Intel first started working on its "Bonnell" microarchitecture in 2004. The Bonnell design team was assigned the task of creating a small, low-power core that could be used in a variety of applications, such as in a many-core CPU or a low-powered Internet device. The team's focus was narrowed in 2005: aim for Mobile Internet Devices (MIDs) and smartphones. MIDs first, with smartphones as an evolution.

Seven years later, Intel's Medfield platform built around the Atom Z2460 system-on-a-chip has scored Intel's first smartphone design wins, with Lenovo shipping a handset to the Chinese market within the next few months and Motorola Mobility shipping smartphones and tablets in the second half of the year.

The modern smartphone market is dominated by chips based on the instruction set, and often designs, developed by British company ARM Ltd. In the early days of the smartphone, ARM's own ARM architecture had competition from the SuperH and MIPS architecture, but these long ago fell by the wayside: any Android, iPhone, or Windows Phone is ARM-powered.

ARM Ltd doesn't make processors itself; it just sells licenses to its designs and instruction set. The ARM processors used in modern smartphones come from a range of vendors, including Samsung, Texas Instruments, and Qualcomm, with some manufacturers using ARM's own designs—notably the Cortex A8, A9, and, soon, A15.

Whether using an ARM Ltd design or not, a common feature of all these procesors is their extremely low power usage: idling at a few tens of milliwatts, drawing no more than about one watt at full power. ARM came to dominate the smartphone market through a combination of low power, (relatively) high performance, and small, highly integrated packages. The challenge for Intel was to produce a chip that could match the ARM processors on all three counts.

Packaging

Bonnell evolved into Saltwell, and the Saltwell architecture is the core of Intel's entire range of Atom processors. Intel has three main lines of Atom processors: Atom N, for netbooks (drawing 6.5-8 W); Atom D, for nettops (drawing 10-13 W); and Atom Z, for MIDs, with the first few Atom Z models drawing around 3 W.

Intel's first attempt at an Atom Z for smartphones was the Moorestown platform in 2010. Moorestown required five chips. Three of these came from Intel: a Lincroft system-on-a-chip, a Langwell I/O controller, and a Briertown power management IC (PMIC). To this, manufacturers had to add another chip for DRAM and another for wireless connectivity.

Moorestown never made it to market in a smartphone. Compared to more highly-integrated ARM designs it just wasn't very compelling; though the performance seemed to be good, the number of chips was high, and the power consumption in certain important workloads, such as video playback, was also high.

Medfield is Moorestown done right. The Medfield platform replaces the two-chip Lincroft and Langwell combination with a single chip named Penwell. Penwell comes in a package-on-package configuration, with DRAM stacked on top of the SoC. Medfield still needs to be coupled with a Briertown PMIC and a wireless chip, but by going from (essentially) three chips into one, it's now much more comparable to ARM SoC designs.

Performance

Compared to Cortex A9, Atom's performance has never been a problem. The basic Saltwell architecture is all but identical to the Bonnell core that Jon Stokes covered four years ago; the biggest change between the two is manufacturing process, with Bonnell on 45 nm and Saltwell on 32 nm. Each Penwell chip contains a single Saltwell core. This core is a simple in-order design with only limited execution resources. There are two floating point arithmetic units: one with SSE support, two integer arithmetic units, and two address generators. The integer units have no multiply capability, instead using the floating point pipe for those operations. The core is hyperthreaded, able to run two threads concurrently in order to make better use of its execution resources.

The Atom Z2460 has a similar kind of turbo boost capability to Intel's mainstream Sandy Bridge processors. Its normal maximum clock speed is 1.3 GHz, but it can ramp up to 1.6 GHz should workloads permit. It can also reduce its clock speed to as little as 100 MHz.

Compared to Intel's mainstream performance-oriented processors, Atom is very weak indeed, but the competition in the smartphone space is currently Cortex A9. The mix of execution resources in a Cortex A9 is not all that dissimilar to Bonnell: A9 has only a single floating point unit, which also supports ARM's NEON SIMD instructions and its mix of integer units is slightly different: instead of two address generation units, it has one AGU and one multiply unit. But overall, it's similarly narrow. However, Cortex A9 has a much shorter pipeline: 8 stages, compared to 16 in Atom, and is an out-of-order processor. Instead of hyperthreading, most Cortex A9 implementations are dual core, with some even quad core.

Cortex A9 clock speeds vary greatly. The design can operate at speeds of up to 2 GHz, but the parts found in smartphones are much slower, typically between 800 MHz and 1.2 GHz.

With a combination of similar execution resources as its competitor and a higher clockspeed, Z2460 beats both high-end Android phones and the iPhone 4S, at least in a limited set of browser-based benchmarks. Its JavaScript performance is particularly strong. Such benchmarks tend to favor single-threaded performance, so the Atom part may well be able to reach its full 1.6 GHz speed; meanwhile, one of the Cortex A9's cores will be essentially unused. Still, this shows an essential truth of processor performance: increased single-threaded performance makes everything faster, whereas adding more cores only makes parallel programs faster.

The GPU performance is less of a standout. Alongside the Saltwell core, each Penwell SoC includes a PowerVR SGX 540 GPU running at 400 MHz and dedicated video encode and decode units from Imagination Technologies. The PowerVR 5 series GPUs are found in a number of competing SoCs; Apple's A5 includes a dual core SGX 543MP2 at 200 MHz, TI's OMAP4430 (found in many phones including the Motorola Droid RAZR and some Samsung Galaxy SII versions) has an SGX 540 at 304 MHz, and TI's OMAP4460, found in the Galaxy Nexus, has a SGX 540 at 384 MHz. The clock speed advantage does give Intel a slight edge over the SGX 540 competitors, but they're all within the same ballpark, and it isn't enough to compete with the much stronger SGX 543MP2.

The performance picture does have a slight wrinkle: the operating system of choice for Medfield smartphones is Android. While most Android applications are written in processor-independent Java and compiled to processor-independent bytecode—and hence equally at home on both ARM and x86 processors—around a quarter of them use native ARM code. Intel has devised a binary translator that will allow x86 phones to run most native code-using Android applications, but compatibility won't be perfect, and performance won't be up to native levels. It's too early to know what the impact will be—we won't get a clear picture of that until the phones ship—but it could well be the case that the faster-in-theory Intel processor becomes slower-in-practice, especially for performance sensitive native code games.

Power

Intel is claiming that the power consumption of Medfield is low, though not quite best-in-class. On some workloads, such as browsing the Web, Medfield scores well—a total draw of 1 W, compared to 1.3 W for an iPhone 4S or 1.2 W for a Samsung Galaxy S II. Others are less favorable; 850 mW for 720p video playback on Medfield, versus 650 mW on the Galaxy S II, and just 500 mW on the iPhone 4S. But even the bad numbers aren't that bad: Medfield is undoubtedly competing in the same space as the Cortex A9 processors—perhaps a little worse in some areas, perhaps a little better in others, but overall it's a wash.

Intel's process technology is instrumental in achieving this low power. Medfield is built on the company's 32 nm LP (low power) process. Compared to Moorestown, built on Intel's 45 nm process, this has allowed Intel to use 43 percent less dynamic power at the same frequency, or alternatively, offer a 37 percent higher frequency for the same power draw.

By contrast, the competing ARM designs are currently built on 45 nm processes. 32 nm and 28 nm parts will start to materialize over the next few months, and these will give a boost to the power and performance figures of the ARM designs. The process shrink gains might be enough to nullify Intel's current performance advantage.

Good enough to compete

With its combination of power, performance, and packaging, Medfield is a genuinely viable smartphone platform. Atom has always had the performance it needed to compete in this space, and now it also has the low power and high integration to tackle Cortex A9 head on. It might not always be the best in class, but it doesn't have to be the best. It has to be competent and credible, and it is both of those things. It is the first Intel Atom design that has deserved smartphone design wins, and it's got them. If Lenovo and Motorola make a succcess of their Intel smartphones, other design wins are likely to follow.

Intel is hoping that Medfield will be enough to at least get a foothold in the smartphone market. It should achieve this.

In the short term, it may struggle to do more. Perhaps surprisingly, it's the performance front that it will meet its toughest challenge. Later this year, the first Cortex A15 devices should trickle onto the market, and they raise the game considerably. Cortex A15 designs will have as many as four cores, and each core will be considerably more capable: as with Cortex A9, Cortex A15 is an out-of-order design, but it has considerably more execution resources than its predecessor, with two floating point units, both supporting NEON, and on the integer side, an extra AGU. Coupled with the shrink to 28 and 32 nm, ARM processors are set to get quite a bit faster in coming months, so Intel's performance lead may prove to be short-lived.

But Intel isn't standing still. By the end of the year, a dual core Medfield variant with an improved GPU—the same dual-core PowerVR SGX 543 MP2 as is used in the Apple A5—should be on the market.

Longer term, Intel's process advantage could prove decisive. In 2013, Saltwell will be replaced by Silvermont, built on the company's 22 nm process. The 22 nm process will allow for substantial improvements in power consumption, frequency, or both. In 2014, the process shrinks again, to 14 nm. This is more aggressive than the path that TSMC, IBM, Globalfoundries, and Samsung plan to take, and from 2013 onwards, Intel could attain a sustained process advantage for its smartphone processors—an advantage that may prove all but insurmountable.

One possible difficulty for Intel may be matching ARM's flexibility and variety. With ARM, a smartphone manufacturer can design a custom SoC by picking and choosing which building blocks and features they want to support. Apple's A5, for example, is similar to other Cortex A9 SoCs, but the exact combination of features (GPU, memory, support for optional parts of the ARM instruction set) is unique to Apple. Intel's business is geared towards mass production of identical units, not shorter runs of customized designs. Matching these capabilities is not part of Intel's usual business model.

Phone manufacturers might also be unwilling to commit to a single-source supplier. Samsung, for example, builds smartphones using processors from Qualcomm, TI, NVIDIA, and its own in-house line. Adding Intel as a fifth processor source might be palatable, but switching to make Intel the sole processor source would be a big risk.

Seven years after deciding to aim for the smartphone market, and at the second attempt, Intel has a platform that can compare favorably with the ARM competition. The company still has much work to do if it wants to turn these early design wins into the same kind of dominance as it has over the desktop market, but in Medfield it has the foundation it needs.