1.2: The FDE Cycle

Exam Board:

Eduqas / WJEC

Specification:

2016 + 

The Fetch - Decode - Execute (FDE) cycle is performed by the CPU millions of times every second.

This cycle is how the CPU processes data and instructions for each program or service that requires its attention.

Important Registers

A register is a small storage space for temporary data in the CPU.

 

Each register has a specific role. There are three essential registers used in the FDE cycle:

Program Counter (PC) 

 

A register that tracks the RAM address of the next instruction to be fetched.

Memory Address Register (MAR) 

 

A register that tracks the RAM address of the instruction that is to be fetched.

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 essential idea of the FDE cycle is that instructions are fetched from RAM, to be decoded (understood) and executed (processed) by the CPU.

1.

The Program Counter (PC) register displays the address in RAM of the next instruction to be processed.

 

This value is copied into the Memory Address Register (MAR).

0054

2.

The PC register is increased by 1.

This prepares the CPU for the next instruction to be fetched.

0055

3.

The CPU checks the address in RAM which matches the address held in the MAR.

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4.

The instruction in RAM is transferred to the Current Instruction Register (CIR).

CIR

5.

The instruction in the CIR is decoded (understood) and executed (processed) by the most appropriate component - such as the ALU for a calculation.

6.

The cycle repeats by returning to the first step and checking the program counter for the address of the next instruction

Questo's Questions

1.2 - The Fetch - Decode - Execute (FDE) Cycle:

1. What is the purpose of the registers? [2]

2. Describe the purpose of each register:

  • a. The Program Counter (PC) [1]

  • b. The Memory Address Register (MAR) [1]

  • c. The Current Instruction Register (CIR) [1]

3. Draw a diagram of the six steps of the Fetch - Decode - Execute cycle. [6]

These are not the only registers but the main three used as part of the FDE cycle.

 

Other registers exist, such as the accumulator, which stores the result of calculations made by the ALU.

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