What is an instruction? What are its components? What is the role of an instruction in a computer? Explain with the help of an example. Where does the instruction reside at the time of execution.

What is an Instruction?

An instruction is a binary-encoded command that tells a computer’s processor what operation to perform. Instructions are fundamental components of a computer program and are executed sequentially (or as per program flow) by the processor to achieve desired computations.

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Components of an Instruction

An instruction typically consists of the following components:

1. Operation Code (Opcode):

• Specifies the operation to be performed, such as addition, subtraction, data transfer, or comparison.
• Example: ADD, SUB, LOAD, STORE.

2. Operands:

• Provide the data or the location of the data required for the operation.
• Operands can be in the form of:
  • Registers: Data stored in CPU registers.
  • Memory Addresses: Data stored in RAM or cache.
  • Immediate Values: Data directly embedded within the instruction.

3. Addressing Mode (Optional):

• Specifies how to interpret the operand (e.g., direct, indirect, or indexed addressing).

Instruction Format Example:

[Opcode] [Operand1] [Operand2] [Addressing Mode]

For instance, ADD R1, R2 means add the contents of register R2 to register R1.

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Role of an Instruction in a Computer

An instruction guides the CPU on:

1. What Operation to Perform: The opcode indicates the specific computation or action to execute.
2. Where to Fetch Data: The operands specify the data or the location of data needed for execution.
3. How to Store Results: The result of the operation is often written back to a specific location in memory or a register.

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Example

Instruction: ADD R1, R2

• Operation Code (Opcode): ADD (perform addition).
• Operand 1: R1 (destination register).
• Operand 2: R2 (source register).

Execution Steps:

1. The CPU fetches the instruction from memory.
2. It decodes the instruction to understand the operation (ADD) and the operands (R1, R2).
3. The CPU retrieves the values stored in R1 and R2.
4. The CPU performs the addition and stores the result in R1.

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Where Does the Instruction Reside During Execution?

At the time of execution, instructions are stored and processed in the following locations:

1. Memory (RAM): The instruction resides in RAM when fetched for execution.
2. Instruction Register (IR): The CPU fetches the instruction from memory and stores it temporarily in the Instruction Register.
3. Program Counter (PC): The Program Counter keeps track of the memory address of the next instruction to execute.

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Conclusion

An instruction is a core element of computer operations, enabling the processor to perform specific tasks. It consists of components like opcode, operands, and addressing mode. During execution, the instruction resides in RAM and is processed through the CPU’s registers, such as the Instruction Register and Program Counter, to ensure smooth execution of tasks.