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Nvidia's nForce 780a SLI chipset

Hybrid SLI comes to the high end
— 8:00 AM on May 6, 2008

AMD launched its Phenom processor nearly six months ago as a part of an ambitious Spider platform that also included a new 790FX chipset and Radeon HD 3800 series graphics. As the first core logic chipset to bring PCI Express 2.0 to AMD processors, and the only one capable of taking advantage of Phenom's HyperTransport 3 link and dual power planes, the 790FX was a particularly important part of the Spider platform. Unfortunately, it was saddled with an old and flaky south bridge chip, and motherboard makers were slow to adopt it.

Only a handful of boards based on the 790FX are currently on the market, which for AMD's flagship processor platform is frankly a little lackluster—not that we're surprised. Nvidia chipsets powered the bulk of the enthusiast-oriented motherboards for the Athlon X2, and the Phenom has been sorely in need of an nForce to call its own—an nForce 780a SLI, to be exact.

Nvidia gave us a first peek at the 780a at CES in January, revealing plans dripping with second-gen PCI Express, HyperTransport 3.0, and all the trappings you'd expect from a high-end Phenom chipset. The 780a also packs what Nvidia has taken to calling a motherboard GPU, enabling Hybrid SLI technology that promises significant power savings, HD video decoding capabilities, and additional monitor outputs.

We've been eagerly anticipating the 780a's arrival for months, particularly since erratum-free B3 Phenoms are now out in the wild, the 780a has finally arrived astride Asus' M3N-HT Deluxe motherboard. Keep reading for the goods on Hybrid SLI and an in-depth look at how the 780a stacks up against AMD's 790FX.

Introducing the mGPU
By far the most novel element to the nForce 780a SLI is its graphics processor, which Nvidia refers to as the motherboard GPU, or mGPU. Putting a graphics processor inside a core logic chipset is hardly a new idea—low-end chipsets have been doing it for years. Nvidia's approach with the 780a's mGPU is quite different, though. Traditional integrated graphics chipsets are designed to provide basic functionality for budget systems without the need for a discrete graphics card. The 780a's mGPU, however, is designed to work in tandem with discrete graphics products in Hybrid SLI configurations.

Before you get too excited about the potential for asymmetrical SLI, we should note that Hybrid SLI isn't quite what it sounds like. Hybrid SLI is an umbrella term that covers two very different technologies: GeForce Boost and HybridPower. The former allows for, er, hybrid SLI configurations that team a motherboard GPU with a discrete graphics card to improve 3D performance. In order for GeForce Boost to improve performance, the motherboard GPU and discrete graphics card must be closely matched in terms of horsepower. Since integrated graphics processors are generally pretty weak, GeForce Boost only works with low-end graphics cards, making it a poor fit for a high-end platform like the nForce 780a SLI.

Hybrid SLI's HybridPower component is far more relevant to the 780a because it's designed specifically to reduce the power consumption of high-end graphics configurations. With HybridPower, the mGPU acts as the primary display adapter, and you plug your primary display right into the motherboard. If the system is idling or you're just messing around on the desktop, not using applications that demand significant graphics resources, the motherboard GPU runs the show while the discrete graphics card lies dormant to conserve power. This power saving mode doesn't rely on fancy clock throttling or special low-power states—it literally turns off the discrete graphics processor.

When additional graphics horsepower is required, HybridPower switches the system into performance mode, awakening the discrete graphics processor from its slumber. The discrete GPU then takes over graphics processing duties. Since it's not actually hooked up to the system's primary display, the discrete GPU still needs to cooperate with the motherboard GPU to put pixels on the screen. This task is achieved by copying the contents of the discrete GPU's frame buffer over to the motherboard GPU's frame buffer, which then outputs them to the display.

Nvidia says that PCI Express 2.0 provides more than sufficient bandwidth and low enough latency for seamless frame buffer replication, particularly because the discrete graphics card is typically rendering a few frames ahead of what's actually being displayed on the screen. The company acknowledges that performance can suffer in certain situations, but insists that "aggregate performance loss is expected to be less than 5%." Such a performance loss might be worth the potentially substantial power savings that HybridPower can provide, and we'll test both a little later in this review.

In addition to the potential for reduced performance, HybridPower comes with some other caveats. The first, and perhaps most important, is that it requires a HybridPower-aware graphics card. The commands used to turn the discrete graphics processor on and off are sent over an SMBus that's part of the PCI Express specification, but the graphics card has to know how to interpret those commands. To date, only Nvidia's high-end GeForce 9800 GTX and GX2 graphics cards are HybridPower-aware. We can expect Nvidia's high-end graphics products to be compatible with HybridPower moving forward, although the company seems less keen on moving the technology down into low-end parts that just don't consume much power. HybridPower is also a Vista-only feature at the moment, and it requires that the motherboard GPU occupy at least 256MB of system memory.

Interestingly, the nForce 780a SLI's motherboard GPU is much more than just a frame buffer middleman—it's a fully functional DirectX 10-class graphics processor. The GPU core has the same architecture as the GeForce 8400 GS discrete GPU and features 16 stream processors. Clock speeds are actually a little higher than the GS, with the 780a's mGPU core running at 500MHz and its shaders clocked at 1.2GHz. The mGPU even includes a PureVideo HD decode engine capable of handling "full" MPEG2, VC-1, and AVC high-definition video decoding duties for the Blu-ray and the now defunct HD DVD standards.

The inclusion of an HD video decode engine seems a little indulgent considering that Nvidia's high-end graphics cards boast similar capabilities, but it makes perfect sense in a HybridPower context. Why spin up a discrete graphics card that's only going to draw power and generate fan noise when movie playback can be handled by the mGPU? There are more limitations here, however. PureVideo HD requires a processor with a HyperTransport 3 interface because of the bandwidth requirements of HD video decoding, and its noise reduction filters won't work if you're running Vista's Aero interface.