The Apple iPad’s A4 Processor Runs an ARM9 Core (or Maybe a Cortex-A9)

Buried under the mounds of hype connected to Apple’s iPad announcement was the tiny piece of information that Apple had designed its own processor, the A4, to run the device. I became interested in this facet of Steve Jobs’ latest assault on the high-tech toy market after reading that several Apple employees responsible for the A4 design had left Apple to found a new processor design company in San Jose called “Agnilux” and I was curious enough to do a little more digging. I discovered that the Apple A4 does not implement a new microprocessor architecture. It reportedly contains a customized ARM9 RISC processor core. (Other reports say it’s an ARM11-derived ARM Cortex-A9 processor core  with an 8-stage pipeline, or multiple Cortex-A9 cores because the Cortex-A9 is intended for multicore designs, which I think makes it less likely to be a Cortex-A9.) Because the A4 processor’s RISC core is reportedly running at 1GHz, this core is clearly not a conventional ARM9 with a 5-stage pipeline. If it is an ARM9 core, the A4 design team added several more pipeline stages to the ARM9’s microarchitecture to win those 1GHz bragging rights, which Steve Jobs then exercised in his iPad rollout. For example, the ARM11, which does hit 1GHz, has eight pipeline stages and the ARM Cortex-A8 employs 13 pipeline stages to hit that magic clock rate that makes the fanboys swoon with techno lust. If it’s a Cortex-A9 core, then the 8-stage pipeline can hit 1GHz in a 65nm, speed-optimized process technology according to Microprocessor Report.