There are two giants in the computer processor industry. One is Intel, which builds most of the processors in today’s PCs and servers. The other is ARM Holdings, in Cambridge, England, which thanks to its vast ecosystem of partners has established near-complete dominance of the market for the core logic inside smartphones and tablets.
But the demand for energy-efficient chips is reshaping the industry. As the PC market flattens, Intel aims to capture a sizable chunk of the rapidly growing mobile market, which rose to nearly half a billion smartphones in 2011. And chip designers in ARM’s camp are eyeing a US $50 billion server market, fueled by the rise of social networking and cloud computing.
The coming months will see a number of volleys exchanged across the line that has traditionally divided the high-performance and low-power chip markets. One of the first will come from a small start-up in Austin, Texas, called Calxeda (pronounced cal-ZAY-dah). The fabless firm will begin shipping chips for servers based on 32-bit ARM mobile processor designs. They’ll soon be joined by AppliedMicro, in Sunnyvale, Calif., which is working on an even speedier, 64-bit ARM-based chip. At the same time, Intel will leap into the mobile game; two big companies—Lenovo and Motorola—plan to release phones based on Intel’s low-power Atom processor by the end of this year. (The very first Intel-based smartphone was launched in April by the India-based firm Lava International.)
Exactly how this competition shapes up will depend not on performance or power consumption but on the ratio between the two: performance per watt. And that metric is fueling a fiery debate over the fundamental differences between Intel’s x86 chips and ARM’s processors.
But the most obvious difference between the two may not actually be the important one, according to experts. ARM processors use reduced instruction set computing (RISC), while x86 processors rely on an older approach, retroactively dubbed complex instruction set computing (CISC).
Both RISC and CISC architectures govern the set of machine-level instructions, compiled from more complex code, that a chip can execute. CISC chips have a wider vocabulary—they can perform certain operations in one step that might require a series of commands on a RISC chip. But RISC chips can better handle speed-boosting tricks like allowing overlapping operations during each clock cycle.
As a result, over the years, Intel has incorporated decoders into its x86 chips to convert CISC instructions to RISC instructions to boost performance. This conversion process takes energy, but it’s unclear whether this added step gives ARM an advantage when it comes to efficiency.
Instead, other differences between ARM and Intel chips may have more of a bearing on the coming competition. One key difference is microarchitecture—the particular way that processor resources such as cache and registers are distributed and instructions are scheduled. Today’s high-performance processors, for example, are designed so instructions can be performed out of order. Every part of a computation is done as soon as possible to boost speed. Chips that employ this approach have built-in bookkeeping to make sure that the results are assembled in the right order at the end of the process.
Such tricks can have a big impact on efficiency and performance, says Benjamin C. Lee, an assistant professor of electrical and computer engineering at Duke University, in Durham, N.C. While a researcher at Microsoft, Lee studied how well the company’s Bing Web search engine performed on Intel’s out-of-order Xeon server chip and its in-order Atom netbook processor. Each core on the Atom chip could handle queries at half the rate of a Xeon chip core but required just 20 percent of the energy per request. However, Atom wasn’t able to handle some of the more complex requests.
Microarchitecture will be a key battleground in any competition between Intel and ARM chips, says microprocessor industry analyst Linley Gwennap. Eking out even a slight improvement in performance can come at the expense of a large boost in power consumption. If ARM-based devices like AppliedMicro’s 64-bit server chip are to compete with Intel chips in the server market, developers will have to accept similar diminishing returns, he says.