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Intel's X38 Express chipset


PCI Express 2.0 arrives
— 11:00 PM on October 9, 2007

Back in May, Intel launched the P35 Express chipset, a replacement for its popular 965P. Classified as a mid-range product, the P35 added official support for front-side bus speeds up to 1333MHz, came bundled with a new ICH9R south bridge chip, and became the first desktop chipset to support DDR3 memory. Enthusiasts lapped it up, and in the months since, we've seen a wave of P35 motherboards that offer great performance and tantalizing overclocking potential.

Intel's mid-range chipsets have traditionally been excellent, so the P35's success wasn't a surprise. That success also raised expectations for Intel's new high-end core logic chipset: the X38 Express. This isn't a new trend; the all-around goodness typical of Intel's mid-range chipsets has long made it difficult for the company's flagship offerings to shine.

Topping the P35 Express won't be easy, but Intel has a few tricks up its sleeve with the X38. Chief among them is next-gen PCI Express 2.0 connectivity—a first for desktop chipsets—with enough lanes for dual-x16 CrossFire configurations. As is customary for its high-end chipsets, Intel has also rolled out memory controller optimizations that promise faster performance and support for higher DDR3 memory speeds.

To find out whether these perks are enough to elevate the X38 Express over its blue-collar P35 sibling, we've run the first X38 boards from Asus and Gigabyte through a relentless series of memory controller, application, and peripheral performance tests. Read on to see how the X38 fares and what you can expect from the first wave of motherboards based on this new chipset.


Introducing the X38 Express
The meat of the X38 Express is in its north bridge chip, which is what sets it apart from the P35 Express. Intel's engineers have built several optimizations into the X38's memory controller, including what Intel calls a "flexible clock crossing architecture" and improved prefetching circuitry. Those optimizations aim to make the X38's memory controller quicker than that of the P35, which is a tall order considering how aggressively motherboard makers push memory performance on their mid-range boards.

Still, the X38 can count official support for DDR3 memory speeds up to 1333MHz as one advantage over the P35. That chipset only supports DDR3 memory up to 1066MHz, which is, well, slower. Thanks to rather quickly ramping DDR3 memory speeds, a number of DDR3-1333 modules are on the market and ready to take advantage of the X38, as well.

In addition to supporting faster DDR3, the X38 chipset can take advantage of memory modules that feature Extreme Memory Profiles, or XMP. XMP adds additional memory timing and clock speed profiles to DDR3 memory, much like Enhanced Performance Profiles (EPP) do for DDR2 memory. These profiles make it easier for less experienced users to tune or even overclock their memory through a series of profiles rather than forcing them to manipulate individual timings and bus speeds.

DDR3 memory prices remain high, though, with 2GB kits costing more than double that of their DDR2 counterparts at the same speed grade. Fortunately, the X38 also supports DDR2, providing motherboard makers with a measure of flexibility and end users with the opportunity to save quite a lot of money when putting together a new system. You probably won't find many X38 boards with both DDR2 and DDR3 memory slots, but we expect mobo makers to offer variants of their boards that support each. With the P35 Express, those models typically used the same basic board design, save for the DIMM slots.

X38 Express P35 Express nForce 680i SLI SPP
Front-side bus 1333/1066/800MHz 1333/1066/800MHz 1333/1066/800MHz
Memory controller DDR2-800/DDR3-1333 DDR2-800/DDR3-1066 DDR2-800
PCI Express lanes 32* 16 18
Multi-GPU support CrossFire CrossFire* SLI
Chipset interconnect DMI DMI HyperTransport
Peak interconnect bandwidth 2GB/s 2GB/s 8GB/s

Nvidia's high-end chipset remains the nForce 680i SLI. As a result, Intel is the only chipset manufacturer offering support for DDR3 memory. The X38 Express also breaks new ground as the first to offer next-gen PCIe 2.0 support. The X38 Express north bridge offers up a total of 32 PCI Express 2.0 lanes, and those lanes can be split evenly between a pair of PCIe x16 slots for multi-GPU configurations like CrossFire. (Although the X38 could almost certainly act as a very good host to a pair of GeForce cards in SLI, Nvidia continues to lock out third-party chipsets in its graphics drivers.)

According to the PCI SIG, the standards body behind PCI Express, increasing demand for graphics bandwidth was the driving force behind PCIe 2.0. It's no surprise, then, that gen-two PCIe lanes offer 1GB/s of bandwidth—double that of their PCIe 1.1 predecessors. This increase in bandwidth comes courtesy of faster signaling rather than wider data paths; PCIe 2.0 lanes push data at 5.0GT/s where their predecessors were only capable of 2.5GT/s.

Gen 2 PCIe has provisions to save power, as well, including support for software controls that can dynamically throttle lane speeds. Don't think PCIe 2.0 means you have to go out and buy a new graphics card, though. PCI Express 2.0 is backward compatible in both directions, so 2.0 cards will work in 1.1 slots and 1.1 cards will work in 2.0 slots.

Thanks to the Radeon HD 2900 XT, you may already be familiar with another recent addition to the PCIe picture, the 300W card electro-mechanical (CEM) spec. This change officially blesses the new eight-pin auxiliary power connectors on cards like the 2900 XT. Those eight-pin aux power connections can deliver up to 150W of power, in addition to the 75W of juice routed through the slot itself and the 75W available in a six-pin aux PCIe power plug. Taken together, these three power sources can add up to a healthy 300W for a single PCIe x16 slot. Technically, one doesn't need a PCIe 2.0 slot in order to make use of 300W worth of power—2900 XTs use six- and eight-pin connectors together now—but we do expect to see the Gen 2 spec and the 300W CEM used more commonly in conjunction.


As you can see in the diagram above, the X38 Express retains Intel's DMI chip-to-chip interconnect. DMI has been around for a while, and despite offering considerably less bandwidth than the HyperTransport link Nvidia uses in the nForce 680i SLI, we've yet to see it suffocate system performance, mostly because Intel doesn't divvy up PCIe graphics connectivity between two chips like Nvidia does. Outfitting the X38 Express with DMI enables it to connect to Intel's existing ICH9 series south bridge lineup. Since the X38 is a high-end offering, you're most likely to find it paired with the ICH9R, which stacks up against the nForce 680i SLI MCP like so.

ICH9R nForce 680i SLI MCP
PCI Express lanes 6 28
Serial ATA ports 6 6
Peak SATA data rate 300MB/s 300MB/s
AHCI Y N
Native Command Queuing Y Y
RAID 0/1 Y Y
RAID 0+1/10 Y Y
RAID 5 Y Y
Matrix RAID Y N
ATA channels 0 1
Max audio channels 8 8
Audio standard HDA HDA
Ethernet 10/100/1000 2 x 10/100/1000
USB ports 12 10

The ICH9R isn't new—it was launched back in May with the P35 Express—so its PCI Express lanes don't have that gen-two sheen. Not that it matters. The south bridge's PCIe lanes will most likely be used for onboard peripherals and expansion cards slots, neither of which require the bandwidth or slot power of PCIe 2.0.

Auxiliary peripheral chips are actually a key ingredient of almost any motherboard that uses an ICH9 series south bridge. Because Intel has jettisoned ATA connectivity in favor of SATA, motherboard makers are forced to farm out ATA support (which they still deem necessary) to third-party silicon. Ethernet duties are usually shouldered by third party peripheral chips, as well. The ICH9R does have a built-in GigE controller, so it should be able to handle Ethernet on its own. However, we've yet to see an ICH9-based motherboard make use of the chip's networking capabilities.