A4 Uses "Lean and Mean" 3GS Cortex A8 Chip?
Relying on unnamed sources, Jon Stokes at Ars Technica is asserting that the Apple's new A4 system-on-a-chip is based on a stripped-down Cortex A8 CPU, and not the faster A9 that some have been speculating about. The iPhone 3GS and iPod touch are based on the A8, though underclocked to 600 MHz to save power. The CPU on the A4 SoC is believed to run at 1 GHz clock speed.
When the A4 was announced along with the iPad in January, many observers theorized that the CPU would be a wholly new design that Apple would have developed in-house with the talent from their acquisition of chipmaker P.A. Semi. After the device's specs were revealed, however, it seemed more likely that the CPU was a dual core Cortex-A9 with advanced power-management technology to achieve the 10-hour battery life that Apple claimed.
According to Stokes's sources, though, Apple was able to run the A8 chip up to 1 GHz and still save power because I/O demands were limited. "With one 30-pin connector on the bottom and no integrated camera of any kind," Stokes writes, "the A4 needs a lot less in the way of I/O support than comparable chips... It's lean and mean to a degree that isn't possible with an off-the-shelf SoC." If this supposition proves correct, it could explain why Apple decided to ship the iPad - at least in its initial version - without a camera. lack of a camera has frequently been mentioned as a disappointment by many observers. The lack of camera support in the hardware allowed Apple to improve performance, Stokes theorizes, suggesting that "Apple has probably ditched some dedicated image processing blocks."
P.A. Semi's role in the development of the A4 chip may have been limited to power management, Stokes thinks. Noting that the new hires had a little time between when P.A. Semi was acquired in April 2008 and the release of the iPad in January 2010, Stokes suggests they brought their "power-gating" expertise from the development of the PWRficient chipset, which was based on two 2GHz superscalar PowerPC cores running at 2 GHz and had a draw of as little as 5 watts. Power gating requires shutting down the parts of a chip that aren't in use, which is difficult to do on high-speed chips. Killing most of the I/O and image processing blocks would have allowed Apple to squeeze a lot more speed out of the A8.
A separate story on VentureBeat early last month also asserted that the P.A. Semi team didn't work on the A4, but rather the VLSI team that had been developing Apple's custom silicon going back to the northbridges on the G4 and G5. Ars's Stokes joins other observers in speculating that the A4 may power a future iPhone release.