The Lincroft SoC (continued)
The Atom processor core in Lincroft isn't much changed, architecturally, from the first generation; it still supports a wide range of alphabet-soup instructions, up to SSE3 but no further, and is 64-bit capable. One core can still track and execute two threads simultaneously via Hyper-Threading, and cache sizes remain steady, with a 32KB L1 instruction cache, a 24KB L1 data cache, and a 512KB L2 cache. The team hasn't made any noteworthy tweaks to improve per-clock instruction throughput or to extend the core's capabilities.
Changes throughout the chip should improve the processor's performance and power efficiency, regardless. Most notably, clock frequencies are upand down. They're down because the chip's SpeedStep dynamic clocking mechanism can now drop the core's frequency as low as 200MHz at idle, versus 600MHz for Silverthorne. They're up thanks to the addition of a "burst mode" in this new Atom similar to the Turbo Boost feature familiar from Intel's recent desktop and server processors.
Burst mode raises clock speeds opportunistically when the thermal headroom of the chip and the device will permit, as high as 1.5GHz in the smart-phone-oriented versions of Lincroft and up to 1.9GHz in the tablet-oriented variants. The Atom core can run in burst mode indefinitely so long as the thermal headroom is sufficient, so larger devices like tablets might spend quite a bit of time in burst mode, whereas smart phones may not.
Lincroft's burst mode differs from Turbo Boost in a couple of notable ways. For one, the advertised clock frequency for a given Atom model will be its peak burst mode frequency, not a lower, guaranteed baseline speed. In fact, we came away from our visit to Austin without any record of the baseline clock speeds of the new Atoms. When we later asked Intel what those base frequencies would be, the firm declined to answer! That base clock is the advertised speed for the Core i7 and friends. Another major difference, one that pervades nearly every component in Moorestown, is the fact that software has extensive control over the CPU's clock frequencies. A handset maker could, for instance, choose to enable burst mode in only in certain scenarios where it might be needed, such as playing back YouTube videos. Alternately, it could decide to leave burst mode enabled generally, but turn it off in specific situations, like during voice calls when the phone must also power a modem.
The Atom's front-side bus uses the same protocol as past Atoms, even though it's now communicating with other components on the same chip. The FSB has gained some of the same clock speed flexibility that the CPU has, though, with its own version of, erm, forced induction. The speed of Lincroft's front-side bus scales dynamically with the speed of the CPU core, peaking at 200MHz or 800 MT/sdouble the max of Menlow and Silverthorne. As I understand it, the FSB clock will remain at 200MHz any time the CPU rages above 1.2GHz. When the CPU drops to between 600 and 1200MHz, the bus will run at 100MHz, and when the CPU goes to 200MHz, the bus can range down to 50 or even 25MHz. The caveat here is that some workloads might use relatively little CPU power and still need a fair amount of bus bandwidth. Either Intel or the device maker will have to tune the platform to make sure this feature doesn't harm performance in such scenarios.
To meet the power requirements of smart phones, Lincroft's memory controller supports a single, 32-bit channel of low-power DDR memory at 400 MT/s. Larger devices like tablets can use DDR2 memory at 800 MT/s, but the lower bandwidth of LP DDR was a particular concern for Lincroft's architects. Belli Kuttanna, Sr. Principal Architect in Intel's Ultra Mobility Group, told us they tried to offset the memory bandwidth lost during the switch to LP DDR RAM by making memory scheduling improvements to increase DRAM utilization. In his assessment, they've more or less "broken even, clock for clock" by doing so.
Lincroft's memory arbiter deals with memory access requests coming in from multiple devices in the system. The arbiter's buffer size is doubled in Lincroft, and it has been tweaked to do a better job of coalescing requests where possible, especially those from the graphics and media subsystems. One reason for the changes, according to Kuttanna, was making sure the chip could deal well with concurrent workloadsfor instance, playing music and display 3D graphics at the same time, perhapsin a bandwidth-constrained environment.
Of course, the memory controller now works harder to keep power consumption low, too, with extensive DRAM power management and more aggressive policies for the closing of DRAM pages.
The graphics processor in Lincroft is the same Imagination Tech IGP used in Menlow, but it now runs at up to twice the clock speed. The Atom team says it made performance optimizations "around" the IGPsuch as the memory coalescing we've just mentionedand power use optimizations "in and around" it. The result should be better graphics performance than Menlow, and Intel claims an advantage of 2-4X over any competitors that use the same graphics core, thanks to the combination of a better implementation, integration, the benefits of Intel's 45-nm process, and better drivers.
Lincroft carries over the Imagination Tech video decoding and playback logic from Menlow, as well, with support for a broad array of formats including MPEG2/4, WMV9, VC1, H.264 and DivX. Intel is promising better video decoding performance this time around, which would be nice given our experience with the Poulsbo-based Acer Aspire One 751. Any improvements are likely due to higher clock speeds, the memory arbiter changes, or both. In fact, the firm says Lincroft can decode H.264 video streams at bit rates of up to 20 Mbps in 1080p resolution with all profiles, putting its capabilities well above competing platforms that top out at 720p or below.
To this decoding prowess, Lincroft adds a new feature: a hardware video encoding engine, also from Imagination Tech, capable of squeezing 720p video into several formats, including MPEG4 and H.264 base profile, at up to 30 FPS. Obviously, the encoder should be useful both for capturing video and for doing video conferencing on a Moorestown-based device.
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