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AMD's 890GX integrated graphics chipset

Introducing the all-new SB850 south-bridge chip

AMD has been on a bit of a hot streak lately. No, I'm not talking about the dominating performance of its peerless Radeon HD 5870 GPU. I'm not referring to quad-core bargains like the nearly-$100 Athlon II X4 630, either. I speak of something far more exciting: integrated graphics chipsets.

Ok, so maybe exciting is a bit of a stretch. But AMD has definitely been on a run in the integrated graphics world, and it all started with a 780G chipset launched two years ago nearly to the day. With a DirectX 10-class graphics core, Blu-ray decode acceleration logic, and gen-two PCI Express, the 780G quickly became our integrated graphics chipset of choice—and our recommended platform for budget desktops and home-theater PCs.

This past summer, AMD replaced its mainstream integrated graphics chipset with the 785G. A refreshed graphics core with tweaked shader units and an updated video decode block punctuated this release, propelling Gigabyte's implementation into TR Editor's Choice territory.

AMD hasn't been content to confine its integrated graphics chipsets to budget microATX motherboards, though. Some six months after introducing the original 780G, a hopped up version of the chipset dubbed the 790GX arrived astride mid-range ATX boards targeted at PC enthusiasts, gamers, and overclockers. The 790GX also brought with it a new SB750 south bridge chip with AMD's first chipset-level RAID 5 implementation and an Advanced Clock Calibration capability that typically gave overclockers an extra few hundred MHz to play with.

Six months have passed since AMD lifted the curtain on the 785G, and right on schedule, an amped-up version is set to debut as the 890GX. Like the 790GX that came before it, the 890GX boasts higher GPU clock speeds and a penchant for full-sized ATX motherboards. It also sports new SB850 south bridge silicon with a 6Gbps Serial ATA controller of AMD's own design, which is very exciting indeed. Naturally, we had to take a closer look.

The core-logic Swiss Army knife
As someone who has long chastised marketing departments for escalating model numbers without merit, I would be remiss not to take issue with the 890GX's primary digit. AMD showed admirable restraint when it updated the 780G with the appropriately named 785G. The 790GX made perfect sense as a tuned-up version of the 780G, too. You'd think, then, that a hopped-up 785G would carry a 795GX model designation. But no, AMD apparently couldn't resist and has dubbed its latest north bridge component the 890GX.

Look past the model number, and you'll find that the 890GX shares the very same north bridge silicon as the 785G. The chip features just over 200 million transistors and is fabricated on a 55-nm process by TSMC. AMD sorts the chips it gets from the Taiwanese semiconductor firm, reserving only the best for the 890GX, while the rest live on as 785Gs.

The 890GX needs the cream of the crop because its Radeon HD 4290 integrated graphics core runs at 700MHz—200MHz faster than the Radeon HD 4200 in the 785G. Apart from the difference in clock speeds, though, the graphics cores are identical. Both share the same RV620 architecture, which serves up 40 DirectX 10.1-compliant stream processors.

Like most integrated graphics components, the Radeon HD 4290 is capable of carving out a slice of system memory for its own use. With such an arrangement, the IGP is forced to share memory bandwidth with the rest of the system. Fortunately, motherboard makers also have the option of pairing the 890GX's integrated Radeon with "sideport" memory. Also referred to by AMD as a DDR3 performance cache, this sideport RAM is typically a single, 128MB DDR3-1333 memory chip. One such chip can be seen sitting next to the 890GX north bridge component in the picture above.

Since graphics chips are responsible for more than 3D pixel pushing these days, I should also note that the Radeon HD 4290's Universal Video Decoder (UVD) block is fully up to date. The UVD supports dual-stream decode acceleration for high-definition MPEG2, VC-1, and H.264 video, which neatly covers all the formats used by Blu-ray movies. Video output can be piped over HDMI with an accompanying audio stream, but there are a few limitations on that front. The 890GX can't pass TrueHD, DTS-HD, or uncompressed multi-channel LPCM audio over HDMI, putting it a step behind some integrated graphics platforms.

An 890GX block diagram. Source: AMD

In addition to its graphics core, the 890GX north bridge features second-generation PCI Express logic. 16 lanes of connectivity are reserved for discrete graphics cards, and unlike the 785G, the 890GX can split those lanes evenly between a pair of x8 links for CrossFire. The 890GX has an additional six PCIe lanes reserved for expansion slots and peripherals, too.

The rest of the chipset's connectivity is consolidated in its new SB850 south bridge component, which is connected to the 890GX via an Alink Express III interconnect that offers 4GB/s of bidirectional bandwidth—twice the bandwidth of Intel's DMI interconnect. (The 2GB/s in the block diagram above refers to one-way speed). Alink Express looks a whole lot like PCIe, and I'd wager the interconnect is little more than four lanes of PCI Express 2.0.

The south bridge has two more PCIe lanes for talking to peripherals, giving the chipset 24 lanes in total. Unlike Intel's P55, H55, and H57 Express Platform Controller Hubs, whose second-gen PCIe lanes signal at the 2.5GT/s rate typical of gen-one implementations, the SB850's PCI Express lanes each boast a full 5GT/s signaling rate.

By far the most interesting element of the SB850 is its Serial ATA "3.0" controller, which supports transfer rates up to 6Gbps—roughly 600MB/s, with overhead taken into account—and all the usual RAID array configs. This is the first 6Gbps SATA controller we've seen make its way into a core-logic chipset, and it's only the second implementation of the new standard currently in the wild. AMD designed the new SATA controller itself, too, which is a departure from previous south bridge chips that used third-party storage controller logic.

AMD's older south bridge chips have a history of Serial ATA performance and compatibility issues, particularly in AHCI mode, which is necessary for features like Native Command Queuing. Developing the SB850's SATA controller itself should give AMD more control this time around, and in a moment, we'll see whether that paid off.

The dearth of storage solutions—including even high-end SSDs—capable of exceeding the bandwidth available with old-school 3Gbps SATA makes the SB850's 6Gbps SATA support feel more like forward-looking insurance than a must-have feature. AMD didn't look to the future when crafting the SB850's USB controller, though. The controller design has changed from older SB700-series implementations and now features 14 ports instead of 12. But they're all USB 2.0 rather than SuperSpeed USB 3.0. Given the speed of today's external storage devices, that strikes me as a little short-sighted.

At least AMD has squeezed a Gigabit Ethernet controller into the SB850 alongside the usual HD audio interface and, surprisingly, an old-school ATA channel. The whole thing is fabricated by TSMC at 65 nm, resulting in a chip that measures about 50 x 70 mm. According to AMD, the SB850 draws just 0.85W at idle, which is a quarter-watt less than the old SB750.