Under the Hood of G84

So the quick and dirty summary of the changes is that the G84 is a reduced width G80 with a higher proportion of texture to shader hardware and a reworked PureVideo processing engine (dubbed VP2 as opposed to G80's VP1). Because there are fewer ROPs, fill rate and antialiasing capabilities will be reduced from the G80 as well. This isn't as necessary on a budget card where shader power won't be able to keep up with huge resolutions either.

We expect the target audience of the 8600 series to be running 1280x1024 resolution panels. Of course, some people will be running larger panels and we will test some higher resolutions to see what kind of capabilities the hardware has, but above 1600x1200 tests are somewhat academic. As 1080p TVs become more popular in the coming years, however, we may start putting pressure on graphics makers to target 1920x1200 as their standard resolution for mainstream parts even if average computer monitor sizes weigh in with fewer pixels.

In order to achieve playable performance at 1280x1024 with good quality settings, NVIDIA has gone with 32 shaders, 16 texture address units, and 8 ROPs. Here's the full breakdown:

GeForce 8600/8500 Hardware
GeForce 8600 GTS GeForce 8600 GT GeForce 8500
Stream Processors 32 32 16
Texture Address / Filtering 16/16 16/16 8/8
ROPs 8 8 4
Core Clock 675 MHz 540 MHz 450 MHz
Shader Clock 1.45 GHz 1.19 GHz 900 MHz
Memory Clock (Data Rate) 2 GHz 1.4 GHz 800 MHz
Memory Bus Width 128-bit 128-bit 128-bit
Frame Buffer 256 MB 256 MB 256MB / 512MB
Outputs 2x dual-link DVI 2x dual-link DVI ?
Transistor count 289 M 289 M ?
Price $200 - $230 $150 - $160 $90 - $130

We'll tackle the 8500 in more depth when we have hardware. For now, we'll include the data as reference. As for the 8600, right out of the gate, 32 SPs mean one third the clock for clock shader power of the 8800 GTS. At the same time, NVIDIA has increased the ratio of Texture address units to SPs from 1:4 to 1:2. We also see a 1:1 ratio of texture address and filter units. These changes prompted NVIDIA to further optimize their scheduling algorithms.

The combination of greater resource availability and improved scheduling allow for increased efficiency. In other words, clock for clock, G84 SPs are more efficient than G80 SPs. This makes it harder to compare performance based on specifications. Apparently stencil culling performance has also been improved, which should help boost algorithms like the Doom 3 engine's shadowing technique. NVIDIA didn't give us any detail on how stencil culling performance was improved, but indicated that this, among other things, was also tweaked with the new hardware.

Top this off with the fact that G84 has also been enhanced for higher clock speeds than G80 and we can expect much more work to be done by each SP per second than on 8800 hardware. Exactly how much is something we don't have an easy way of measuring as changes in efficiency will vary by the algorithms running on the hardware as well.

With 256 MB of memory on a 128-bit bus, we can expect a little more memory pressure than on the 8800 series. The 2 x 64-bit wide channels provide 40% of the bus width of an 8800 GTS. This isn't as cut down as the number of SPs; remember that the texture address units have only been reduced from 24 on the 8800 GTS to 16 on the 8600 series. Certainly the reduction of 20 ROPs to 8 will help cut down on memory traffic, but that extra texturing power won't be insignificant. While we don't have quantitative measurements, our impression is that memory bandwidth is more important in NVIDIA's more finely grained unified architecture than it was with the GeForce 7 series pipelined architecture. Sticking with a 128-bit memory interface for their mainstream part might work this time around, but depending on what we see from game developers over the next six months, this could easily change in the near future.

Let's round out our architectural discussion with a nice block diagram for the 8600 series:



We can see very clearly that this is a cut down G80. As we have discussed, many of these blocks have been tweaked and enhanced to provide more efficient processing. The fundamental function of each block remains the same, and the inside of each SP remains unchanged as well. The features supported are also the same as G80. For 8500 hardware, based on G86, we drop down from two blocks of Shaders and ROPs to one each.

Two full dual-link DVI ports on a $150 card is a very nice addition. With the move from analog to digital displays, seeing a reduction in maximum resolution on budget parts because of single-link bandwidth limitations, while not devastating, isn't desirable. There are tradeoffs in moving from analog to digital display hardware, and now an additional issue has a resolution. Now we just need to see display makers crank up pixel density and improve color space without reducing response time and this old Sony GDM-F520 can finally rest in peace.

In the video output front, G84 makes a major improvement over all other graphics cards on the market: G84 based hardware supporting HDCP will be capable of HDCP over dual-link connections. This is a major feature, as a handful of larger widescreen monitors like Dell's 30" only support 1920x1080 with a dual-link connection. Unless both links are protected with HDCP, software players will refuse to play AACS protected HD content. NVIDIA has found a way around the problem by using one key ROM but sending the key over both links. The monitor is able to handle HDCP connections on both links, and is able to display the video properly at the right resolution.

As for manufacturing, the G84 is still an 80 nm part. While G80 is impressively huge at 681M transistors, G84 is "only" 289M transistors. This puts it at nearly the same transistor count as G71 (7900 GTX). While performance of the 8600 series doesn't quite compare to the 7900 GTX, the 80 nm process makes smaller die sizes (and lower prices) possible.

In addition to all this, PureVideo has received a significant boost this time around.

Index The New Face of PureVideo HD
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  • kilkennycat - Tuesday, April 17, 2007 - link

    (As of 8AM Pacific Time, April 17)

    See:-

    http://www.zipzoomfly.com/jsp/ProductDetail.jsp?Pr...">http://www.zipzoomfly.com/jsp/ProductDetail.jsp?Pr...

    http://www.zipzoomfly.com/jsp/ProductDetail.jsp?Pr...">http://www.zipzoomfly.com/jsp/ProductDetail.jsp?Pr...
    Reply
  • Chadder007 - Tuesday, April 17, 2007 - link

    Thats really not too bad for a DX10 part. I just wish we actually had some DX10 games to see how it performs though.... Reply
  • bob4432 - Tuesday, April 17, 2007 - link

    that performance is horrible. everyone here is pretty dead on - this is strictly for marketing to the non-educated gamer. too bad they will be disappointed and probably return such a piece of sh!t item. what a joke.

    come on ati, this kind of performance should be in the low end cards, this is not a mid-range card. maybe if nvidia sold them for $100-$140 they may end up in somebody htpc but that is about all they are good for.

    glad i have a 360 to ride out this phase of cards while my x1800xt still works fine for my duties.

    if i were the upper management at nvidia, people would be fired over this horrible performance, but sadly the upper management is more than likely the cause of this joke of a release.
    Reply
  • AdamK47 - Tuesday, April 17, 2007 - link

    nVidia needs to have people with actual product knowledge dictate what the specifications of future products will be. This disappointing lineup has marketing written all over it. They need to wise up or they will end up like Intel and their failed marketing derived netburst architecture. Reply
  • wingless - Tuesday, April 17, 2007 - link

    In the article they talk about the Pure Video features as if they are brand new. Does this mean they ARE NOT implemented in the 8800 series? The article talked about how 100% of the video decoding process is on the GPU but it did not mention the 8800 core which worries the heck outta me. Also does the G84 have CUDA capabilities? Reply
  • DerekWilson - Tuesday, April 17, 2007 - link

    CUDA is supported Reply
  • DerekWilson - Tuesday, April 17, 2007 - link

    The 8800 series support PureVideo HD the same way GeForce 7 sereis does -- through VP1 hardware.

    The 8600 and below support PureVideo HD through VP2 hardware, the BSP, and other enhancements which allow 100% offload of decode.

    While the 8800 is able to offload much of the process, it's not 100% like the 8600/8500. Both support PureVideo HD, but G84 does it with lower CPU usage.
    Reply
  • wingless - Tuesday, April 17, 2007 - link

    I just checked NVIDIA's website and it appears only the 8600 and 8500 series support Pure Video HD which sucks balls. I want 8800GTS performance with Pure Video HD support. Guess I'll have to wait a few more months, or go ATI but ATI's future isn't stable these days. Reply
  • defter - Tuesday, April 17, 2007 - link

    Why you want 8800GTS performance with improved Purevideo HD support? Are you going to pair 8800GTS with $40 Celeron? 8800GTS has more than enough power to decode H.264 at HD resolutions as long as you pair with modern CPU: http://www.anandtech.com/printarticle.aspx?i=2886">http://www.anandtech.com/printarticle.aspx?i=2886

    This improved Purevide HD is aimed for low-end systems that are using a low end-CPU. That's why this feature is important for low/mid-range GPUs.

    Reply
  • wingless - Tuesday, April 17, 2007 - link

    If I'm going to spend this kind of money for an 8800 series card then I want VP2 100% hardware decoding? Is that too much to ask? I want all the extra bells and whistles. Damn, I may have to go ATI for the first time since 1987 when I had that EGA Wonder. Reply

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