x86 Everywhere: Intel Announces Medfield Phones

The SoC: Single-Core Atom + PowerVR

Intel's official slide on Medfield isn't big on details, but the company has given us permission to tell you more about the chip than what's printed above.

The GPU portion of the Z4260 is based on PowerVR's SGX540 with a target core clock of ~400MHz. That puts the chip's GPU performance in line with Texas Instruments' OMAP4460, which uses the same GPU and is clocked at 384MHz. Medfield integrates support for three displays, including 1900x1080 output via HDMI.

The chip supports dual-channel LPDDR2 667-800MHz and encodes video at 30 fps in 720p.

Penwell is in the upper-right hand corner, with the two chips from Moorestown (Lincroft and Langley) for comparison

The CPU core at the heart of Medfield is named Saltwell; it's the first new iteration of the architecture since Intel debuted Bonnell, the 45nm variant, back in 2008. At a high level, most of the core's features are unchanged. Saltwell is a single-core chip with HyperThreading. Like Bonnell, it's an in-order core capable of decoding up to two instructions per clock cycle with 56K of L1 and 512K of L2 cache.

There were, however, a few low-level improvements. Previously, Atom used a 4K table for gshare branch prediction. Saltwell stores 8K worth of entries in single-threaded mode and 4K of data per thread when HyperThreading is in use. Increasing the number of entries lowers the number of mispredicts and can help prevent thread stalls.

Intel's other performance improvements to Saltwell include faster memory copy routines, "improved performance of certain microcode flows", and a reduction in instruction scheduling restrictions. All of these improvements are extremely low-level, but Atom's in-order nature makes them more important than they might be otherwise. Unlike a conventional desktop or laptop processor, Atom can't re-order code for optimum execution. Relaxing scheduling restrictions helps improve core utilization and performance per watt efficiency.

Saltwell and the Medfield SoC are designed to open this new range of products

One fact about Saltwell that caught us by surprise is the chip's operating frequency. The chip runs at the same 1.6GHz that's been the hallmark of Atom since it debuted in netbooks 3.5 years ago. Its ability to dynamically adjust its clock frequency relative to workload, however, has been significantly expanded (more on this in the Power Consumption section later on).

Can a Single-Core x86 Processor Keep Up?

Intel's decision to opt for a single-core x86 chip bucks the market's general trend towards multi-core phones, but it's a good strategy for multiple reasons. HyperThreading doesn't deliver the same performance improvement as a second core, but we've seen it improve Atom's performance by 30-50 percent in a wide range of non-smartphone tests. The benefit of being able to schedule twin threads for simultaneous execution are low level enough that Android should see similar benefits.

Atom's in-order architecture makes HT particularly useful when it comes to improving core utilization and efficiency

Intel's other ace card is Atom's inherent performance advantage relative to its ARM counterparts. Benchmarks between the two are admittedly hard to come by, but the test results that are available suggest that Atom's single-threaded performance is significantly better than that of its ARM-based counterparts.

The best way to understand Saltwell's relative performance is as a balance between clock speed, multi-threading capability, and x86's inherently higher efficiency as compared to ARM. Against 1-1.2GHz dual-cores, Medfield's higher clock speed and HT should keep it in the running. Later this year, the chip faces stiff competition with the next generation of hardware expected to emerge, but remember, Medfield's primary goal is to compete with ARM products, not blow the doors off.

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