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The central processing unit (CPU) consists of six main components:

• control unit (CU)
• arithmetic logic unit (ALU)
• registers
• cache
• buses
• clock

All the components work together to allow processing and system control.

Control unit (CU)

The CU provides several functions:

• it fetches, decodes and executesinstructions
• it issues control signals that control hardware components within the CPU
• it transfers data and instructions around the system

Arithmetic logic unit (ALU)

The ALU has two main functions:

• it performs arithmetic and logical operations (decisions).
• it acts as a gateway between primary storage and secondary storage - data transferred between them passes through the ALU.

Registers

Registers are small amounts of high-speed memory contained within the CPU. They are used by the processor to store small amounts of data that are needed during processing, such as:

• the address of the next instruction to be executed
• the current instruction being decoded
• the results of calculations

Different processors have different numbers of registers for different purposes. Most have some, or all, of the following:

• program counter (PC)
• memory address register (MAR)
• memory data register (MDR)
• current instruction register (CIR)
• accumulator (ACC)

Cache

Cache is a small amount of high-speed random access memory (RAM) built directly within the processor. It is used to temporarily hold data and instructions that the processor is likely to reuse. This allows for faster processing, as the processor does not have to wait for the data and instructions to be fetched from the RAM.

Clock

The CPU contains a clock which, along with the CU, is used to coordinate all of the computer's components. The clock sends out a regular electrical pulse which synchronises (keeps in time) all the components.

The frequency of the pulses is known as clock speed. Clock speed is measured in hertz (Hz). The greater the speed, the more instructions can be performed in any given moment of time.

In the 1980s, processors commonly ran at a rate of between 3 megahertz (MHz) and 5 MHz, which is 3 million to 5 million pulses or cycles per second. Today, processors commonly run at a rate of between 3 gigahertz (GHz) and 5 GHz, which is 3 billion to 5 billion pulses or cycles per second.

Buses

A bus is a high-speed internal connection. Buses are used to send control signals and data between the processor and other components.

Three types of bus are used.

• Address bus - carries memory addresses from the processor to other components such as primary storage and input/output devices. The address bus is unidirectional.
• Data bus - carries the data between the processor and other components. The data bus is bidirectional.
• Control bus - carries control signals from the processor to other components. The control bus also carries the clock's pulses. The control bus is unidirectional.