Explanation:
When using a subroutine call instruction in a program, it is necessary to clear the instruction register.

Subroutine Call Instruction:
A subroutine call instruction is used to transfer control from the main program to a subroutine. This allows the subroutine to be executed, and then control is returned to the main program after the subroutine finishes its execution.

Importance of Clearing the Instruction Register:
Clearing the instruction register is necessary before executing a subroutine call instruction for the following reasons:

1. Preventing Unexpected Behavior: The instruction register holds the current instruction being executed by the microprocessor. If the instruction register is not cleared before executing the subroutine call instruction, there is a possibility that the previous instruction's opcode or operand could interfere with the subroutine call instruction's opcode or operand. This can lead to unexpected behavior and incorrect execution of the subroutine.

2. Avoiding Execution of Incorrect Instructions: If the instruction register is not cleared, the microprocessor may attempt to execute the previous instruction instead of the intended subroutine call instruction. This can result in executing incorrect instructions, leading to undesired outcomes and potential errors in the program.

3. Ensuring Proper Transfer of Control: Clearing the instruction register ensures that the subroutine call instruction is properly fetched and executed by the microprocessor. This allows for the correct transfer of control from the main program to the subroutine, ensuring the subroutine is executed as intended.

4. Maintaining Program Integrity: Clearing the instruction register before executing the subroutine call instruction helps maintain the integrity of the program by preventing any interference or conflicts between instructions. It ensures that each instruction is executed in the correct sequence, without any residual effects from previous instructions.

Therefore, clearing the instruction register is necessary in a program using a subroutine call instruction to prevent unexpected behavior, avoid executing incorrect instructions, ensure proper transfer of control, and maintain program integrity.