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Choosing a CPU


Choosing a CPU for your homebuilt computer is one of the very first things you will want to do during the planning stage. The CPU you choose will affect every subsequent decision you make about the components you choose, and will be a crucial factor in how your completed machine performs.

When choosing a CPU for a desktop, there are many factors to consider. The most important of these are number of cores, form factor, speed, on-board cache, and reputation. Increasingly, computer builders are also concerned about power consumption and energy conservation.

Bye-Bye 32-bit. With this revision of this page, I am dropping the discussion of the differences between 32-bit and 64-bit processors. That's because 64-bit architecture is now the norm and will remain so for the foreseeable future, so there is no reason to build a 32-bit machine unless you specifically need to build an obsolete computer (for example, to run some sort of legacy industrial equipment). For the rest of us, 32-bit is ancient history; and so is the 32-bit vs. 64-bit discussion that used to be here.


Things to Think About when Choosing a CPU

Number of Cores

Of particular interest to most computer builders is the number of cores, so we may as well start here.

The "core" is the actual CPU that is mounted on a processor chip. It's the part that actually crunches numbers, which is all any CPU actually does when you get down to it. Originally, processors had one core, and high-end servers and other computers that needed more processing power would employ two or more processors.

Then processor manufacturers got the bright idea of mounting two cores on one chip, providing more processing power at lower cost and in a smaller package. And so we had the Athlon 64 X2 series on the AMD side, and the Core 2 Duo series on the Intel side. Each core on a multi-core chip is recognized as a CPU by the computer, so dual-core chips represented a major leap ahead for ordinary users: They could now enjoy the advantages of simultaneous multi-processing at a fraction of the cost of multiple-processor machines.

Now we have quad-core processors (such as AMD's Phenom II X4 and Intel's quad-core i3, i5, and truly awesome i7 line ), hex-core processors (such as AMD's Phenom II X6 and Intel's Core i7 Extreme Edition 980X), and even octa-core processors (such as AMD's FX-8150 and Intel's Xeon E7-2820) readily available and limited only by your budget. In fact, you can use combine multiple, multiple-core processors on the same motherboard. For example, four octa-core processors will result in a 32-processor machine.

But will it be faster?

Of course, the question is, how much noticeable difference will all these additional cores make to the user?

At the time of this revision (in July of 2012), my answer is that for average users, four cores is probably the sweet spot as far as performance per dollar is concerned. That's because hex- and octa-core processors are still very expensive, and very few software programs on the market assume that very many people have more than quad-core processors; so most software will run just fine on a good quad-core CPU.

Will the same software run even faster on a hex or an octa than a quad, though? Usually, yes, Almost all modern software will distribute CPU load over as many cores as it finds. But with the exception of a few very high-end programs, the user probably won't notice as much of a difference between a quad-core and a hex-core or octa core, as they would between a dual-core and a quad-core.

Of course, that will change: History shows that no matter how fast and powerful hardware gets, software programmers will always try to push that hardware to its limits. But for the next few years, I think a good quad-core processor will be perfectly satisfactory for the vast majority of users.


Form Factor

The form factor of a CPU refers to several things, the most important of which for the do-it-yourself computer builder is what socket it fits in. You must purchase a motherboard that supports your processor's form factor, otherwise it simply won't fit. (Conversely, if you're upgrading an existing computer or using a mobo you already have, you must purchase a processor that your mobo supports.)

The form factor also determines the type of CPU cooler you'll need, although there's a bit more standardization in this area.


CPU Speed

The speed of a CPU determines how many computations it can perform per second. (In the case of a multi-core CPU, it determines how many computations each core can perform in a second.)

Obviously, a faster chip can perform more computations. If you plan to use your computer only for pretty routine, low-resource applications like surfing the web, word processing, and checking email, you can save yourself some money by buying a somewhat slower chip -- for example, one or two notches below the top of that line of processors. If you're a gamer, do video or music editing, or use your computer for high-end graphics or CAD/CAM apps, then set your sights higher.


On-Board Cache

The L2 (or Level 2) cache bridges the gap between the very fast CPU and the much slower system RAM bus (and the even slower hard drive) by anticipating and storing data right on the CPU itself. This dramatically increases performance at a given clock speed. With L2 cache, more is always better; and sharing reduces performance. Look for a chip with at least 1 MB of L2 cache per core. Although this increases the cost of the CPU, it's worth the investment.

Many processors also have L3 cache built into the chip (traditionally, it resided on the motherboard). Unlike L2 cache, L3 cache commonly is shared between the cores because the data that resides there may be required by more than one core. In other words, sharing L3 cache can improve performance, whereas sharing L2 cache between cores usually decreases performances.



As with any major purchase, do your homework before buying. Check Internet message boards and forums to see how satisfied other users have been with the chips you are considering, and read the reviews often posted on retailers' sites. If you're a gamer or use other high-end software, check with forums dedicated to that software or with the software publisher, as well. Some software works better on some chips than it does with others.


Intel vs. AMD

This is one of the first questions a computer builder usually ponders, unless they've already developed a loyalty for one company over the other. It's not a question with a simple answer. Some people swear by one company's products or the other, while others look only at the numbers.

My take on the question as someone who has used both companies' chips is this:

Both AMD and Intel make excellent chips toward the top of their respective lines. AMD, being the underdog, tends to be more on the bleeding edge than Intel, which has resulted in their chips sometimes having the early edge over Intel's. They also tend to be less expensive at most ranges of performance. But in my experience, Intel's somewhat more conservative approach has often given them the stability advantage.

Consequently, when building machines for my own use, I've often gravitated toward AMD because of their lower cost and tendency to be a little more aggressive in seeking the bleeding edge. But when building machines for customers (especially businesses), I've always leaned toward Intel because I've typically found their chips to be a bit more stable.

Another factor to consider is the motherboards available for the two companies' computers. Generally speaking, when building an Intel-based machine, I've had better success using motherboards based on Intel chipsets (including Intel's own motherboards, which tend to be very solid and stable performers). With AMD chips, you have to be a little more careful. No doubt there are many excellent motherboards for AMD processors, but there are some truly horrible ones, as well. So be careful. Read the reviews on the boards you're considering, and buy only from reputable merchants.

Finally, consider what you plan to do with the computer. For example, because of the traditional popularity of AMD chips with hobbyists, gamers, and overclockers, many games, in particular, simply run better on AMD chips. On the other hand, many high-end business applications have been specifically designed around Intel's chips and tend to run better on Intel-based machines.

So check around. Ask the manufacturer of the most demanding software that you use what their recommendations are, and ask people on forums what their experiences with various chips have been before making your decision. Ultimately, the most important factor in making your decision is how well the machine will work for you.

Revision date: July 10, 2012


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