The vast majority of computer processors operate based on a clock rate. The clock rate is a measure of the frequency of oscillations of the clock generator of the processor. These clock pulses are used to synchronize the operations of the processor and are a reasonable indicator of processor speed. In other words, it’s the rate at which the CPU can do specific functions.
The clock rate is measured in cycles per second using the SI unites Hertz. Modern CPUs and GPUs are typically measured in Gigahertz (GHz), or billions of cycles per second. Historically, Megahertz (MHz) and even Kilohertz (kHz) have been used when processer clock rates were lower.
The Clock Isn’t Where You Think It Is
You might think that the actual clock generator used to set the clock rate of a CPU is on the CPU itself. The clock generator is located in the CPU chipset on the motherboard. The chipset sets the base clock. This is typically exactly 100MHz. The CPU then sets its clock speed by applying a multiplier to the base clock.
The core oscillator that sets the clock rate is a quartz crystal oscillating at precisely one frequency when an electric charge is applied. The use of a multiplier means that it’s possible to change the actual CPU clock rate at will. This can come in handy when trying to save power when idling or when trying to boost higher when under load. Overclocking is the process of manually increasing this multiplier.
Some motherboards offer a second base clock that can run at 125MHz. This forms a second physical quartz crystal that oscillates at a faster rate. As you might expect, this can increase system performance, even on CPUs with a locked multiplier, because it’s now closed to multiplying a larger value. Unfortunately, this can cause stability issues with other components as basically everything assumes a 100MHz base clock. Your mileage may vary, but this generally isn’t advisable.
Allowing for Speed Limits
Electrons in electrical circuits can travel quite fast, typically two-thirds of the speed of light. That might sound fast, but there are some issues with clock rates in the GHz range. At a clock speed of 5GHz, the CPU clock oscillates once every 0.2 nanoseconds. The universe’s absolute speed limit is the speed of light in a vacuum. The speed of light is very fast, almost 300 million meters per second. Still, in 0.2 nanoseconds, light only travels 6 centimeters or 2.4 inches.
Now CPUs aren’t particularly big, but they are relatively close to six centimeters in size. The path a – slower than light – electron would take through a CPU is hardly straight. This leads to coherence problems as – with a single clock – one side of the CPU would simply get the clock pulse later. To combat that, CPUs have multiple clocks that are all carefully synchronized but cover a much smaller area within the overall CPU. This allows modern high-speed CPUs to stay synchronized.
CPUs are designed to run at a specific clock speed. Manufacturers sell them with a guaranteed clock speed. The faster models will almost always be more expensive. Even without defects, manufacturing tolerances lead to slight variations impacting performance. Before every CPU is sold, it is tested to confirm its capabilities. It is sorted into a high-performance “bin” if it can reach the highest clock rate.
Similarly, CPUs that don’t reach the peak speeds but can reach the speeds intended for lower processor tiers are sorted into lower performance bins. This process is called “binning” and generally means that more expensive CPUs are likely to be able to run at higher clock rates. It can be possible for CPUs from lower bins to perform better than their advertised tier. However, they may not be able to exceed it by much as they typically weren’t placed in higher bins.
Not every CPU comes out perfect, though, and manufacturing defects can simply prevent a CPU from ever operating. These manufacturing defects can sometimes be minor enough that certain features can simply be disabled. For example, if a CPU has a tiny fault, this may prevent a single core from working while the rest of the CPU is fine.
To sell the product, the manufacturer will typically disable affected parts – and if necessary, to meet a product tier – even some perfectly functional parts. This can allow the manufacturer to sell what was, for example, a six-core CPU as a four-core CPU, which still earns them more money than simply discarding an expensive product. Typically, this doesn’t directly affect the clock rate, though it may mean that what would have been a top bin CPU gets placed in a lower tier simply because some parts were disabled.
The clock rate is a critical factor in CPU performance, though it may not be directly comparable between CPU architectures. The clock rate of a CPU is actually set indirectly. A standard 100MHz base clock is used in almost all computers.
The CPU then sets a multiplier on this base clock to get its actual clock rate. CPUs are sold with a guarantee to operate at a specific clock rate or below. In many cases, they can be pushed beyond that via overclocking. However, that often requires good cooling as it draws more power and generates more heat.