Fetch decode execute cycle

A computer is an electronic device that takes input, processes data and delivers output


FETCH stage:

  • the program counter is checked, as it holds the address of the next instruction to be executed

  • the address stored is then copied into the memory address register (MAR)

  • the address is then sent along the address bus to main memory, where it waits to receive a signal from control bus

  • the control unit sends a read signal along the control bus to main memory

  • the content stored in the memory can now be sent along the data bus to the memory data register

  • the data received by the MDR from memory now gets copied into the CIR


DECODE stage:

  • the Instruction held in the CIR is split into opcode and operand and the opcode is used to determine the type of instruction and what hardware to use to execute it.

  • the operand holds either:

— the address of the data to be used with the operation, which is then copied to the MAR

— the actual data to be operated on, which will be copied to the MDR

— the data to be operated on may be passed to the ALU/ACC


EXECUTE:

  • the appropriate instruction/opcode is carried out on the operand