A5: All Apple, part mystery
Paul Boldt and Don Scansen
4/13/2011 12:01 AM EDT
How Apple did on the A5
There were two pieces of information to come out of the early reverse engineering from the big RE houses. First, the A5 die is dramatically larger than the A4. Both UBM Techinsights and Chipworks found the A5 die to be 12.1 mm x 10.1 mm, giving a die size of 122 mm2. This compares to 53 mm2 for the A4. The die size of the A5 is therefore 2.3 x larger than the A4.
Let's look closer at the A4 and A5 floorplans and consider what might be behind such a dramatic increase in die size for the A5. For this comparison we look to the floorplans published by Chipworks. The two ARM cores of the A5 combine to consume approximately 14 percent of the total die area. This is the roughly the same percentage as the single ARM core in the A4.
What about the GPU? The published floorplan of the A4 does not specifically identify it. However, if you assume it is a moderate size block with a fair amount of cache there is logic core 4, which is the largest outside the CPU and logic core 5. It is also noted that there are numerous blocks of approximately the same size as logic core 5. For the sake of the discussion let's assume core 4 is the GPU. In terms of absolute areas, If the CPU and GPU areas are added and the total subtracted from the overall die size there are 41 mm2 left for other digital blocks, analog and I/O for the A4.
Now to the A5
We are not aware of any published floorplan, including the one cited above, that explicitly identifies the GPUs, so some assumptions will be made. There are three sets of two identical blocks clustered together. Two of these are labeled as the ARM cores and four are labeled as "Processor Data Path." There are no two other blocks that appear the same. For a dual core system one would anticipate some sort of arbitration has to occur between them, agreeing with the labeling. However in the absence of two other blocks that are the same, it is believed that the two GPUs are within these blocks, or area. In the end, again for the sake of argument, these six blocks representing CPU+GPU+arbitration consume approximately 40 percent or 47 mm2 on the A5 die, leaving 75 mm2 for other stuff.
Die size and architecture comparison between
larger dual core Apple A5 APU and the first
Infrared backside die image of Apple's A4 applications processor with single ARM core indicated.
Die size and architecture comparison of larger dual core Apple A5 applications processor.
The second piece of early information was that both the A4 and A5 are manufactured in Samsung's 45-nm process. This point makes a comparison of the published A4 and A5 floorplans that much easier. Because the two dies are fabbed with the same process node the analog blocks came out looking pretty much the same.
Thus, one can readily identify the Wi-Fi and audio blocks in the A4 die photo as they are labeled and appear the same, apart for minor layout differences, on the A5. Finally, for the sake of argument it is assumed that the IP cores on the A4 are also present on the A5, and any differences in area consumed for I/O and basic IP cores are minor in the overall die area. One then arrives at 34 mm2, or 64% of the total A4 die size, in additional real estate for digital logic on the A5. While the measurements for the above numbers contain some assumptions, they highlight the scale of the differences.
The somewhat simplified area analysis indicates that there is more to the increased die area than just the upgraded and expanded CPU+GPU and the arbitration circuitry. One can arrive at the same conclusion by simply counting the blocks. There are nine digital blocks in addition to the CPU+GPU on the A4. This number increases to 12 for the A5. It is unclear whether we see the same digital blocks on the A5 as we did on the A4. There will likely be some of the same IP cores on the two die. However, by comparing memory array layout in various blocks it is apparent that the layout is different. Of particular note is logic core 6 of the A4, which appears to be a sea of gates with no apparent memory. There is no similar block on the A5. If we combine this observation with the increased number of blocks one readily concludes that there is a lot more going on in the A5 than the switch to a dual core CPU and GPU.
Now it's time to put on our thinking caps. If you were Apple what would your strategy be? We do know there is quite a bit of real estate on the A5 beyond the CPU+GPU+arbitration, and the necessary IP blocks for memory control, I/O etc. So what to do with this extra real estate.