During the Instruction Cycle, which specific register holds the instruction that is currently being decoded and executed by the C…

Question

During the Instruction Cycle, which specific register holds the instruction that is currently being decoded and executed by the CPU?

1. A. Accumulator (AC)
2. B. Memory Address Register (MAR)
3. C. Program Counter (PC)
4. D. Instruction Register (IR) (Correct answer)

Correct Answer

Option D — Instruction Register (IR)

Detailed Solution & Explanation

The correct answer is Instruction Register (IR).

Key Points

• The Instruction Register holds the instruction currently being decoded and executed, after it has been fetched from memory.
• The other registers have distinct roles:
  • Program Counter (PC) holds the address of the next instruction
  • Memory Address Register (MAR) holds the address being accessed in memory
  • Accumulator (AC) holds intermediate arithmetic and logic results

Additional Information

• The instruction cycle runs Fetch → Decode → Execute: the PC supplies an address to the MAR, the instruction returns via the MBR/MDR into the IR, the control unit decodes it, and the ALU executes it.
• The Memory Buffer Register (MBR), also called the Memory Data Register, holds the data being transferred to or from memory.
• The PC increments automatically during fetch so it points at the following instruction, which is what makes sequential execution work; a jump instruction overwrites it.
• Registers are the fastest storage in the machine, sitting above cache, main memory and secondary storage in the memory hierarchy.