FeDEx cycle

• The program counter (PC) keeps the address of the next instruction to be executed.
• The contents of the PC are copied to the memory address register, which is connected to the address bus.
• The address of the next instruction to be executed is placed on the address bus.
• The control unit instructs a memory read operation to allow the contents of the memory location to be transferred to the processor.
• The instruction that is stored at that address is transferred using the data bus from the main memory to the processor, and is saved in the memory data register.
• The contents of the memory data register are copied to the current instruction register.
• The program counter (PC) is incremented by 1.
• Decode 

  The control unit decodes the instruction that is kept in the current instruction register. This involves splitting the instruction into operand and opcode to determine what type of instruction needs to be carried out, checking if additional data are required from memory, and figuring out where these are kept in main memory. 
• Execute 

  The exact sequence of operations depends on the type of instruction that is being executed.  

  For example, for an adding two numbers together any required data are fetched from the main memory, then the calculation is executed by the Arithmetic logic unit and the result of the instruction is stored in the accumulator.  
• control unit:

  In charge of organising the sequence in which program instructions are executed, followed by decoding the instructions.