Mobile CPUs Explained – Tech Explained
Have you ever wondered how people like Apple and Samsung are able to fit 8 core CPUs inside your phone, when a desktop 8 core is this massive? Or how they can be so damn efficient? Well in this video I want to explain just that – but first if you haven’t already, consider subscribing for more videos like this one every monday wednesday and friday!
So, how do you go from an 8 core CPU being this size, to something that fits inside a phone, and doesn’t drain your battery in an instant? Well, theres a few key differences. First, this 8 core CPU isn’t actually ‘this big’ – most of this is the integrated heat spreader (IHS) and the PCB with the pins/pads, the actual die, the “chip”, is pretty tiny by comparison.
But that alone doesn’t make it fit or run inside a phone – the important thing to know is that mobiles CPUs – while still very impressive – are nowhere near as powerful as desktop chips. The Ryzen 3700X – their lowest power 8 core – still draws over 80W when in full use, with a TDP or thermal design power rating of 65W. Compare that to the Snapdragon 855 with it’s TDP of just 5W, you can imagine the performance implications there.
To illustrate that, I installed Geekbench 5 on both my main PC, a Ryzen 2700 admittedly without closing any of the 4 web browsers, 4 VSCode windows or any of the other applications open – so almost a worst case scenario for the desktop, and also installed it on my phone, a oneplus 7t pro. The results were interesting, single core performance was actually pretty close, but multithreaded… The PC was lightyears ahead. That’s to be expected of course, but interesting to see in action.
So they are lower power, is that it? Not quite. See a mobile processor isn’t just a CPU – it’s a CPU, a GPU, a modem, a wifi and bluetooth controller, an image processor, security controller and accelerator, all in one. That’s known as a “SoC” or System on chip. This block diagram gives you an idea, but here’s a good comparison. The number of transistors – think tiny switches that make up all processors – in a Snapdragon 855 is 6.7 billion. Compare that to a desktop chip, and while it’s close, a 2700X for example has just shy of 5 billion, the desktop chip is only a CPU, not everything else too.
That means that only a fraction of those 6.7 billion can be used as CPU cores, making for a much less powerful design. One way they get around this limitation though is by using the ARM architecture. ARM is a british company who actually designs the cores, then licenses people like Qualcomm to integrate them into full chips and manufacture them. ARM use RISC, or reduced instruction set computing, which effectively is a way of processing instructions in a more simplified way so that each core can be more efficient.
For example, your desktop processor might want to multiply the numbers in two blocks of memory. It would run the command “MULT 2:3, 5:2” – and that would be that. On RISC, you’d have to “manually” load each value, “LOAD A, 2:3”, “LOAD B, 5:2”, then do your arithmetic, “PROD A,B” then store that value again, “STORE 2:3, A”. That might sound less efficient, but when you ask the desktop processor to multiply, it still has to do all the things the RISC chip does, but it has to know to do that based on the instruction, rather than to just do exactly what you tell it to. Because of this, mobile CPUs are impressively efficient with their lower transistor counts.
So for the most part, that’s how you get a CPU to work in your phone. While there are some exceptions, Apple’s own designs for example are remarkably powerful especially in comparison to Qualcomm’s efforts, they still mostly follow the same pattern.
