What is a CPU?

The CPU is your PC's most important component, but what does it really do?
By Posted on March 8, 2018 12:45 pm

Bill Roberson/Digital Trends

A central processor, or CPU, is arguably the most important component of any computing device. It handles basic instructions and allocates the more complicated tasks to other specific chips to get them to do what they do best. It's the core of your PC, smartphone, or tablet. and it's what makes the whole device run as it should.

What makes a CPU a CPU?

The CPU itself is a core component of what makes a computer a computer, but it isn't the computer itself -- it's just the brains of the operation. It's a small computer chip that sits atop the main circuit board (motherboard) of a computer, whether that's a desktop PC, laptop, or tablet. It's distinctly separate from the memory, which is where information is stored, and the graphics card or graphics chip, which handles all rendering of video and 3D graphics to your monitor or screen.

CPUs are built by placing billions of microscopic transistors onto a single computer chip. Those transistors allow it to make the calculations it needs to run programs that are stored on your system's memory.

One of the most common advancements of CPU technology is in making those transistors smaller and smaller. That's resulted in the improvement to CPU speed over the decades, often referred to as Moore's Law.

In the context of modern devices, a desktop or laptop will have a dedicated CPU which performs a number of processing functions for the system. Mobile devices and some tablets instead utilize a "System on Chip" (SoC) which is a chip that contains its  CPU alongside other components. Intel and AMD both offer CPUs with graphics chips and memory stored on them too, meaning they can do more than just standard CPU functions.

What does a CPU actually do?

At its core, a CPU takes instructions from a program or application and performs a calculation. This process can be broken down into three key stages: Fetch, decode, and execute. A CPU fetches the instruction from a system's RAM, then it decodes what the instruction actually is, before it is executed by the relevant parts of the CPU.

The executed instruction, or calculation, can involve basic arithmetic, comparing certain numbers together, or moving them around in memory. Since everything in a computer is represented by numbers, those kinds of simple tasks equate to what a CPU does. It's what facilitates everything from starting up Windows, to watching a YouTube video.

In modern systems, the CPU doesn't do everything, but it still has to feed specialized hardware the numbers they need to do their job. It needs to tell the graphics card to show that explosion because you clicked on that fuel barrel, or grab the contents of your Office document from the local memory

Cores, clocks and costs

Originally, processors had a single processing core. Today's modern processors are made up of multiple cores which allow it to perform multiple instructions at once. They're effectively several CPUs on a single chip. Almost all CPUs sold today are at least dual core, but at the higher end, you'll see four (quad) core CPUs, and even six, eight and 12 core CPUs in some cases. Some processors also employ a technology called multi-threading, which creates virtual processor cores. They aren't as powerful as physical cores, but they can help improve a CPU's performance.

Clock speed is another number that's thrown around a lot with CPUs. That's the "gigahertz," (GHz) figure that you'll see quoted on CPU product listings. It effectively denotes how many instructions a CPU can handle per second, but that's not the whole picture when it comes to performance. Clock speed mostly comes into play when comparing CPUs from the same product family or generation. When all else is the same, a faster clock speed means a faster processor, but a 3GHz processor from 2010 isn't going to be as fast as a 2GHz processor from 2018.

So, how much should you be paying for your CPU? We have a number of guides to give you some suggestions for the best CPUs you can buy, but for a general outline, unless you're a hardcore gamer or someone looking to do photo or video editing, you don't need to spend more than $200. Stick to a recent generation. For Intel chips that means 6th, 7th or 8th-generation chips, and for AMD, its Ryzen 1000 or 2000 CPUs.

How important is the CPU?

Although the CPU isn't as important for overall system performance as it once was, it still plays a major part in making a device run quickly. Since it is solely responsible for executing commands within programs, the faster your CPU is, the faster many applications will run.

That said, a fast CPU isn't everything. A processor, no matter how powerful, can't easily render the latest 3D games, nor can it store information. That's where other components, like graphics cards and memory, come into play.

In short, the CPU isn't everything, but it is important. In general, a faster CPU will mean that your system or device will run faster. At the very least it won't be a bottleneck in its own right. Multiple cores and threads can help you do more things at once.

Would you like a little more help to buy your next CPU? Here is our guide to the best chips from AMD and Intel.

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