With the Galaxy Alpha and its Exynos 5430, Samsung launched its first 20nm processor. That chip was still based on the ARMv7 architecture, using four 1.3 Ghz Cortex A7 cores and four 1.8 Ghz Cortex A15 cores for its CPU.
Samsung's first ARMv8 chip, with Cortex A53 and Cortex A57 cores, was also built on 20nm, and it was put inside the Galaxy Note 4. Samsung called it Exynos 5433, which sounds like it's only slightly different than the Exynos 5430 in the Galaxy Alpha, despite the fact that it was the company's first 64-bit chip on the new ARMv8 architecture. That's an achievement one would think Samsung would want to promote more when it first used this processor.
There have been some theories for why Samsung didn't give this chip a new name. One is that it's a 64-bit ARMv8 chip that wouldn't be well supported on KitKat – at least not without some effort from Samsung's engineers. With Android Lollipop just months from arriving on Samsung's devices, the company probably thought that there's no need to try to reinvent the wheel and that it's better to wait for Android 5.0 Lollipop to get that full ARMv8 support.
Another theory is that Samsung didn't want to create a situation where it had a "32-bit Galaxy Note 4" and a "64-bit Galaxy Note 4 on the market. Samsung uses the Snapdragon 805 SoC for Note 4 in some markets, and that chip is 32-bit only. The Exynos 5433 supports both 32-bit and 64-bit modes but has the 64-bit mode disabled. Whether this will only last until the Note 4 gets its Android 5.0 update or not remains to be seen.
Chances are that Samsung will continue to want everyone to think that the Note 4 – as in both models – is only a 32-bit device. For new devices that will use the Exynos 5433, Samsung will call the chip the "Exynos 7 Octa." If you're wondering where "Exynos 6 Octa" is, it seems Samsung is pulling a Windows 10 here and skipping a number, most likely for marketing purposes. (Plus, the number 7 has positive connotations in many cultures.)
The Exynos 7 Octa, just like the Exynos 5433, comes with Mali-T760, which according to Samsung offers a 74 percent increase in graphics performance over the Mali-T628 MP6 inside the Galaxy Alpha. That may be necessary to power the Note 4's 1440p resolution (78 percent more pixels than previous 1080p Samsung devices).
It's unlikely we'll see this chip in another Samsung smartphone until the Galaxy S6 arrives in spring next year, but we may see it in a successor to the Galaxy Note 10.1 2014 Edition, or in some other high-end Samsung tablet in the meantime.
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The difference is 64-bit also means it supports ARMv8 instructions sets and follows the ARM architecture enhancements that go with it. This speeds up certain operations without any effort, and speeds up even more if programs are compiled to use ARMv8 architecture.
64-bit is borderline useless on mobile today, as the biggest change you'll notice is probably increased power usage and a larger footprint.