A desktop PC used to need a lot of different chips to make it work. You had the big parts: the CPU that executed most of your code and the GPU that rendered your pretty 3D graphics. But there were a lot of smaller bits too: a chip called the northbridge handled all communication between the CPU, GPU, and RAM, while the southbridge handled communication between the northbridge and other interfaces like USB or SATA. Separate controller chips for things like USB ports, Ethernet ports, and audio were also often required if this functionality wasn't already integrated into the southbridge itself.

As chip manufacturing processes have improved, it's now possible to cram more and more of these previously separate components into a single chip. This not only reduces system complexity, cost, and power consumption, but it also saves space, making it possible to fit a high-end computer from yesteryear into a smartphone that can fit in your pocket. It's these technological advancements that have given rise to the system-on-a-chip (SoC), one monolithic chip that's home to all of the major components that make these devices tick.

The fact that every one of these chips includes what is essentially an entire computer can make keeping track of an individual chip's features and performance quite time-consuming. To help you keep things straight, we've assembled this handy guide that will walk you through the basics of how an SoC is put together. It will also serve as a guide to most of the current (and future, where applicable) chips available from the big players making SoCs today: Apple, Qualcomm, Samsung, Nvidia, Texas Instruments, Intel, and AMD. There's simply too much to talk about to fit everything into one article of reasonable length, but if you've been wondering what makes a Snapdragon different from a Tegra, here's a start.

Putting a chip together

There's no discussion of smartphone and tablet chips that can happen without a discussion of ARM Holdings, a British company with a long history of involvement in embedded systems. ARM's processors (and the instruction set that they use, also called ARM) are designed to consume very small amounts of power, much less than the Intel or AMD CPUs you might find at the heart of a standard computer. This is one of the reasons why you see ARM chips at the heart of so many phones and tablets today. To better understand how ARM operates (and to explain why so many companies use ARM's CPU designs and instruction sets), we first must talk a bit about Intel.

Intel handles just about everything about its desktop and laptop CPUs in-house: Intel owns the x86 instruction set its processors use, Intel designs its own CPUs and the vast majority of its own GPUs, Intel manufactures its own chips in its own semiconductor fabrication plants (fabs), and Intel handles the sale of its CPUs to both hardware manufacturers and end users. Intel can do all of this because of its sheer size, but it's one of the only companies able to work this way. Even in AMD's heyday, the company was still licensing the x86 instruction set from Intel. More recently, AMD sold off its own fabs—the company now directly handles only the design and sale of its processors, rather than handling everything from start to finish.

ARM's operation is more democratized by design. Rather than making and selling any of its own chips, ARM creates and licenses its own processor designs for other companies to use in their chips—this is where we get things like the Cortex-A9 and the Cortex-A15 that sometimes pop up in Ars phone and tablet reviews. Nvidia's Tegra 3 and 4, Samsung's Exynos 4 and 5, and Apple's A5 processors are all examples of SoCs that use ARM's CPU cores. ARM also licenses its instruction set for third parties to use in their own custom CPU designs. This allows companies to put together CPUs that will run the same code as ARM's Cortex designs but have different performance and power consumption characteristics. Both Apple and Qualcomm (with their A6 and Snapdragon S4 chips, respectively) have made their own custom designs that exceed Cortex-A9's performance but generally use less power than Cortex-A15.

The situation is similar on the graphics side. ARM offers its own "Mali" series GPUs that can be licensed the same way its CPU cores are licensed, or companies can make their own GPUs (Nvidia and Qualcomm both take the latter route). There are also some companies that specialize in creating graphics architectures. Imagination Technologies is probably the biggest player in this space, and it licenses its mobile GPU architectures to the likes of Intel, Apple, and Samsung, among others.

Chip designers take these CPU and GPU bits and marry them to other necessary components—a memory interface is necessary, and specialized blocks for things like encoding and decoding video and processing images from a camera are also frequent additions. The result is a single, monolithic chip called a "system on a chip" (SoC) because of its more-or-less self-contained nature.

There are two things that sometimes don't get integrated into the SoC itself. The first is RAM, which is sometimes a separate chip but is often stacked on top of the main SoC to save space (a method called "package-on-package" or PoP for short). A separate chip is also sometimes used to handle wireless connectivity. However, in smartphones especially, the cellular modem is also incorporated into the SoC itself.

While these different ARM SoCs all run the same basic code, there's a lot of variety between chips from different manufacturers. To make things a bit easier to digest, we'll go through all of the major ARM licensees and discuss their respective chip designs, those chips' performance levels, and products that each chip has shown up in. We'll also talk a bit about each chipmaker's plans for the future, to the extent that we know about them, and about the non-ARM SoCs that are slowly making their way into shipping products. Note that this is not intended to be a comprehensive look at all ARM licensees, but rather a thorough primer on the major players in today's and tomorrow's phones and tablets.

Listing image by Andrew Cunningham / Aurich Lawson