Article

Fetch-Decode-Execute

Wednesday, 19 November 2025

computer-science hardware

See: Thinking Concurrently

Fetch

  • The program counter (PC) passes the address of the current instruction to the memory address register (MAR)
  • It then increments to hold the address of the next instruction to be executed
  • The address (in MAR) is then sent along the memory bus to main memory, where it waits to receive a signal from the control bus
    • To ensure that the data is stored in the correct memory address, the control unit sends a read signal along the control bus to main memory
      • The control bus sends a grant signal to permit reading the address
      • Then a read signal is sent
      • The control bus sends a grant signal to permit reading the data at the address
  • The data is then sent along the data bus to the memory data register (MDR)
    • The data will then be copied over to the current instruction register (CIR)

Decode

See: Little Man Computing, Memory Addressing

  • Control unit (CU) decodes instruction content
    • I.e., 0101 11 01
    • Opcode: 0101 = load
    • Addressing Mode: 11
    • Operand: 101 = pointer/data

Execute

May be done by either the CU or arithmetic logic unit (ALU).

  • If needed, send the operand address to the MAR, send along, … repeats data read actions
    • Data is now stored in MDR
    • MDR is now coped into accumulator

Drive-Thru Analogy

Warning

This analogy was generated by AI.

1. Fetch (Getting the Order)

  • PC (The Queue Number): Tells you which car is up next. As soon as you take car 1’s order, you increment your counter to car 2.
  • MAR (The Menu Board): This points to the location of the food item.
  • MDR (The Food Tray): The physical tray where the food sits while it is being moved from the kitchen to the customer.
  • CIR (The Cash Register): The order is locked into the system here so you can read it.

2. Decode (Understanding the Order)

  • CU (The Worker’s Brain): You look at the order screen. You split it up:
  • Opcode: The action (“Make a burger”).
  • Operand: The target (“Add extra cheese”).

3. Execute (Making the Food)

  • ALU (The Kitchen Grill): You actually cook the burger.
  • Accumulator (The Bag): Where the finished meal is stored temporarily before handing it over.