What Does GHz Mean in a Computer Processor?

by Steve Lander, Demand Media Google
A gigahertz rating essentially tells you how fast a CPU can work.


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One of the most frequently touted measures of processor performance is a given chip's speed in gigahertz. Processors with higher GHz ratings can, theoretically, do more in a given unit of time than processors with lower GHz ratings. However, the processor's speed rating is just one of many factors that impact how fast it actually processes data. Given that some specialized applications can be very computationally demanding, choosing the fastest computer is more important than buying a machine with the highest clock speed.

System Clocks

Processors work according to a clock that beats a set number of times per second, usually measured in gigahertz. For instance, a 3.1-GHz processor has a clock that beats 3.1 billion times per second. Each clock beat represents an opportunity for the processor to manipulate a number of bits equivalent to its capacity -- 64-bit processors can work on 64 bits at a time, while 32-bit processors work on 32 bits at a time.

Internal vs. External

The clock that usually gets included in marketing materials is the internal clock, but a processor also has an external clock that determines how quickly the processor can communicate with the outside world. The internal clock represents how quickly the processor can manipulate the data it already has, while the external clock specifies how quickly it can read the information it needs to manipulate or how quickly it can output the manipulated data. As of the date of publication, external clocks are frequently significantly slower than internal clocks. For example, while a processor may run at 3 GHz, its external clock could be anywhere from a few hundred MHz to 1 GHz. Since the external clock determines how quickly the processor can communicate with the system's memory, it has a significant effect on your processor's real-world speed.

Clocks and Instructions

The difference between a processor's internal and external clock speeds is one limitation on its performance. Another is the number of clock ticks it takes to execute an instruction. While some instructions can be completed in one clock tick, it could, for instance, take four ticks to complete a multiplication operation. This would turn a processor that can, for instance, add at 4 GHz into one that multiplies at an effective speed of 1 GHz.

Putting It All Together

The three factors identified here work together to determine how fast a given processor will operate. Sixty-four-bit chips work on twice as much data at once as 32-bit chips, giving them a significant performance boost. Processors with faster external clocks can also exchange data with the computer faster than those with slower external clocks. Finally, processors with more efficient instruction sets that can do more work in fewer clock cycles run more quickly than those that need more cycles to finish an instruction. Once you've made all of those factors equal, compare processors to see which is faster by looking at the internal clock speed's gigahertz rating.

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