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ARM Vs. x86: The Secret Behind Intel Atom's Efficiency

Putting Hard Numbers Behind Mobile Power Consumption

Andrew Ku has been in the process of reviewing a couple of new tablets: Samsung's ATIV Smart PC 500T, with an Atom Z2760 inside, and Samsung's ATIV Tab, with Qualcomm's APQ8060A. I've been playing with Acer's Atom-powered Iconia W510, and my initial thoughts on that will go live soon.

One thing both Andrew and I are impressed with is that the Atom-based tablets, running full versions of Windows 8, compete readily with Microsoft's Tegra 3-powered Surface when it comes to battery life, and doubly so when you consider Acer's W510 only has a 26.6 Wh battery compared to the Surface's 31.5 Wh power source.

The sensitive stuff is blacked-out to protect the innocent

As far as performance goes, the Atom Z2760 (code-named Clover Trail) trades blows with Nvidia's Tegra 3 in our Geekbench numbers, but then completely blows it away in our Web browsing metrics. The fact that it's able to run x86-based apps fairly well is a clear benefit, too.

So, how do you get dramatically better performance in certain tests, and then competitive run time in those same workloads, all from a smaller battery? Answering that question would have been quite a challenge. But then we asked Intel to show us some of the advanced equipment in its own lab, which allows the company to take very granular measurements.

While we were up in Santa Clara last week, Intel presented us with data its performance analysis team had generated, showed us how it generated that information, letting us play with the test equipment ourselves, and discussed with us in great depth Clover Trail's strengths and weaknesses. Again, the following numbers come from Intel's team, not the Tom's Hardware lab. We did confirm that all displays were standardized to 200 cd/m2 (nits) using a Gossen Luminance meter, though, and power consumption is reported as the average over a 50 millisecond time slice. During the course of this piece, we'll break down Intel's numbers and compare them what we've done in our own offices, looking for connections.

Idle: Microsoft Surface
Platform (W)CPU (W)GPU (W)Memory (W)Panel Backlight (W)Everything Else (W)
System Idle (Ideal): Windows 8 UI2.820.00380.210.250.981.38
System Idle (Ideal): Desktop3.000.00470.
System Idle: Split Screen3.640.290.310.371.111.56
Idle: Acer W510
Platform (W)CPU (W)GPU (W)Memory (W)Panel Backlight (W)Everything Else (W)
System Idle (Ideal): Windows 8 UI2.470.
System Idle (Ideal): Desktop2.760.020.110.301.091.24
System Idle: Split Screen3.370.180.240.361.141.45

For each tablet, Microsoft's Surface and Acer's W510, we're looking at power consumption on the Windows 8 UI, the Windows 8 Desktop, and a split-screen scenario, where a Windows 8 app is open on one side and the Desktop is open on the other. In this case, the Windows 8-style app is the primary screen and the Desktop functions as the sidebar.

The first column to pore over is the usage of each CPU. Tegra 3 delivers a very impressive idle processor draw of 0.0038 W in the Windows 8 UI interface, compared to the Atom's .02 W. But running in split-screen mode uses a lot more power. Tegra 3 jumps to .29 W, ahead of the Atom's .18 W.

Next up is the GPU. Now, we already know that the single-core PowerVR SGX545 in Intel's chip is quite a bit slower than Tegra 3 in 3D graphics. But at least in the 2D applications you run on both SoCs, the Atom's simpler graphics engine uses less power across the board.

Wired up to the requisite microsolder points.

The first hint of Intel's efficiency advantage comes from the memory column. At first glance, the Atom looks worse-off than Tegra. But remember that the Atom Z2760 sports two 32-bit memory controllers with LPDDR2-800 support, delivering up to 6.4 GB/s of bandwidth. The T30 found in most devices boasts a single-channel controller able to address DDR3-L at up to 1500 MT/s, yielding up to 6 GB/s. In theory, though, Intel's Atom has to do more work, suggesting its memory subsystem also operates more efficiently.

The rest of the platform includes the screen itself and panel electronics (like the LVDS transmitters), along with wireless radios, audio codecs, NAND, and anything else going on under the hood. Even in a scenario where Acer’s backlight eats up more power (operating in split screen mode), the rest of its platform is still 0.10 W more efficient.