GCSE Computer Science (CCEA)

Topic Summaries

COMPUTER SYSTEMS: Systems architecture

The fetch-execute cycle

  • The CPU accomplishes its tasks by running something called the fetch-execute cycle, sometimes also called the fetch-decode-execute cycle.
  • The faster the CPU can run this cycle, the faster the computer can run tasks. The average computer can run this cycle billions of times a second!

  • Fetch: 
    • The program counter (PC) holds the address of the next instruction and sends this to memory access register (MAR). 
    • The instruction is retrieved from main memory and placed into the memory data register (MDR), and copied to the current instruction register (CIR).
  • Decode:
    • The control unit (CU) interprets the instruction stored in the CIR.
    • The CU identifies the required operation and the necessary data, registers, or addressing mode.
  • Execute:
    • The indicated operation is carried out: arithmetic/logic operations (via the ALU), data transfer, or control flow. 
    • Results are written back to registers or memory as required.
    • The cycle then repeats for the next instruction.

Unlock The fetch-execute cycle

Sign up to unlock the rest of this topic — plus every other topic, video, flashcard set and eBook guide in your SnapRevise+ subscription.

Cancel anytime — instant access the moment you sign up.

Previous Factors affecting CPU performance Next Von Neumann architecture