1.2 The F-D-E Cycle

Important Registers

Below are three important registers that are used in the fetch - decode - execute cycle.

  • Program Counter (PC) - A register that tracks the RAM address of the next instruction to be executed.
     

  • Memory Address Register (MAR) - A register that tracks the RAM address of data that is currently being accessed.
     

  • Current Instruction Register (CIR) - A register that stores the instruction that has been fetched from RAM, and is about to be decoded or executed.

 

Fetch - Decode - Execute Cycle

The FDE Cycle is a sequence of steps that a CPU performs, again and again, to process currently running programs. 

1. Check the Program Counter (PC) register.

This states the address of the next instruction.

2. The instruction is fetched from the address in RAM and transferred into the registers.

3. The PC increases by 1 to show where the next instruction is.

5. The instruction is executed using the relevant components, e.g. ALU for calculations.

4. The instruction is decoded by the control unit to understand what actions must be taken.

The cycle repeats.

Questo's Corner

Registers:

a. Explain the role of three different registers.

FDE Cycle: 

a. Describe, in as much detail as possible, the different steps of the fetch decode execute cycle.

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