How The Exynos 5 Octa CPU Works, Inside of The Recently Unveiled Galaxy S4

Samsung officially unveiled its new flagship Galaxy S4 smartphone on 14 March after weeks of speculation, leaks, and strange ad campaigns. The company's presentation was focused mostly on the software side of the equation, with all of the hardware information rattled off in just a few minutes at the beginning of the presentation.

Despite the fact that the S4 looks a lot like its predecessor, there's quite a bit of new hardware under the hood. Today, we wanted to take a quick look at the chip that powers the international versions of the phone, Samsung's new Exynos 5 Octa system-on-a-chip (SoC). We should note: the US versions of the S4 likely won't include this chip, but if precedent tells us anything, American consumers will eventually get their hands on it, possibly in the form of a future Samsung tablet.

It's a given that this chip will be faster than the Exynos 4 Quad that powered the international Galaxy S III, but the new chip's architecture also brings a few interesting things to the table. Let's take a look.

Eight cores (technically)
Most mentions of the Exynos 5 Octa simply say that it has eight CPU cores. This isn't untrue -- the chip actually does have eight distinct CPU cores -- but not all of these cores are created equal.

The biggest issue in designing a chip for a smartphone or tablet is balancing performance and power consumption, and most modern chips attempt to do both -- the chips can use multiple cores and higher clock speeds when higher performance is called for, but will typically disable cores and lower clock speeds during light or idle use. The Octa attempts to solve this problem using a CPU configuration that ARM calls " big.LITTLE".

Big.LITTLE pairs two distinct CPU cores, one larger and faster (in this case, a Cortex-A15 running at 1.2GHz) and one that is smaller and more power-efficient (a Cortex-A7 running at 1.6GHz). These two cores support the same instruction sets and can execute all of the same code, so speed and power consumption are the main differences between them. Lighter tasks like web browsing and email checking will be executed on the power-saving Cortex-A7 cores, while more computationally intensive tasks like gaming will be sent over to the Cortex-A15s.

The core switching is controlled by a firmware layer that sits in between the software and the chip itself. Operating systems can be tweaked to better support big.LITTLE's particular arrangement of cores, but any OS that supports power state switching for CPUs (any mainstream operating system from the last decade or so) can take advantage of big.LITTLE without any additional changes.

Different versions of this idea have existed in shipping products for some time now -- the most prominent example is probably Nvidia's Tegra chips, which include a single "companion" or "shadow" core that kicks in for light use so that the more power-hungry main CPU cores can switch off. Big.LITTLE simply takes it further, pairing each high-end CPU core with a slower one. Since the Exynos 5 Octa is the first big.LITTLE chip to ship, we don't have much real-world evidence that one approach is superior to the other, but in both cases the concept is similar.

There are two different implementations of big.LITTLE that hardware makers can use: one in which the Cortex-A7 and A15 cores can be active at the same time (called "big.LITTLE MP" in ARM's documentation) and one in which a Cortex-A7 core powers down when its corresponding A15 core powers up and vice versa. By all appearances, the Octa uses the latter implementation.

Samsung's demo video for the chip has some CPU usage examples toward the end, and as long as the examples used here are representative of how the chip actually works, the A7 and A15 cores can't both be used at the same time -- the chip has eight cores, but only four of them can be active at any one time. The upshot of this is that the Exynos 5 Octa's maximum performance will be consistent with a quad-core Cortex-A15 chip like Nvidia's Tegra 4.

Benchmarks aside, the relatively low clock speed of the Octa in the S4 is a byproduct of using a chip like the A15 in a smartphone. In a tablet with more room to dissipate heat and a larger battery to compensate for the resultant bump in power usage, there should be plenty of room to ramp the clock speed up a bit. Given that the Exynos 4 Quad from the international Galaxy S III later made its way into both the Galaxy Note 10.1 and the Galaxy Note 8.0, it's a safe bet that we'll see some version of the Octa make its way into future Samsung tablets.

An Apple-esque GPU
There's (thankfully) much less to say about the GPU in the Exynos 5 Octa. It's a triple-core Imagination Technologies PowerVR SGX 544MP3, which is a bit of a departure for Samsung -- its past Exynos chips (including the Exynos 5 Dual) have largely depended on ARM's Mali GPUs, but the Octa is using something much more similar to the GPUs that Apple uses in its A-series SoCs.

The 544MP3 is in fact very similar to the triple-core 543MP3, with the only real difference being API support (the 544MP3 supports DirectX 9 and OpenGL 2.1, suggesting that this chip may find its way into Windows phones and tablets going forward). Again, we weren't able to run any specific GPU benchmarks, but depending on the GPU's clock speed we should be looking at performance in the same ballpark as the Apple A6, albeit in a phone with a much higher-resolution screen.

Just from this chip, it's not clear whether going with Imagination's graphics technology over ARM's is a new direction for Samsung or a one-off decision for the Octa in particular, but in any case it's a big win for Imagination. Its graphics technology isn't quite as widespread as Qualcomm's Adreno GPUs (at least not in the United States), but to have their technology included in so many high-profile, top-selling phones has got to be good for their bottom line.

Sorry, North Americans and Brits, but this probably doesn't apply to you (yet)
The Exynos 5 Octa is an interesting chip, but consumers in North America and the UK probably aren't going to get to see it, at least not in the Galaxy S4. As is often the case, the American and UK version of the phone will instead use a 1.9GHz quad-core Snapdragon 600 SoC. This is a fairly common practice for most of the major smartphone players, since Qualcomm's LTE modems are the most mature in the market at the moment, but with companies like Intel and Nvidia catching up, we should hopefully start seeing a little more variety later this year and into next.

We weren't able to run our full suite of benchmarks on the Snapdragon-equipped S4 we briefly handled, but we were able to get a couple of quick browser benchmarks that we can use to compare the S4 to both the US version of the Galaxy S III and the Tegra 4 reference tablet we spent some time with at Mobile World Congress in February -- this chip is a quad-core Cortex-A15 chip, and so at the same clock speed should share at least similar performance to the Exynos 5 Octa.

Despite running at the same clock speed, the Tegra 4 numbers show off the advantages of using four Cortex-A15 CPU cores. The Exynos 5 Octa's cores will be clocked significantly lower, but I'd be interested to see how the Octa in the international version of the phone compares to the Snapdragon 600 in the US version.

Compared to the dual-core 1.5GHz Snapdragon S4 in the Galaxy S III, though, the S 4 acquits itself reasonably well -- both the CPU and the GPU will be substantially upgraded over last year's model. This will almost certainly be the model of the phone that we get for our review, and rest assured that we'll be benchmarking it more thoroughly when the time comes.

As for the Exynos 5 Octa, as we mentioned earlier it's a fair bet that this will make it to American shores in the form of a future tablet from the company, or possibly even something like the ARM Chromebook (which uses the Exynos 5 Dual). When we get our hands on a device that uses the chip, we'll be paying special attention to the battery life under various workloads to see whether big.LITTLE lives up to its promises -- as companies continue trying to push the performance envelope without destroying their devices' battery lives, chips like the Octa are only going to become more common. [Source]

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