Central Processing Unit (Processor)

The processor is the part of the computer system that handles the instructions used to ensure that hardware and software respond as expected. Processors can handle millions of instructions per second.

Processors must be able to:

  • fetch, decode and execute instructions from RAM
  • perform arithmetic calculations
  • perform logical operations
  • control read, write, clock, interrupt and reset lines

Arithmetic and Logic Unit (ALU)

All calculations and logical decisions are handled by the ALU. Processing real numbers would be an example of an action performed by the ALU.

  • performs all calculations
  • performs comparisons on numeric values
  • makes decisions based on logic (AND, OR, NOT)


  • temporary storage locations within the processor
  • stores addresses, data or instructions

Control Unit

The control unit is responsible for managing the clock, read, write, reset and interrupt lines.

The control unit has a clock line that sends out a signal to synchronise the fetch/execute cycle. Clock speed is measured in Hertz (Hz) and is a good indication of how quickly the processor can complete tasks. Modern processors for desktop PCs operate in GHz.

Read and Write lines are used to either read data from RAM or write data to RAM.

The reset line clears all registers while the interrupt line is used to handle interruptions to a process during execution.

RAM (Random Access Memory)

When a program is running, the data that will be processed by the processor is temporarily stored in RAM. RAM will hold data until it is needed by the processor.

When a program is closed the data held in RAM is deleted.

When the computer system shuts down, RAM is cleared completely.

There are many different locations in RAM and each location has its own unique address. Each address has a unique value that is used by the address bus during the fetch/execute cycle. The more RAM that is available, the more programs it is possible to run concurrently (at the same time) without slowing system performance. New desktop PCs will usually have between 8GiB and 16GiB of RAM.