1.1 - Architecture of the CPU

• Fetch

  The instruction is fetched from main memory and stored in the processor (the CPU). The control unit can then access the instruction so that it can be decoded and executed.
• Decode

  The instruction needs to be decoded before it can be run. This is the process the control unit uses to work out what signals to issue to the other components for the instruction to be executed.
• Execute

  The control unit will send signals to the relevant components so that the instruction is carried out.
• The arithmetic and logic unit (ALU) is responsible for performing arithmetic calculations and logical operations that include:

  • Addition, subtraction, multiplication, division
  • Logical bitwise operations, such as AND, OR, NOT, and XOR
  • Comparisons between values, such as greater than, less than, equal to
  • Shifting binary patterns to the left or right
• The control unit organises the sequence of micro-operations that need to be performed in order to carry out an instruction, including managing the other components in the processor. The control unit is responsible for the following tasks:

  • Decodes every instruction to determine what needs to be done
  • Sends and receives control signals to and from other components
  • Checks that signals have been delivered successfully
  • Makes sure that data goes to the correct place at the correct time
  • Ensures the execution of instructions in the correct sequence