Question

Arrange the following operations of control unit of a CPU in the order of their execution: A. Decode, B. Fetch, C. Write Back, D. Execute.

• A. A, B, D, C
• B. A, D, C, B
• C. B, A, C, D
• D. B, A, D, C

Hint:

The basic CPU instruction cycle is Fetch, Decode, Execute, and Write Back.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:

• This question requires us to arrange the primary operations of a CPU's execution cycle (also known as the Instruction Cycle or Machine Cycle) in their correct chronological order.

• We need to analyze the steps involved when a processor executes a computer program instruction.

Step 2: Detailed Explanation:

• The CPU instruction cycle represents the sequence of operations that the processor performs to execute a single program instruction. It consists of the following standard sequential steps:

• 1. Fetch (B): The Control Unit retrieves the instruction's binary code from the computer's main memory (RAM) using the address stored in the Program Counter (PC). This is the first step.

• 2. Decode (A): Once retrieved, the instruction is loaded into the Instruction Register. The Control Unit interprets (decodes) this binary code to understand what operation is to be performed (addition, subtraction, jump, etc.) and identifies any required operands. This is the second step.

• 3. Execute (D): The CPU's Arithmetic Logic Unit (ALU) or other processing units perform the actual mathematical, logical, or data-movement operation specified by the decoded instruction. This is the third step.

• 4. Write Back (C): The final result generated during the execution phase is written back into a CPU register or the main memory for future use. This is the fourth step.

• Therefore, the correct execution sequence is: Fetch (B) $\rightarrow$ Decode (A) $\rightarrow$ Execute (D) $\rightarrow$ Write Back (C).

• This corresponds to the sequence: B, A, D, C.

• This matches Option (D).

Step 3: Final Answer:
The correct option is (D).