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Fetch Execute Decode Cycle

The main job of the CPU is to execute programs using the fetch-decode-execute cycle (also known as the instruction cycle). This cycle begins as soon as you turn on a computer. To execute a program, the program code is copied from secondary storage into the main memory.

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What happens in the cycle?

The fetch-execute cycle (also known as fetch-decode-execute cycle) is followed by a processor to process an instruction. The cycle consists of several stages.

  • The memory address held in the program counter is copied into the MAR.
  • The address in the program counter is then incremented - increased - by one. The program counter now holds the address of the next instruction to be fetched.
  • The processor sends a signal along the address bus to the memory address held in the MAR.
  • The instruction/data held in that memory address is sent along the data bus to the MBR/MDR.
  • The instruction/data held in the MBR/MDR is copied into the CIR.
  • The instruction/data held in the CIR is decoded and then executed. Results of processing are stored in the ACC.
  • The cycle then returns to step one.


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