“X” branch was introduced at the “5” generation with the A5X in March
2012. Its die size grew by 35% compared to the A5, i.e. the same
generation of the other branch. At least some of this extra area was
due to a doubling of GPU real estate.
The increase in graphics
horsepower compared to its generational counterpart emerged with a
four-fold increase in the number of pixels from the iPad2 to the
Further evidence of the bifurcation of the A-series family came
in the digital blocks outside of the CPU, GPU and GPU spine. There are
12 for the A5 and 15 for the A5X.
What does this bifurcation
look like at the “6” generation of the A-series family?
As pointed out
above there is again much more real estate devoted to the GPU in the A6X
compared to the A6. We also know about differences in the layout,
interfaces and PLLs. It is time to look at the digital blocks outside
of the CPU and GPU.
There is most likely considerable block
commonality between the two processors. Again referring to Chipworks’
A6X article, “Other than the CPU, it appears all the other digital cores
have new layouts,” and “many of the analog and interface cores have
been reused from the A6, however there are some new interface blocks.”
But are there digital blocks on one die and not the other? We saw no
comment on this. They did mention that the A6X sported considerable new
design and was not just a tweak. It would, however, be interesting to
consider whether there is evidence of a design overhaul in the form of
different digital blocks, not just layout differences.
the die photographs, the A6 and A6X appear to have 17 and 16 digital
blocks outside of the CPU, GPU and GPU spine, respectively.
there are blocks that are the same on both parts. The first thing to
notice is that the shapes of the blocks differ between A6 and A6X.
Obviously, floorplanning of the entire device is dependent on the number
and location of the largest blocks (an additional GPU core in the A6X)
leaving smaller blocks to be shoehorned into the available space. With
no intention of minimizing the task of re-shaping circuitry to fit major
architectural changes, this floorplanning required more than a copy and
There do however appear to be other differences,
again beyond the ’PUs. We noted above there appears to be a difference
in the number of digital blocks.
Is there more?
Given a desire and
the concomitant budget, hard-core reverse engineering is possible. Beyond reverse engineering the question of differences can be considered
in two ways. One can qualitatively analyze the blocks to consider the
amount of cache, for example. With this approach the block labeled “B”
in the A6 die photograph does not appear on the A6X, even considering
One can also look at this more
quantitatively, yet without a large reverse engineering project. Of the
five digital blocks (see die photographs) most easily identified between
the A6 and A6X, four occupy areas between eight and 10 percent larger
or smaller than on the A6. The differences are at the upper end of
measurement errors (we are relying on CW published die dimensions and
the accuracy of the associated image files posted on web sites), so are
believed to represent minor design changes within the blocks. The fifth
digital block analyzed varied significantly between the two designs. The
A6X version of this digital block was just over three times that of the
Getting back to processor cores for a moment, there are
four GPUs on the A6X compared to three on the A6 (as noted by
Chipworks). The design within each of these also differs significantly.
The GPU blocks on the A6 are 58% bigger than their A6 counterparts.
Accounting for the GPU cores along with the block that could either be a
dedicated decoder or perform a GPU load balancing operation, there is
nearly twice the die area directly devoted to graphics processing on the