The Technology

There is a very significant distinction to be made between NVIDIA's implementation of multi-GPU on a single card and previous attempts. In the past, solutions that drop two GPUs on one PCB (printed circuit board) have relied on the capability of an SLI motherboard to configure a single physical X16 PCIe connection into two X8 data paths. While this solution works, it is not optimal in bringing multi-GPU performance to the masses. Requiring not only a chipset that will allow dynamic PCIe lane configuration, but also restricting NVIDIA based graphics boards to NVIDIA core logic based motherboards really cuts down on the potential market.

With its first in-house multi-GPU design, NVIDIA has lifted the requirement for an SLI chipset and enabled the use of their 7950 GX2 on any motherboard with an X16 PCIe slot (provided the manufacturer has proper BIOS support, but more on that later). This chipset agnostic implementation works is by incorporating a PCIe switch which acts as a bridge between the system's X16 interface and the two GPUs. Because of the way PCIe works, the operating system is able to see the two graphics cards as if they were independent parts. You can think of this as being similar to connecting a USB hub to a single USB port in order to plug in multiple devices. Only in this case, the devices and switch are all in one neat little package.

The PCIe switch itself is a 48 lane device, capable of routing each of the three x16 connections to any one of the other two depending on its intended destination. On their 7900 GX2, NVIDIA takes full advantage of this, but for the 7950 GX2, only 8 lanes are routed from the switch to each GPU. The end result is that what the chipset would have had to manage, NVIDIA's 7950 GX2 moves on board.

We mentioned BIOS compatibility, which can be a potential problem. The reason we could see some issues here is that, while PCI Express switches are perfectly valid and useful devices, we haven't seen any real commercial attempt that takes advantage of them on an add-in board. Combine this with the fact that many motherboard makers only recognize graphics hardware in their x16 PCIe slots, and we end up with some wrinkles which need to be smoothed. The system BIOS must be able to handle finding a PCIe switch, and furthermore it must be able to recognize that a graphics card is beyond the switch in order to load the video BIOS.

NVIDIA has been working hard with the rest of the industry to help get BIOS updates ready and available for launch. The list is relatively long at this point, and we can confirm that the 7950 GX2 will actually run in many ATI based motherboards right now with the proper BIOS update. Inevitably, there will be some systems which will not run the 7950 GX2 at launch. Just how large a problem this is remains to be seen, but we can't put too much of the burden on NVIDIA's shoulders for this problem. Motherboard makers do need to support more than just graphics devices in their X16 slots, and the proper handling of PCIe switches is important as well. It just so happens that NVIDIA has become the catalyst for vendors to roll out support for this type of device. While we do worry about some customers being left out in the cold, often this is the price of admission to the high-tech bleeding edge of computing. To be safe, we strongly recommend interested buyers confirm that their motherboard has proper support before purchasing.

This is also the first NVIDIA product line that will fully and completely support HDCP over DVI. This means that, when combined with a monitor or TV that also supports HDCP over DVI, content which requires HDCP to play will not have any problem. While the entire lineup of NVIDIA and ATI GPUs has been capable of supporting HDCP, no full product lines have actually implemented the required solution.

The reason this is a first is due to the requirements of HDCP. Not only must the hardware be capable of transmitting HDCP content, but it also must provide a vendor specific key. These keys are only provided to vendors after paying a hefty fee. Until now, with the lack of protected content and compatible display devices, graphics board makers have not wanted to shell out the cash for HDCP keys. These keys are actually stored on a chip that must be integrated on the graphics card, so even though older cards have the potential for HDCP, the lack of the HDCP chip means that they cannot support the feature.

While we could take a few thousand words here to editorialize the wastefulness of content "protection" in consumer markets, we'll keep our thoughts brief. Real pirates will always find a way to make their money by selling stolen content. Cost or technical barriers are not sufficient deterrents to people who make their living through illegal distribution of content. If it can be seen or heard in a decrypted format, it will always be possible to copy. Until it is mandatory that decryption hardware and software with a private key for everyone be implanted into our brains, media designed for mass distribution can never really have full protection from copying. Content protection is a flaming pit into which an industry terrified of change is demanding hardware designers, programmers and governments toss as much money as possible.

That being said, the inclusion of HDCP support on the 7950 GX2 is a good thing. There's no reason to make it more difficult on the end user who just wants to watch or listen to the media they paid for. If content providers are going to go down this route either way, then it is certainly better to be prepared. While we have not spoken with every vendor, NVIDIA assures us that every 7950 GX2 will have HDCP key hardware onboard.

Index The Card and The Test
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  • bluebob950 - Tuesday, June 27, 2006 - link

    thinking about getting a 7950 but i cant find monitor any monitors that do 2048 my boss says the dell 21" at work do but i have yet to see it what do you use? and where can i find them online?
  • bluebob950 - Tuesday, June 27, 2006 - link

    thinking about getting a 7950gt but i cant find monitor any monitors that do 2048 my boss says the dell 21" at work do but i have yet to see it what do you use? and where can i find them online?
  • JNo - Wednesday, June 7, 2006 - link

    I think the review at bares a very interesting negative conclusion, somewhat in contrast to anandtech (not that I'm dissing anandtech), and provides food for sobering thought...">

    Think seriously about what's been presented to you in this article thus far, concerning GeForce 7950 GX2, and you should (if I can do my job properly) come to a conclusion like this: NVIDIA GeForce 7950 GX2 is probably the most caveat-laden graphics purchase yet released.

    The conditions that have to be satisfied before it makes sense to get one are pretty much as follows:

    * Are you willing to live with less-than-absolute best image quality from the high-end generation right now?
    * Are you sure the games you play all have SLI support, or you're at least happy to wait for support to come in a future driver?
    * Do you have a PC platform that supports it properly, which is realistically just nForce SLI of some flavour?
    * Do you have a very well ventilated PC chassis, able to assist in the significant cooling challenges it presents?
    * Do you own a high resolution PC display, since it's built for at least 1600x1200 in current supported games?
    * If you run dual displays, are you happy for one to go blank when in multi-GPU mode?

    Be sure and really consider the first two questions, and ponder the fact that ATI Crossfire is arguably even worse at satisfying the second, given its lack of a user-adjustable game profiling system. Done so? Good.

    Now given yes to all of the above, are you then willing to spend £450 on a single graphics board? You are? Brilliant, they're available today, you'll enjoy the framerates and overall IQ, happy shopping!
  • Tephlon - Wednesday, June 7, 2006 - link

    I just wanted to point out... that list isn't just particular to this card. That's all SLI setups.

    All have to watch the heat. Drivers don't allow any SLI setup to dual-view+multi-GPU. SLI does dominate primarily in Hi-Res configs. and so on and so on.

    Its still a great list to consider... but please remember that it isn't limited to just the 7950GX2.
  • DerekWilson - Wednesday, June 7, 2006 - link

    except the 3rd point doesn't apply at all to 7950 GX2 --

    Do you have a PC platform that supports it properly, which is realistically just nForce SLI of some flavour?
    It seems there has been a lot of confusion around the support requied for this card. It will run on Intel, ATI, VIA, SiS, an NVIDIA chipsets as long as the BIOS supports non-video devices in x16 PCIe slots along with supporting add-in PCIe switches. Suprisingly many boards support these features as of this week.

    the 7950 has a bunch of advantages over a 7900 GT SLI setup, not the least of which is performance. The lenient platform requirements, power draw less than an x1900xt, potential for expansion to quad sli, and it's really very transparently plug and play (just plug it in, install drivers, and no more tweaking is required for it to work as expected).
  • Jeff7181 - Tuesday, June 6, 2006 - link

    On page three in the chart it says the 7900GTX core clock is 700 MHz (650 for vertex core). Is that accurate? So what I'm seeing and adjusting in the driver settings with coolbits is actually the speed of the vertex core?
  • DerekWilson - Tuesday, June 6, 2006 - link

    you know, I borrowed that chard from NVIDIA's reviewers guide on the 7950 GX2 ...

    I am under the impression that the vertex core of the 7900 GTX is 700MHz. As this is NVIDIA's chart, I'm not sure if vertex clock is listed first or second ...
  • Jeff7181 - Tuesday, June 6, 2006 - link

    Based on the other values in the chart, and the row/column titles, I assumed the number given was supposed to be the core clock, and the one in parenthasis was supposed to be the vertex clock. I guess it could be a type-o... otherwise I want a new 7900GTX since the core speed of mine is only 650 MHz and it should be 700 according to that chart. :D
  • v12 - Tuesday, June 6, 2006 - link

    Could you post comparative scores of the X1900XTX as well? does it beat the 7950 or not? I cant tell from this review.

  • AnnonymousCoward - Tuesday, June 6, 2006 - link

    Thanks for the sweet article. 2 GPUs on a non-SLI board is the way to go...until dual-core GPUs. When are those coming out, anyway?

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