CPU performance

The CPU has a clock that sends out pulses at a steady rate. Each pulse is a cycle, and the CPU can carry out a step of the fetch–execute cycle on each one. Clock speed is the number of cycles per second, measured in hertz (Hz). A clock speed of 3.5 GHz means 3.5 billion cycles every second.

Higher clock speed: more instructions are carried out each second, so programs run faster.

Its limits:

• A faster clock makes the CPU hotter and uses more power, which drains a laptop or phone battery faster. Too much heat can damage the CPU, so it needs better cooling.
• The CPU still has to wait for slower parts, such as RAM, so doubling the clock speed doesn't double the speed of every program.

Cache is very fast memory inside the CPU. It holds the instructions and data the CPU is using most, so it doesn't have to fetch them from RAM, which is much slower.

Bigger cache: more instructions and data can be kept close at hand, so the CPU waits for RAM less often and runs faster.

Its limits:

• Cache is expensive and takes up space on the CPU chip, so it is small compared with RAM.
• Once the instructions and data a program uses most already fit in the cache, adding more makes little difference.

A core is a complete processing unit, able to fetch, decode and execute instructions on its own. A multi-core CPU has several cores on one chip: a quad-core CPU has four, so it can work on four instructions at the same time.

More cores: more instructions can be processed at once, so the computer can run several programs smoothly, or split one big job (such as editing a video) between the cores.

Its limits:

• The software must be written to split its work between cores. If it isn't, extra cores sit idle.
• Some tasks have to be done in order, as each step needs the answer to the one before, so they can't be shared out.
• The cores have to coordinate, which takes time, so doubling the cores doesn't double the speed.

Exam questions often compare two CPUs. Take these two:

• For editing video, which splits easily between cores, CPU A is likely to be faster: it can process eight instructions at once, and its bigger cache means fewer waits for RAM.
• For an older program that only uses one core, CPU B may be faster, as each of its cores carries out more cycles per second.

The characteristics work together: no single number decides which CPU is faster for every job.