4.09 – The CPU

Computer Science Revision

The Central Processing Unit (CPU) processes all of the data and instructions that make the computer system work.

The CPU has five main components:

• The Control Unit (CU)
  The CU coordinates all the activities taking place inside the CPU. Its main job is to manage the Fetch-Execute cycle which will be explored on the next page.
• The Arithmetic and Logic Unit (ALU)
  The ALU carries out all the calculations. This includes addition and subtraction, binary shifts, logical comparisons like greater than and less than, and boolean operations like AND, OR, NOT, and XOR.
• The Clock
  The clock is located inside the CU. Its job is to synchronise the actions of the CPU. It sends out a signal at a constant rate. The number of clock cycles (or "ticks") per second is called the clock speed.
• The Buses
  The buses are a collection of wire that are used to transmit data between the components of the CPU, and to other parts of the computer system.
• The Cache and Registers
  The registers hold a small amount of data which is needed by the CPU. They are the quickest form of memory to read from and write to. The cache is memory which is slower than the registers, but faster than RAM. It stores regularly used data so that the CPU can access it quickly the next time it's needed.

Why isn't all of a computer's data stored in cache?

Tap/click to reveal Cache is volatile, so it loses its contents when the power is turned off. Cache is also very expensive, so it would not be practical to use a lot of it.