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[ The PC Guide | The PC Buyer's Guide | Designing and Specifying PC Systems and Components | Detailed Considerations and Tips for Specifying Particular Components ]

System Processors (CPUs)

Description: The system processor, also called the central processing unit or CPU, is the "brain" of the PC, the device that actually performs the majority of the calculations within the system--it is what runs the software you run. The CPU probably gets more attention than any other component within the PC, and it does deserve a great deal of attention, though perhaps not quite as much as it receives.

CPUs are fascinating devices that I could talk about at length. However, most of the interesting aspects of them are related to their internal architecture and construction, which don't matter much to most PC buyers. What matters with a CPU is that it runs fast and works reliably. To that end my discussion will be geared more towards what the processor does than what it is.

Tip: For much additional information on CPUs, including more discussion of how they work, as well as many of the technical details, criteria and features mentioned below, see the Reference Guide section on the system processor.

Role and Subsystems: The system processor is a major component in the system processing core. It performs the processing of the system and connects to other key system devices such as the chipset, system memory, video and system buses.

Related Components: The processor is strongly related to the motherboard (most specifically, the chipset on the motherboard that interfaces and works with the CPU) and the system memory as well. The CPU, motherboard and system memory type are typically selected at the same time. The processor is also related to the power supply in that some newer CPUs require a great deal of power.

Key Compatibility Selection Criteria: For many system designers, the CPU is not selected specifically to match other components, but the other way around: they start with the CPU they want to use and match the other components to it. The CPU and motherboard are particularly tightly connected; the selection of CPU immediately defines your motherboard options. Conversely, if you are adding a CPU to an existing motherboard, you can only use whatever CPU models and speeds that motherboard supports. The selection of the CPU type and its "siblings" such as the chipset, motherboard and system memory is sometimes called choosing a platform.

When it comes to CPUs, and in fact the entire system platform, the biggest decision that system designers may make today is this: Intel or non-Intel? Over the last few years companies such as AMD have produced CPUs that are every bit as good as Intel's, and in some cases offer better performance per dollar. The decision of what CPU manufacturer to choose is largely a matter of personal taste and the particulars of the market at any given moment in time. (It also often comes down to politics and personal bias as much as it does anything of a technical nature.) Unfortunately, since around 1997, Intel and its competitors have been making CPUs that use different motherboards and chipsets, making this even more an "either-or" decision than in the past, when Intel and non-Intel CPUs used the same motherboards.

The CPU must be matched to the motherboard in terms of both its interface (system bus) speed and its physical packaging. Certain speeds of some processors are available in more than one package style (for either slots or sockets) and in more than one system bus speed. In some cases BIOS upgrades are needed to support newer CPUs. See below for more.

Note: To be sure of compatibility, you may need to find out the exact model number of the CPU you are considering. For example, "Pentium III XXX MHz" doesn't tell you all you need to know because different Pentium IIIs running at XXX MHz may have slightly different electrical characteristics. A good idea is to purchase the CPU and the motherboard from the same source.

Another less significant compatibility issue is that some CPUs have rather demanding power and cooling requirements. You may need to consult with a compatibility list or check the output requirements of the supply to be sure the CPU will get all the juice it needs to run properly.

Performance and Capacity Selection Criteria: Here are the performance issues that define the performance characteristics of the CPU:

  • Clock Speed: Measured in MHz or GHz, this represents the raw number of instructions or instruction steps that the CPU can execute each second (MHz is millions of cycles per second, GHz is billions). A CPU with a faster clock speed runs more quickly than one of the same type with a slower clock speed. However, there is more to CPU speed than just clock speed (see the paragraphs that follow for more.)
  • Architecture: The general design of the CPU affects the speed of the CPU. This includes features such as the number of internal processing units, the amount of primary cache and other details that I already said I wasn't going to go into. :^) As a buyer you do not need to analyze these engineering issues, but you do need to recognize that the design of the CPU is in many ways as important as its clock speed--you can't compare two different CPU types just on the basis of their MHz or GHz rating.
  • System Bus Speed: The bus speed of the CPU, sometimes called the front side bus or FSB, refers to the clock speed of the interface between the CPU and the system memory. Faster buses improve performance, but not by a tremendous amount.
  • Secondary Cache Design And Amount: Most systems have two sets of cache, which refers to buffers that improve performance by holding recently-used information for near term use by the CPU. The primary cache is integrated into the CPU and is the fastest; I consider it an architectural feature. The secondary cache was traditionally a component on the motherboard, but modern CPUs now integrate it (first it was put into a module with the CPU, and now it has been incorporated into the physical processor package, a design first used with the Pentium Pro in the mid-1990s). The size and bus speed of the secondary cache influence overall performance, to a degree.
  • Overclockability: Some CPUs are easier to overclock than others, which is important if you are overclocking.

Quality Selection Criteria: Quality is actually not much of a selection criterion for CPUs; some might find that surprising. The reason is that there are only a few large companies making CPUs, and they are all very good when it comes to reliability and quality issues.

Probably the only real quality concern when it comes to CPUs isn't even really related to the CPU! It's a matter of the way in which it is cooled. Be sure that the processor either comes with a heat sink and fan appropriate to it, or that you purchase one that is appropriate.

Important Features: There really aren't any major "extra features" associated with CPUs. The only thing that really falls into this category is the extra instruction sets included in various CPUs by their manufacturers. Since Intel and its competitors can't agree on these extensions to the standard PC ("x86") instruction set, these have never been standardized. Support for them is not consistent and it's not clear to me yet if support for any of them will ever become essential. If there's a particular game that is important to you that takes advantage of the instruction set of a specific CPU, this may influence your decision.

"Magic Numbers" To Watch For: CPU clock speed is The Big One, the Mother Of All Magic Numbers. :^) There are people that will try to sell an entire PC as, for example, a "700 MHz system". It's true that a 700 MHz system will be faster than a 400 MHz one, but it's really a gross simplification to focus overly on CPU clock speed--a PC with a 550 MHz CPU can easily be faster than one with a 700 MHz chip if the former PC has better hardware in other areas. Be sure to pay attention not just to other components, but to the other performance characteristics of the CPU and its interfaces. Also see the next paragraph.

Performance Impact: The CPU is definitely an important performance component. However, its impact on overall performance is often overstated. Large differences are important; small ones are not. Increasing your CPU's speed from 400 MHz to 800 MHz will result in noticeable improvement, but upgrading from 400 MHz to 500 MHz is probably a waste of money. Even though the CPU will be 25% faster, the system overall won't be, because there is much more to system performance than just CPU clock speed. There are diminishing returns, especially as you increase CPU speed while leaving the rest of the system the same. Unless you're a hard-core gamer, one of the most important ways to keep a PC under budget is not to overspend on the CPU; see here for the reason why.

Retail, OEM and Gray Market Issues: CPUs are typically sold as either boxed retail pieces or OEM units. Gray market CPUs are also everywhere, usually as OEM units. If you buy a boxed retail CPU you will normally get with it a full multi-year warranty from the manufacturer and a heat sink and fan designed for the chip. OEM units carry only whatever warranty the vendor is willing to grant, often as little as 30 days.

Importance of Manufacturer: Not very important from a quality standpoint, as I discussed above, but some people have a tendency to get very evangelical about their favorite CPU maker. :^)

Typical Component Lifetime: A CPU will last for over a decade if it is properly installed and cooled. As for obsolescence, it's really a matter of what your needs are. After as little as six months even the fastest CPU around is displaced by something newer; that doesn't mean the CPU needs to be replaced however. Most people can use a system for at least three years before the need to upgrade becomes more pressing. CPU upgrades are relatively easy until you reach the fastest CPU that can run in a particular motherboard. At that point you are looking at a new motherboard and CPU--a platform change that will radically alter the system as a whole.

Warranty Issues: As mentioned above, full warranty is only provided from the manufacturer on boxed units--if you buy an OEM or gray market part from a vendor you only get whatever warranty the vendor provides. I would not say that CPUs are a component to be especially concerned about when it comes to warranty matters: the vast majority of CPUs work fine for years straight out of the box, and the vast majority of those that are bad will fail within the first few days of operation. (What you do not want to do is buy an OEM processor and then let it sit around beyond the vendor's warranty period before you thoroughly test it...)

Driver Support Issues: Not applicable, though sometimes bugs occur in games or even operating systems that require patches. There are only so many different CPUs out there so problems of this sort are usually uncovered rather quickly.

Special Specification Considerations: In addition to the above, here are a few issues to keep in mind when buying CPUs:

  • Be sure to buy from a reputable dealer to avoid trickery. There have been cases of people buying CPUs and getting sent slower units "by accident", of course with the heat sink already attached by the vendor as a "convenience" (so you can't see its markings). Of course, CPU remarking (unscrupulous people misrepresenting the speed of processors) is a problem that has been widely publicized.
  • Prices on CPUs are reduced regularly, according to well-publicized schedules. Right before those times availability of processors becomes spotty as vendors try to get rid of inventory so it doesn't "devalue" during the price drop. Plan your purchase accordingly.
  • If you are planning to use multiple CPUs in a system, there may be specific additional requirements you need to look at. For example, you may need two identical CPUs of the same stepping (revision) level. A good vendor can help you with this.

Next: System Memory (RAM)

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