Question

The control unit of a computer is designed to go through an instruction cycle that is divided into following phases.
• [(a)] Fetch the instruction from memory
• [(b)] Decode the instruction
• [(c)] Execute the instruction

• A. Only (b) and (c) are correct
• B. Only (c) is correct
• C. Only (a) is correct
• D. (a), (b) and (c) are correct

Hint:

The instruction cycle can always be remembered as: \[ \text{Fetch} \rightarrow \text{Decode} \rightarrow \text{Execute} \] Every CPU instruction must pass through these three fundamental stages.

Answer 1

The Correct Option is D

Step 1: Understanding the Fetch Phase. The first step is fetching the instruction from memory. The Program Counter (PC) stores the address of the next instruction to be executed. The Control Unit reads this instruction from memory and places it into the Instruction Register (IR). Therefore, statement (a) is correct. \[ {\text{Fetch Phase}} \]

Step 2: Understanding the Decode Phase. After the instruction is fetched, the Control Unit interprets its meaning. The opcode is analyzed so that the processor can determine what operation is required. This process is called decoding. Therefore, statement (b) is correct. \[ {\text{Decode Phase}} \]

Step 3: Understanding the Execute Phase. Once the instruction has been decoded, the processor performs the required operation. The Arithmetic Logic Unit (ALU) or other functional units carry out the task. This phase is called execution. Therefore, statement (c) is correct. \[ {\text{Execute Phase}} \]

Step 4: Determining the final answer. Since fetching, decoding, and execution are all mandatory phases of the instruction cycle, \[ (a),\ (b),\ (c) \] are all correct. Hence, the correct answer is: \[ {(D)\ \text{(a), (b) and (c) are correct}} \]