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Factors Affecting CPU Performance


There are many factors that affect processor performance. Understanding some of these factors will help you make the proper choices when designing your homebuilt computer.

The most important factors affecting processor performance are:


Instruction Set

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This is the processor's built-in code that tells it how to execute its duties. It's something that's coded into the chip when it's manufactured and that you can't change. But together with processor architecture, it does affect performance across a given line of CPU's. The processor's architecture and instruction set determine how many cycles, or ticks, are needed to execute a given instruction.

In other words, some instruction sets are more efficient than others, enabling the processor to do more useful work at a given speed. This is why just looking at the numbers doesn't always tell the whole story of how well a processor will function in the real world. It's also why when choosing a processor, benchmark tests that measure the chip's abilities to do real-world work can be very useful.


Clock Speed

The clock speed (or clock rate) is stated in megahertz (MHz) or gigahertz (GHz), and refers to the speed at which the processor can execute instructions. The faster the clock, the more instructions the processor can complete per second.

All else being equal, processors with faster clock speeds process data faster than those with slower clock speeds. The clock speed is also the first number you will see in advertisements for CPUs and is often integrated into their model numbers. But as mentioned previously, the efficiency of the processor's architecture determines how much actual work a processor can do with the same number of cycles.

Long story short, don't select a CPU based on clock speed alone. It's only one of the factors (albeit an important one) that determines how well a CPU will perform in real-world situations. Again, benchmarking tests are your friend.


Measured in bits, the bandwidth determines how much information the processor can process in one instruction. If you were to compare data flow to the flow of traffic on a highway, then clock speed would be the speed limit, and bandwidth would be the number of lanes on the highway.

The current bandwidth standard for desktop and laptop PC's is 64 bit. 32-bit is officially a thing of the past.


Front Side Bus (FSB) Speed

The FSB is the interface between the processor and the system memory. As such, the FSB speed limits the rate at which data can get to the CPU, which in turn limits the rate at which the CPU can process that data. The CPU's FSB speed determines the maximum speed at which it can transfer data to the rest of the system.

Other factors affecting data transfer rates include the system clock speed, the motherboard chipset, and the RAM speed.


On-Board Cache

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The on-board (or "on-die") cache is a relatively small amount of high-performance SRAM built directly into the processor. It enables the CPU to access repeatedly used data directly from its own on-board memory, rather than repeatedly requesting it from the system RAM. It was a rather early development in the history of computing that was necessitated by advances in CPU technology happening much more rapidly than advances in memory technology. To put it simply, CPU's were getting faster, but memory wasn't. Engineers addressed that problem by putting tiny amounts of the best memory then known to man (or woman) right on the processor itself.

The L1 cache typically is the smallest and fastest RAM on the computer. It stores the information that a particular core of the processor is most likely to need to complete its present task. The L2 cache is larger, but not quite as fast. It holds the information the processor core is most likely to need to complete its next task. L3 is much larger, but slower (though still faster than requesting the information from system RAM), and is shared by all the cores. It holds information that is most likely to be needed by any of the cores for their next tasks.

Grossly oversimplified, when the processor core needs some piece of data, it looks for it in L1 first, then in L2, and then in L3. If it doesn't find it, only then does it request it from system RAM. That's why all else being equal, a processor with more onboard cache will outperform a processor with less onboard cache.


Heat and Heat Dissipation

When processors run too hot, they can start doing funky things like throw errors, lock up, or even burn up. Installing an inadequate cooling system can cause your homebuilt computer project to go sour in a big (and possibly expensive) way. So don't skimp on the CPU cooler and case fans.

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