The iPad’s A4 chip under a microscope

The iFixit teardown wizards teamed up with the Ottawa, Ontario-based reverse engineering company Chipworks to cross-section the A4 system-on-a-chip that powers the iPad.
According to both parties, the A4’s CPU die is basically a single-core ARM Cortex-A8 design. While dual-core Cortex-A8 designs based on ARM’s CPU blueprint can be found in other mobile platforms, Apple probably opted for a single-core version in order to keep the number of transistors and power consumption down. Chipworks noted the following:

The Apple A4 processor is packaged just like the Apple iPhone processors using package-on-package technology. One for the microprocessor and one for the two DRAM die.

Such a design improves the speed and efficiency of the chip while allowing Apple to source memory from any manufacturer, iFixit noted. Apart from a higher frequency that gives the A4 at least twofold performance jump over the iPhone 3GS, it owes its performance to a combined graphics, I/O, memory controller, and RAM parts – basically everything that’s needed to run apps.

THE A4 CPU DIE, CROSS-SECTIONED
The first two rectangles represent the RAM dies and a bigger center rectangle is the CPU die. Because the RAM dies are close to the CPU die, both latency and power consumption are reduced.
Those findings confirm what the pundits have been saying all along – Apple’s A4 chip is essentially the same silicon found inside the iPhone 3GS, only overclocked. In fact, the upcoming iPhone HD is also expected to run a variant of the A4 chip.

A closeup of Samsung's DRAM die on Apple's A4 chip that powers the iPad.

They also discovered a few tidbits that we didn’t know before, including the following items:

• the DRAM in the A4 is made by Samsung, provided as two 1Gb mobile DDR SDRAM dies marked K4X1G323PE
• the A4 chip is an Apple-branded baby – you won’t find Samsung’s part numbers anywhere on the silicon, unlike on the iPhone 3GS’s processor
• Apple could have clocked the A4 chip above 1GHz, but the iPad’s battery life would have taken a significant toll as a result
• the A4 “sips power” – according to iFixit, in order to get ten hours of battery life, “the entire iPad (including display) has to pull less than 2.5 Watts on average”
• the iPad design wins include Samsung, Broadcom, Texas Instruments, NXP, Linear Technologies, STMicroelectronics, LG, Intersil, Atmel, and others