GCSE Computer Architecture 4 - Fetch Decode Execute Part 2

Summary

This video provides an in-depth explanation of the fetch-decode-execute cycle, a fundamental process in a CPU. It breaks down each stage, starting with the fetch phase, where the program counter directs the instruction to the memory address register. The decode stage follows, where the control unit verifies the instruction. Finally, the execute stage performs the operation, utilizing components like the arithmetic logic unit (ALU) and accumulator. The video highlights how this cycle repeats billions of times per second in a computer's operation, demonstrating how instructions are fetched, decoded, and executed in a seamless and rapid manner.

Takeaways

• 💻 The video explains the Fetch-Decode-Execute cycle in detail, describing how different parts of the CPU work together during each stage.
• 📥 In the Fetch stage, the program counter provides the memory address of the instruction, which is placed in the memory address register (MAR) and fetched to the memory data register (MDR).
• 📜 The instruction from the memory data register is copied to the current instruction register (CIR), and the program counter increments by one.
• 📝 The Decode stage involves the control unit reading the contents of the current instruction register (CIR) and verifying that the instruction is part of the CPU's instruction set.
• ⚠️ If the instruction is invalid, the program will crash. If valid, the CPU proceeds to the Execute stage.
• ⚙️ In the Execute stage, the instruction is carried out, potentially involving the arithmetic logic unit (ALU), accumulator, and fetching additional data from memory if necessary.
• ➕ An example given is the instruction 'add 5', where the ALU adds 5 to the value in the accumulator, changing its value from 0 to 5.
• 🔄 The Fetch-Decode-Execute cycle repeats billions of times per second during the operation of a computer, enabling all software and applications to function.
• 📊 Each step in the cycle involves specific registers and CPU components like the program counter, MAR, MDR, CIR, and ALU.
• 📝 The video encourages viewers to review the steps, take notes, and understand how the contents of various CPU registers change during the process.

Q & A

What is the first stage of the fetch-decode-execute cycle?

The first stage is the fetch stage, where the address of the instruction is moved from the program counter to the memory address register, and the instruction is transferred from memory to the memory data register.

What happens to the program counter during the fetch stage?

After the instruction is copied to the current instruction register, the program counter is incremented by one to point to the next instruction in the sequence.

What is the purpose of the memory address register (MAR)?

The memory address register holds the address of the memory location that contains the next instruction to be executed.

What is the role of the memory data register (MDR) in the fetch cycle?

The memory data register temporarily stores the instruction fetched from memory before it is copied into the current instruction register.

What does the control unit check during the decode stage?

During the decode stage, the control unit checks whether the instruction in the current instruction register is valid by confirming it belongs to the CPU's instruction set.

What happens if the instruction is invalid during the decode stage?

If the instruction is invalid, the program will crash, as the CPU cannot continue processing the invalid instruction.

What occurs during the execute stage of the fetch-decode-execute cycle?

In the execute stage, the instruction is carried out by the CPU, which may involve operations like using the arithmetic logic unit (ALU), fetching additional data from memory, or updating registers such as the accumulator.

How does the execute stage handle arithmetic operations?

For arithmetic operations, such as 'add 5', the arithmetic logic unit (ALU) reads the current value in the accumulator, adds the specified value, and updates the accumulator with the new result.

What is the role of the program counter when a jump instruction is executed?

If a jump instruction is executed, the program counter is updated to point to a completely different address instead of incrementing sequentially.

How often does the fetch-decode-execute cycle occur during computer operation?

The fetch-decode-execute cycle happens billions of times per second, allowing the CPU to process instructions rapidly and ensure software and apps run smoothly.