Computers have numerous specifications, measured in capacities or speeds. Storage devices are measured by how much data they can store and how fast the data can be accessed. Other components also have to access data, and can be measured by how fast that access occurs and often how fast it can perform functions with that data.
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Most computer components can be measured by speed. Hard drives spin at certain speeds, or RPMs. A computer communicates across its motherboard at a particular bus speed and optical drives access data at certain speeds. The CPU also operates at a particular speed, its clock speed.
The general term, computer speed, refers to CPU clock speed. This is how fast the CPU operates, usually measured in gigahertz (GHz). One gigahertz is equal to 1 billion hertz, or cycles per second. So a 1 GHz computer has a CPU that operates at 1 billion cycles per second.
Operating at 1 GHz does not mean a computer can do 1 billion operations every second. Different CPUs use different instructions that affect how many cycles it takes to complete an operation or how many operations can be completed in one cycle. So two different CPUs might not operate practically at the same speed even if their clock speeds are identical.