Fetch-Execute Cycle (Cambridge (CIE) A Level Computer Science) : Revision Note

F-E stages

What is the fetch-execute cycle?

• The fetch-execute cycle is the process that the CPU goes through repeatedly to process instructions

• There are 3 stages:

  • Fetching an instruction from memory - supplying the address and receiving the instruction from memory

  • Decoding the instruction - interpreting the instruction and then reading and retrieving the required data from their addresses

  • Executing the instruction - the CPU carries out the required action

How are registers used in the fetch-execute cycle?

• In the section below, registers and CPU components appear in bold and assembly language is in italics

• During the fetch-execute cycle, the following steps happen:

• Fetch

  • The PC is loaded with 0

  • The value from the PC (0) is copied to the MAR

  • The address in the MAR is sent via the address bus, and a read instruction is sent on the control bus

  • The data from that memory location is sent down the data bus to the MDR

  • The PC is incremented by 1, ready for the next instruction

• Decode

  • The data from the MDR is copied to the CIR

  • The CIR splits the instruction into opcode and operand

  • The instruction is sent to the CU, which decodes what to do next

• Execute

  • The registers used will depend on the instruction:

    • INP – value from input device is stored in the ACC

    • OUT – value currently in the ACC is sent to the output

    • LDA – loads value from RAM into the MDR, then to the ACC

    • STA – takes the value from the ACC, moves it to the MDR, then stores it in RAM (location in MAR)

    • ADD/SUB – value in RAM is loaded into the MDR, then the calculation happens in the ALU, result stored in the ACC

    • BRA/BRZ/BRP – branching instructions are handled by the ALU to check if conditions are met, and the PC is updated accordingly

Register Transfer Notation (RTN)

• Register Transfer Notation (RTN) is a way of describing how data moves between registers in a CPU during the fetch–execute cycle

• It uses symbols to represent registers, and the arrow symbol ← to show data being transferred from one place to another

• It is a shorthand way to show what's happening inside the CPU

Fetch

MAR ← PC ; Copy address from Program Counter to MAR

MDR ← RAM[MAR] ; Read instruction from memory into MDR

PC ← PC + 1 ; Increment PC for the next instruction

CIR ← MDR ; Copy instruction from MDR to CIR

Decode

• No specific register transfer, the CU decodes the instruction in the CIR

Execute examples

• LDA X (Load value from memory location X into ACC):

MAR ← operand ; Load address X into MAR

MDR ← RAM[MAR] ; Load data from RAM into MDR

ACC ← MDR ; Copy data into ACC

• STA X (Store value from ACC into memory location X):

MAR ← operand ; Load address X into MAR

MDR ← ACC ; Copy data from ACC to MDR

RAM[MAR] ← MDR ; Store MDR value into memory

• ADD X (Add value from memory location X to ACC):

MAR ← operand

MDR ← RAM[MAR]

ACC ← ACC + MDR

• BRZ X (Branch to X if ACC = 0):

If ACC = 0 then PC ← operand