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AMD's 790GX chipset


A new south bridge boosts Phenom overclocking
— 11:01 PM on August 5, 2008

AMD built one heck of a north bridge chip with the 780G. Not only does it feature the fastest integrated graphics core around in the Radeon HD 3200, but the 780G is also capable of full Blu-ray decode acceleration for silky smooth 1080p playback with nominal CPU utilization. The 780G is an energy-efficient affair thanks to advanced 55nm fabrication technology, and it's loaded with 26 second-gen PCI Express lanes should your gaming aspirations grow beyond modest resolutions and in-game detail levels.

This tiny piece of silicon is so good that AMD's decided to spin it into a new model: the 790GX. With its graphics core boosted from 500 to 700MHz and the addition of sideport memory riding shotgun, the 790GX is perhaps best thought of as a 780G Type R. The graphics upgrades hit the PCI Express front, too, with the 790GX arriving on motherboards designed to accommodate dual-x8 CrossFire configurations.

The 790GX hasn't come alone, either. This chipset also brings with it AMD's new SB750 south bridge chip. This upgraded SB700 adds RAID 5 functionality and an Advanced Clock Calibration (ACC) feature that AMD says allows Phenom processor to overclock higher. Read on for the skinny on ACC, how well it works, and whether AMD has made a great chipset even better with the 790GX.


A new south bridge, with a twist
Unlike the 780G that came before it, the 790GX is not targeted at mainstream audiences. Instead, AMD is gearing this chipset toward the performance and multimedia crowd. That's an odd segmentation line because the 790GX doesn't actually bring any new multimedia capabilities to the table. The chip itself is identical to the 780G, so it has the same UVD video block, with full Blu-ray decode acceleration for MPEG2, H.264, and VC-1 formats.

What AMD has done with the 790GX is cherry-picked 780G chips that run comfortably with a 700MHz integrated graphics core. At this new clock speed, the graphics core gets a model number upgrade to the Radeon HD 3300. It also gets sideport memory that AMD has now taken to calling a performance cache. Chipset-integrated Radeons have long supported caches of onboard memory as a faster front line, backed by main memory, but few motherboard makers have taken advantage of the capability. That will apparently change with the 790GX, which has a 16-bit memory interface that can work with either DDR2 or DDR3 RAM.


AMD expects motherboard makers to offer performance caches this time around, and the company says most are likely to use DDR3 memory. Our Gigabyte 790GX board features a 1Gb Elpida EDJ1115BASE-DJ-E DDR3-1333 memory chip, giving the Radeon HD 3300 128MB of local memory that should be quicker to access than reaching out to main memory through the CPU.

Like its Radeon HD 3200 predecessor, the 3300 supports Hybrid CrossFire configurations with Radeon HD 3400 series discrete graphics cards. According to AMD, the 790GX's graphics clock boost and performance cache memory allow Hybrid CrossFire setups that incorporate a Radeon HD 3470 to offer performance close to that of a Radeon HD 3650. A closer look at the relative cost of each configuration makes that claim considerably less impressive, though. You can find Radeon HD 3650 cards for between $60 and $80 online. Although a Radeon HD 3470 costs only $50, 790GX-based motherboards aren't going to be cheap. AMD expects most 790GX boards to run around $150, which puts them right in the same price range as boards based on Nvidia's nForce 750a SLI chipset. The 750a and 790GX are actually quite similar products. Both offer integrated graphics with Blu-ray decode acceleration, and both support their camps' respective SLI and CrossFire multi-GPU schemes.


Thanks to AMD's new SB750 south bridge, however, the 790GX has one new trick that the 750a can't match—Advanced Clock Calibration, or ACC. AMD's chipset division worked closely with the company's processor department on this one, gaining an understanding of the Phenom's low-level CPU interface and then proceeding to exploit it. The SB750 actually connects directly to spare pins on the Phenom CPU, through which it can change internal processor timings. The Phenom's factory defaults are designed for stock frequency thermals, but through ACC, they can be tweaked for better over- or underclocking.

Citing "competitive reasons," AMD won't be more specific about exactly how ACC fiddles with Phenom's dials. With the requisite car analogy, AMD likens its approach to taking a normal, street-going car and tuning it up for the track. Changing the Phenom's default timings apparently won't curtail the processor's expected lifespan—or at least no more than overclocking does all on its own. AMD is, of course, quick to point out that overclocking success can vary from one CPU to another, but it expects ACC to bolster a processor's top speed by at least 200MHz.


A 200MHz processor clock boost thanks to a south bridge upgrade would be very special sauce indeed, and we'll see how it pans out with our own Phenom X4 9850 Black Edition in a moment. For now, let's turn our attention back to the SB750. This is actually a new chip, but apart from ACC, we're looking at the same core logic as the SB700, which is essentially a die-shrunk SB600. Unfortunately, AMD's longstanding issues with AHCI Serial ATA controller configurations persist in the SB750, all but forcing users to run the south bridge in plain old IDE mode. That's not the end of the world, but IDE mode doesn't support Serial ATA perks like hot swapping and Native Command Queuing.

AMD 790GX Nvidia nForce 750a SLI
Processor interface 16-bit/2GHz HyperTransport 16-bit/2GHz HyperTransport
PCI Express 2.0 lanes 26* 19
Multi-GPU support CrossFire 2-way SLI
Chipset interconnect PCIe 1.1 x4 NA
Interconnect bandwidth 2GB/s NA
Serial ATA ports 6 6
AHCI Y Y
Native Command Queuing Y Y
RAID 0/1 Y Y
RAID 0+1/10 Y Y
RAID 5 Y Y
ATA channels 2 1
Max audio channels 8 8
Audio standard AC'97/HDA HDA
Ethernet N 10/100/1000
USB ports 12 12

Rather than addressing the AHCI issue, AMD turned its attention to the SB750's RAID functionality. Support for RAID 5 arrays has been added to the mix, and AMD claims it can easily outrun Intel's ICH10R with three-drive arrays, although oddly, not with four-drive configs.

That's it for new south bridge features in the SB750. The chip really only needed RAID 5 support to achieve parity with the nForce 750a, and the 790GX has more PCI Express 2.0 lanes than the nForce, anyway. Keep in mind that four of those lanes are consumed by the 790GX's chipset interconnect, though; only 22 PCIe lanes are available for onboard peripherals and expansion slots.