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Intel's Core i7 comes to notebooks

Clarksfield arrives in Clevo's W870CU mobile gaming rig

Intel's Nehalem microarchitecture has been a revelation thus far. This latest "tock" in the chip giant's microprocessor development cycle debuted late last year in the form of Bloomfield-based Core i7 CPUs, whose blistering performance knocked everything else out of the high-end processor market. Then, less than a month ago, Intel trickled Nehalem down to the mainstream with more affordable Core i7 and i5 CPUs based on fresh Lynnfield silicon. As expected, these new processors outclassed the competition, broadening Nehalem's dominance of the market.

Up next: notebooks. At the recent Intel Developer Forum, and not even a month after Lynnfield burst onto the desktop scene, a mobile version of the chip dubbed Clarksfield was unveiled inside larger laptops and portable gaming rigs.

Clarksfield might as well have been called Lynnfield for laptops, because that's exactly what it is. Both squeeze 774 million transistors into a 296-mm² die area using Intel's high-k, 45-nm fabrication technology. They do come in different packages, though. Lynnfield uses LGA1156, while Clarksfield rides a 988-pin package dubbed PGA988A. Despite having fewer pins, PGA988A still measures 37.5 mm square, so it's actually the same size as LGA1156.

Given Clarksfield's roots, it's no surprise that the new CPUs based on it will be known as Core i7 mobile processors. Intel is making three models available to start: the Core i7-720QM and i7-820QM will serve the "quad-core performance" market while the Core i7-920XM fills in as the token Extreme Edition offering for those with deeper pockets. All three are quad-core designs, and they can all execute eight threads in tandem thanks to Hyper-Threading.

Cores Threads Base clock speed L3 cache TDP Price
Core i7-920XM 4 8 2.0GHz 8MB 55W $1054
Core i7-820QM 4 8 1.73GHz 8MB 45W $546
Core i7-720QM 4 8 1.6GHz 6MB 45W $364

Each of Clarksfield's cores has 32KB L1 data and instruction caches. That's backed by 256KB of L2 cache per core, with a shared L3 cache serving as a last line of defense before main memory. The 920XM and 820QM both have 8MB of L3 cache, but the 720QM must make do with only 6MB.

Obviously, the thermal challenges inherent to mobile applications put a bit of a damper on Clarksfield clock speeds. The range-topping 920XM has a base clock speed of 2GHz—a good 666MHz shy of its slowest Lynnfield-based desktop counterpart.

Of course, these base values are just the tip of the iceberg. Thanks to Turbo Boost, Clarksfield CPUs can dynamically range above their base clock speeds when adequate thermal headroom is available. Turbo Boost is most aggressive when only one or two cores are occupied, but it'll also push the issue a little if all cores are engaged, thermals permitting.

LFM clock speed Base clock speed Peak Turbo Boost speed
4 active cores 3 active cores 2 active cores 1 active core
Core i7-920XM 1.2GHz 2.0GHz 2.26GHz 2.26GHz 3.06GHz 3.2GHz
Core i7-820QM 1.2GHz 1.73GHz 2.0GHz 2.0GHz 2.8GHz 3.06GHz
Core i7-720QM 933MHz 1.6GHz 1.73GHz 1.73GHz 2.4GHz 2.8GHz

Interestingly, the mobile Core i7s have much more Turbo Boost range than their desktop counterparts. The most aggressive Lynnfield-based desktop CPU can raise its clock speed by 667MHz above its baseline, which translates to a healthy 18% increase. Yet that pales in comparison to the Core i7-820M, which can push its core a full 1.33GHz higher for a whopping 77% increase in clock speed. The 920XM and 720QM can each boost their clock speeds by 1.2GHz, or around 60%.

Intel doesn't guarantee that every CPU will hit its Turbo Boost peak, however. I suspect success will largely depend on the effectiveness of the notebook's cooling system. Clarksfield certainly presents a challenge on that front. The 920XM has a Thermal Design Power (TDP) rating of 55W, while the other two are rated for 45W. Those TDPs are much lower than Lynnfield's 95W rating, of course, but they're still high enough to keep Clarksfield out of smaller notebook designs.

Clarksfield employs numerous tricks to save power, including shutting down individual cores when they're not in use. The chip can also opportunistically cut power to its cache, memory controller, and I/O components, too. The 920XM and 820QM are capable of throttling back their clock speeds via SpeedStep to 1.2GHz in what Intel calls Lowest Frequency Mode (LFM). The 720QM can drop even lower, down to just 933MHz. When in LFM, mobile Core i7 CPUs have TDP ratings between 35 and 37W.

To put things into perspective, consider that all of Intel's Core 2-based mobile quad-core CPUs have TDP ratings in the 45W range, and they're just CPUs. Clarksfield combines its four cores with traditional north-bridge components, including a dual-channel DDR3-1333 memory controller and 16 lanes PCI Express 2.0 connectivity. On the off chance you're looking for a portable system with CrossFire or SLI, the PCIe lanes can be split evenly between a pair of x8 links, too.

The CPU also has a 2GB/s DMI interconnect that interfaces with Intel's PM55 chipset—a mobile version of the P55 that launched alongside Lynnfield. As one might expect, the PM55 is largely identical to its desktop counterpart. Both offer six Serial ATA RAID ports, 14 USB ports, an embedded Gigabit Ethernet MAC, an HD audio interface, and eight PCI Express lanes. The PCIe lanes are tagged as 2.0, which is technically true, but still a little disingenuous. While they support all the features of the PCIe 2.0 specification, the lanes only signal at 2.5 GT/s, which is half the data rate of a full-bandwidth 2.0 link. This limitation shouldn't be much of an impediment for mobile applications, though.

So what's different in the PM55? The packaging, which is two millimeters narrower than the desktop part. Also the TDP, which, at 3.5W, is 1.2W lower than the P55.