How computers work
By Charlotte Jones
The Control Unit
The Immediate Access Store
The Arithmetic Logic Unit
CPU Speed
A computer’s speed is heavily influenced by the CPU it uses. There are three main factors that affect how quickly a CPU can carry out instructions:
- clock speed
- cores
- cache
Clock Speed
It might seem like CPUs can perform many instructions simultaneously, since it is possible for you to do homework, read instant messages and listen to music at the same time. However, the CPU is able to carry out instructions at such speed that it can seem like it is simultaneous.The speed at which the CPU can carry out instructions is called the clock speed. This is controlled by a clock. With every tick of the clock, the CPU fetches and executes one instruction. The clock speed is measured in cycles per second, and one cycle per second is known as 1 hertz. This means that a CPU with a clock speed of 2 gigahertz (GHz) can carry out two thousand million (or two billion) cycles per second.
Cores
A CPU is traditionally made up of a processor with a single core. Most modern CPUs have two, four or even more cores.
A CPU with two cores, called a dual core processor, is like having two processors in one.
A quad core processor has four cores and can carry out even more instructions in the same period of time. The main downside of using quad core processors is that they are more expensive to design and make, and they also use more power than single or dual core processors. Another disadvantage is that the instructions have to be split up to decide which core will execute them and the results have to be merged together again at the end, which slows the processor down a little.
Cache
The Fetch-Execute Cycle
Fetching The Instruction
The first step the fetch-execute cycle carries out is fetching the instruction, which is either a program or data. The CPU fetches this from the main memory (the hard drive) and stores it in the CPU temporary memory, the immediate access store (the registers).
Once the instruction has been fetched, the CPU will need to understand the instruction to action it. This is called deconding.