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Statements associated with registers of a CPU are given. Identify the false statement.

  1. The program counter holds the memory address of the instruction in execution
  2. Only opcode is transferred to the control unit
  3. An instruction in the instruction register consists of the opcode and the operand
  4. The value of the program counter is incremented by $1$ once its value has been read to the memory address register
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3 Answers

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PC holds the address of the next instruction to be fetched.

MAR holds the address of the instruction being executed.

So, A is False (Answer)


Control Unit needs only the Opcode field of the instruction. The other information (Addressing Mode, and the Operands) are required by the ALU to perform the involved computation.

So, B is True.


Instruction Register (IR) is the register that holds the instruction itself (NOT the address of it).

We say an instruction is fetched when we successfully load it in the IR. Of course, it will consist of the Opcode and the Operands.

So, C is True.


Memory Address Register (MAR) holds the address of the current instruction in execution. When the contents of PC are loaded into MAR, PC is incremented where the next instruction is.

Iff the size of an instruction is 1 word, only then PC would be incremented by 1.

Option D is True under this assumption.

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$\underline{\textbf{Answer:}\Rightarrow}\;\mathbf{a.}$

The program counter (PC) holds the address of the next instruction, not the current instruction.
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Answer: A) The PC holds the memory address of the instruction in execution.

PC actually holds the address of the next instruction.
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5 Comments

Is option a correct?

Do program counter holds the memory address of instruction in execution?

Or program counter holds the memory address of next instruction.

And moreover in option c , if we think of immediate addressing mode where operand is present in the instruction directly, then IR do contain opcode and operand both.
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I actually marked A) only but in made easy answer key it is given as C). Let the official answer key come.
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Correct answer should be option A.
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Yes..
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Option (d) should be incorrect as well.

The value of the program counter is incremented by $1$ once its value has been read to the Memory Address Register

It's incremented by $1$ word in RISC or multiple words in CISC (as instructions can span multiple words) 

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Answer:

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