and spend out again to get 3D functionality.

    Nvidia have made cards as low spec as the GT220 & GT240 3D bluray compatible, just by updating that support in drivers back to those cards. That AMD can't even do that for its flagship card, the 5970 which folk have have spent £400 - £500 on, it's just a sin.

    AMD fans seem to be getting fleeced. Despite Nvidia's aggresive pricing (a good thing for everyone) and their PR machine kicking into gear in reaction, at least they keep adding functionality to their existing line up of cards.

    As for Nvidia not dominating a single segment, neither do AMD. The price /performance statistics show they are even, there is no outright winner, it's a score draw. If you're an AMD fan you're getting a good deal buying their card, if you're an Nvidia fan, you're getting a good deal buying their card.
  • 1 Hide
    sam_p_lay , 27 October 2010 05:01
    LePhuronn - Radeon HD 7000 had occurred to me, but I hope to God they don't do it! Reckon they've got their money's worth out of the Radeon name and should come up with something new... was disappointed with nVIDIA for sticking with GeForce after the 9 series, though I suppose it makes sense with such a strong brand profile.
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