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High-end Socket AM3 chipsets under the microscope

Throwbacks updated for the Phenom II

AMD's Phenom processor had a rough first year and a half. The CPU debuted with a troublesome TLB erratum and lower-than-expected clock speeds, ruining the coming out party for AMD's quad-core architecture. Competition was stiff, as well, with Phenoms facing off against a formidable lineup of Core 2 processors that were just hitting their stride with a new 45nm Penryn core. Even the fastest Phenoms available were no match for the best Core 2 Quads, forcing AMD to cede the high end of the market to Intel. AMD still had flagship CPUs, of course, but they've effectively been flying at half-mast ever since.

The lack of high-margin, high-end desktop processors may have hurt AMD financially, but it hasn't necessarily been a bad thing for enthusiasts. We may covet ultra-expensive CPUs like Intel's Extreme Editions, but we're far too frugal to shell out a grand or more for what is ultimately a modest bump in clock speed. We're more inclined to seek out the best bang for our buck, which usually means picking from the mid range and overclocking a little.

AMD's current top-of-the-line CPU, the Phenom II X4 955 Black Edition, fits nicely into the middle of the market with a street price of under $250. The 955's performance and power consumption are competitive with Intel's previously unflappable Core 2 Quads, whose LGA775 socket is now all but a dead end as far as upgrade paths go. The 955 Black Edition is also cheaper than the most affordable Core i7, and it has an unlocked multiplier that makes for easy overclocking with even the most basic of motherboards.

But you don't want just a basic motherboard for a CPU like the 955. If you were really looking to cut corners, you'd have probably settled for a dual- or triple-core CPU. Fortunately, high-end Socket AM3 motherboards cost a lot less than one might expect. Asus' M4A79T Deluxe is based on AMD's finest core-logic chipset, the 790FX, yet it sells for less than $190. Even more affordable is the M4N82 Deluxe, which uses Nvidia's top-end nForce 980a SLI chipset but can be had for less than $165.

Apart from pulling from different core logic camps, Deluxe flavors of the M4A79T and M4N82 are actually quite similar. Naturally, that raises the question: is the M4A79T, with its 790FX chipset, worth a $25 premium over the M4N82 and its nForce MCP? Let's find out.

Dueling core logic
If AMD's 790FX chipset sounds familiar, that's because it's been around since November of 2007. Originally introduced for Socket AM2, the 790FX has since been updated to support Socket AM3 CPUs. The south bridge component has also been upgraded from the SB600 to the SB700 and, more recently, the SB750. These tweaks haven't dramatically altered the 790FX's capabilities, though—it's essentially the same core logic package as a year-and-a-half ago.

Nvidia's nForce 980a SLI sounds like a fresher product. After all, it has 200 more, er, nForces than the green team's previous high-end chipset for AMD processors, the nForce 780a SLI. However, the 980a is new in name only. It's the same silicon as the 780a, now with a BIOS update to support Socket AM3 CPUs. Nvidia says it changed the chipset's model number to "clearly convey support for AM3 and DDR3," never mind that Phenom memory support is determined by the processor's on-die memory controller rather than the chipset or that Socket AM3 processors will happily plug into Socket AM2+ motherboards.

The nForce 780a SLI was released in May of last year, which makes the 980a's roots a little fresher than those of the 790FX—not that it matters much. Both adhere to the same HyperTransport 3, PCI Express 2.0, Serial ATA 3Gbps, USB 2.0, and "Azalia" high-definition audio specifications. The nForce does differ by its inclusion of a DirectX 10-class integrated graphics processor, though. This so-called motherboard GPU has the same architecture as what you'll find in a desktop GeForce 8400 GS, complete with a 500MHz core clock and 16 stream processors running at 1.2GHz. The 980a's mGPU also has a PureVideo HD video block capable of decode acceleration for MPEG2, VC-1, and H.264 high-definition video content.

The 980a's integrated graphics component may allow Nvidia to pad its GPU shipment numbers, but I don't see the mGPU bringing much value to a high-end chipset that will all but certainly be paired with a reasonably powerful discrete graphics card. With Nvidia's promising HybridPower tech dead on the desktop, there's really nothing for the motherboard GPU to do anymore. Asus evidently agrees, because the M4N82 Deluxe essentially ignores the 980a's integrated graphics component.

AMD 790FX Nvidia nForce 980a SLI
Processor interface 16-bit/2GHz HyperTransport 16-bit/2GHz HyperTransport
PCI Express 2.0 lanes 38 35
Multi-GPU support CrossFire SLI
Chipset interconnect PCIe 1.1 x4 PCIe 2.0 x16
Interconnect bandwidth 2GB/s 16GB/s
Serial ATA ports 6 6
Native Command Queuing Y Y
RAID 0/1 Y Y
RAID 0+1/10 Y Y
ATA channels 2 2
Max audio channels 8 8
Audio standard AC'97/HDA HDA
Ethernet N 10/100/1000
USB ports 12 12

Multi-GPU configurations made up of multiple discrete graphics cards are far more common in the systems targeted by the 980a and 790FX. The 790FX has a total of 38 PCI Express lanes to spare, all of which stem from the chipset's north bridge component. CrossFire support is a given, of course, and the 790FX can even split its PCIe lanes between four x8 links to facilitate exotic four-way configs.

The nForce 980a divides its 35 PCI Express between the chipset's MCP component and its nForce 200 SLI chip. Three PCIe lanes primed for auxiliary peripherals and expansion card slots branch out from the MCP. The remaining 32 lanes hang off the nForce 200 and can be arranged as a pair of x16 links or in an x16/x8/x8 config for three-way SLI.

A 16-lane PCI Express 2.0 link connects the nForce 200 with the MCP, providing 16GB/s of bi-directional bandwidth and introducing a potential choke point for graphics performance. However, Nvidia is quick to point out that the nForce 200 contains a couple of features designed to make better use of the available bandwidth. The chip supports a posted-write shortcut that allows commands to be passed between graphics cards without hitting the CPU or main memory. Also, a broadcast function is capable of replicating commands sent to one graphics card across multiple cards, reducing the burden on the CPU and the MCP/nForce 200 interconnect.

Differing approaches to PCIe connectivity aside, the 790FX and 980a have nearly identical peripheral payloads. Both sport six 3Gbps Serial ATA ports with RAID functionality, and each has a couple of "parallel" ATA channels. You get a dozen USB ports with each chipset, too. However, the nForce has an integrated Gigabit Ethernet controller, while the 790FX lacks a networking component. Motherboard makers have to rely on third-party chips to bring GigE to the 790FX.