Apple's iPad twice as fast as iPhone 3GS, tests show

Results consistent with single-core ARM Cortex-A9 processor, adds expert

Apple's iPad is about twice as fast as the current iPhone, according to recently-published benchmark tests.

The results fit the assumption that the iPad is powered by a single-core ARM processor, an Apple product repair expert said today.

On average, the iPad executes native applications about twice as fast as does the iPhone 3GS, said Craig Hockenberry, who works for Iconfactory, best known as the developer of Twitterific, a Twitter client. Iconfactory released an iPad version of Twitterific last week.

After comparing the performance of iPad and iPhone native applications, as well as the two devices' JavaScript execution speeds, Hockenberry posted results on his personal blog last Saturday.

Hockenberry measured the speed that the iPad and iPhone 3GS each ran an application written with Apple's Cocoa Touch API (application programming interface) using the company's SDK (software developers toolkit). The two devices rely on different versions of Apple's iPhone operating system, however; iPad runs iPhone 3.2, while the iPhone 3GS uses 3.0.

The iPad's JavaScript performance increases were less impressive, said Hockenberry. "The same 2x improvement is not seen for the same benchmarks when executing JavaScript code in Web pages," he said. "Things have definitely improved [in the iPad], but there was a wide variation in results when performing the tests. I suspect that just-in-time compilation or other similar types of caching are affecting the results."

Just-in-time compilation, or JIT, is a technique that improves runtime performance by compiling JavaScript it into native code just before executing, rather than letting the JavaScript run through an interpreter. Apple added a new JIT compiler to Safari's JavaScript engine with version 4.0 of the browser, which launched last summer.

Hockenberry's results for the iPad are consistent with the performance boost one would expect from a single-core ARM processor running at 1GHz, said Aaron Vronko, the CEO of Michigan-based Rapid Repair. Vronko, whose company repairs and supplies parts for Apple's iPod, iPhone and iPad, disassembled an iPad last weekend to get a better idea of what was inside.

"This performance increase over the iPhone 3GS is exactly in line with what I've expected, based on a single-core ARM Cortex-A9 CPU running at 1GHz," Vronko said today. Last January, Vronko said he would expect the iPad to run software 85%-to-90% faster than the iPhone 3GS.

Vronko explained that the iPad's processor -- which like the iPhone's is actually integrated within a system-on-a-chip, or SOC -- is 67% faster in clock speed than the iPhone 3GS' ARM Cortex-A8 CPU running at 600MHz. "So in terms of speed increases, this new CPU sees both a 67% faster clock rate, in addition to being able to complete more operations per clock cycle thanks to innovations in ARM's latest CPU design," said Vronko, citing such factors as native support for an expanded processor instruction set and a shorter code execution pipeline.

He estimated the non-clock speed increase of a single-core A9 at between 25% and 30% for most applications, bringing the total near 100%, or double that of ARM's single-core Cortex A8.

The innards of the Apple SoC -- dubbed "A4" by Apple -- have been of intense speculation by some, including Vronko, since Apple announced the tablet last January. His take: The A4 uses a single-core ARM Cortex-A9.

Vronko based his opinion on the 256GB of system memory that the iPad contains. "That was a pretty big surprise," he said last Saturday after finishing his iPad teardown. "I expected 512GB or more. The 256GB shows that it's a single-core processor [inside the A4]."

If Apple had equipped the iPad's A4 with more system memory, Vronko continued, it would be a good clue that the company would implement full multitasking at some future date, perhaps with a software update to the tablet's operating system. Apple is, in fact, set to unveil iPhone 4.0 on Thursday, and although many expect to see some measure of multitasking added to the OS, one analyst yesterday said he thought that some form of limited multitasking would be the likeliest move.

"With 512GB, it would almost be a crime not to do multitasking, even with a single-core processor," Vronko said.

Others have gone even deeper than Vronko in examining the A4 and its internal architecture. iFixit, a site that specializes in writing self-repair guides for Apple's hardware, teamed with Chipworks , a semiconductor reverse-engineering company, to peek inside the A4. On one count, the iFixit-Chipworks investigation agreed with Vronko: The iPad uses a single-core processor . iFixit-Chipworks, however, said that the processor running the iPad "must be the ARM Cortex A8."

Vronko disagreed. Although ARM's A9 architecture supports multi-core, there's no rule that says it cannot be used to design a single-core chip. "There are no off-the-shelf ARM chips, there are only blueprints," he noted, referring to the design specifications that companies, like Apple, license from ARM.

"The speed increases [in the benchmarks] aren't possible with just the clock speed increase of the A4," he added, using the 67% boost provided by the iPad's 1GHz A4 to argue that the SoC integrates an ARM Cortex-A9, not an A8.

According to ARM's Web site , the Cortex-A9 is available in either single- or multi-core processor configurations.

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Gregg Keizer

Gregg Keizer

Computerworld (US)
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