Clock speed

What clock speed means

Clock speed (sometimes called clock rate) is the number of fetch–execute cycles the processor completes every second. One cycle is a single trip round the fetch–execute cycle, in which the processor fetches an instruction, decodes it and carries it out. Because roughly one instruction completes per cycle, clock speed is often simplified to mean the number of instructions a processor carries out each second.

Clock speed is measured in hertz (Hz). A chip running at 1 Hz completes one cycle every second. Real processors work far faster than that, so kilohertz, megahertz and gigahertz are used instead. Hertz counts cycles, not bytes, so a clock speed on its own says nothing about how much storage a computer has.

Faster and slower clock speeds

More cycles each second gives the processor more chances to work through instructions, so software finishes sooner. Reduce the clock speed and every instruction waits longer for its turn, so tasks take more time. A chip marked 1.2 GHz completes 1,200,000,000 cycles in one second — the same as 1,200 MHz.

Power and heat

A high clock speed comes at a cost: the processor draws more power and gets hotter, so it needs stronger cooling, such as a bigger heatsink, a larger fan or a liquid cooler. That is why the biggest GHz number is not automatically the best buy — it has to be weighed up against electricity use, temperature and the other characteristics that shape performance.