This lesson focuses on the Fetch-Decode-Execute (FDE) cycle, the process the CPU uses to retrieve, interpret, and execute instructions. This cycle forms the backbone of how a processor operates, allowing it to run programs and process data. By understanding the FDE cycle, you will gain insight into how the CPU interacts with memory and executes tasks step by step.
I can describe the three stages of the Fetch-Decode-Execute cycle.
I can explain how key registers (PC, MAR, MDR, CIR, ACC) are used in the FDE cycle.
I can understand how buses are involved in transferring data and instructions during processing.
I can analyze the role of the Control Unit in managing the cycle.
Fetch-Decode-Execute (FDE) Cycle: The process the CPU follows to retrieve, interpret, and execute instructions.
Fetch: The stage where the CPU retrieves an instruction from memory.
Decode: The stage where the CPU interprets the fetched instruction.
Execute: The stage where the CPU carries out the instruction.
Program Counter (PC): Tracks the memory address of the next instruction.
Memory Address Register (MAR): Holds the address in memory of the instruction or data being fetched.
Memory Data Register (MDR): Temporarily stores data or instructions fetched from memory.
Current Instruction Register (CIR): Holds the instruction currently being executed.
Accumulator (ACC): Stores intermediate results of calculations performed by the ALU.
Control Unit (CU): Decodes instructions and directs CPU operations.
Data Bus: Transfers data between the CPU and memory or peripherals.
Address Bus: Transfers memory addresses between the CPU and memory.
Control Bus: Sends control signals (e.g., read/write commands) to coordinate CPU actions.
Instruction Set: The collection of operations a CPU can execute.
Clock Pulse: Synchronizes the timing of all CPU operations.
The FDE cycle is divided into three stages:
Fetch:
The Program Counter (PC) holds the address of the next instruction.
The address is sent to the Memory Address Register (MAR).
The MAR retrieves the instruction from memory, which is stored temporarily in the Memory Data Register (MDR).
The instruction is then transferred to the Current Instruction Register (CIR).
Decode:
The Control Unit decodes the instruction stored in the CIR.
It determines which components (e.g., ALU, Registers) are needed for execution.
Execute:
The decoded instruction is carried out.
If it involves calculations, the ALU processes the data, with results stored in the Accumulator (ACC).
The Program Counter (PC) is updated after each cycle to point to the next instruction.
Registers such as the MAR, MDR, CIR, and ACC play specific roles in managing data and instructions during the cycle.
Buses (Address, Data, and Control) are critical for transferring memory locations, instructions, and data between the CPU and memory.
The Control Unit acts as the manager, ensuring the smooth execution of each instruction and directing data flow within the CPU.
The cycle repeats billions of times per second in modern processors, allowing for rapid multitasking and efficient processing.
Clock pulses generated by the CPU synchronize the operations of all components during the FDE cycle.
The instruction set defines the types of operations (e.g., ADD, LOAD, STORE) the CPU can perform during execution.
Advanced processors use techniques like pipelining (covered in L5) to improve the efficiency of the FDE cycle.
Each stage of the FDE cycle is essential for ensuring programs run as intended, with precise communication between memory and the CPU.
This video breaks down the Fetch-Decode-Execute cycle, explaining how instructions flow through the CPU using registers and buses, and how each stage works.
Structure: Divide your notes into four sections: Overview, Registers, Stages of the Cycle, and Buses.
Section 1: Overview
Write a short summary defining the Fetch-Decode-Execute cycle and its importance.
Section 2: Registers
Create a table listing each register (PC, MAR, MDR, CIR, ACC), its role, and an example of how it is used in the cycle.
Section 3: Stages of the Cycle
Write a detailed paragraph for each stage (Fetch, Decode, Execute), including the roles of specific components.
Section 4: Buses
Draw a labeled diagram showing the flow of data, memory addresses, and control signals through the Address, Data, and Control buses.
Details:
Include specific examples of instructions, such as how the CPU would process a "LOAD" or "ADD" command.
Use diagrams to make the flow of data and instructions clear.
What are the three stages of the Fetch-Decode-Execute cycle?
How does the Program Counter (PC) contribute to the Fetch stage?
What is the role of the Control Unit during the Decode stage?
What happens to the results of calculations in the Execute stage?
How do the Address, Data, and Control buses help the FDE cycle function?
Flowchart Creation: In pairs, create a flowchart of the Fetch-Decode-Execute cycle. Include arrows to show the roles of the PC, MAR, MDR, CIR, ACC, and buses at each stage. Annotate each step.
Register Roleplay: Act out the FDE cycle in small groups, with each student taking the role of a specific register or bus. Demonstrate how data flows through the CPU.
Instruction Analysis: Use an example instruction (e.g., LOAD 5), and work individually to describe what happens during each stage of the FDE cycle. Present your findings to the class.