JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II COMPUTER ORGANIZATION Previous Question Paper (August/September - 2022 - R16)

An instruction cycle is the fundamental process by which a computer retrieves a program instruction from its memory, determines what actions the instruction requires, and carries out those actions. The cycle is composed of several distinct phases:

1. Fetch Phase: The CPU fetches the instruction from the memory address stored in the Program Counter (PC). The instruction is loaded into the Instruction Register (IR), and the PC is incremented to point to the next instruction.
2. Decode Phase: The control unit interprets the opcode in the IR to determine the operation to be performed and identifies the necessary operands.
3. Execute Phase: The CPU performs the actual operation, such as arithmetic, logical, or data transfer tasks, using the ALU or internal registers.
4. Store/Write-back Phase: The result of the execution is written back to memory or a register if required.

These steps repeat continuously until the program terminates or the system is powered off.

An interrupt is a signal sent to the processor by hardware or software indicating an event that needs immediate attention. When an interrupt occurs, the CPU suspends its current execution flow to handle the request, ensuring efficient system management.

The process of handling interrupts involves the following steps:

1. Detection: The CPU checks for interrupt signals at the end of every instruction cycle.
2. Context Saving: The CPU saves the current state of the program, including the Program Counter (PC) and register values, onto the stack.
3. Branching: The CPU jumps to the address of the Interrupt Service Routine (ISR), which is a specific program designed to handle that particular interrupt.
4. Execution: The ISR performs the necessary tasks, such as I/O data transfer or error recovery.
5. Restoration: After the ISR completes, the CPU restores the saved state from the stack and resumes the original program from the point of interruption.

Interrupts are classified into Hardware interrupts (triggered by external devices like keyboards or timers) and Software interrupts (triggered by specific instructions or system calls). They are essential for multitasking and handling asynchronous events in a computer system.