System Interrupts

Overview

Interrupts are signals sent to the CPU that temporarily halt the current process to give priority to an urgent task or event. Interrupts play a crucial role in managing how a computer system responds to events, ensuring efficient multitasking and timely responses. When an interrupt occurs, the CPU pauses its current work to execute an Interrupt Service Routine (ISR), which handles the specific task of the interrupt. Understanding interrupts, their purpose, and their relationship with the fetch-decode-execute cycle is essential to see how a computer system manages multiple tasks seamlessly.

Purpose of Interrupts

• Definition: An interrupt is a signal that informs the CPU of an event needing immediate attention, causing the CPU to temporarily pause its current execution to address the new task.
• Purpose: Interrupts allow a computer to respond to important events, like user input or hardware issues, without waiting for the current process to finish. This is essential for multitasking and ensures that time-sensitive tasks are handled promptly.
• Types of Interrupts:
  • Hardware Interrupts: Triggered by external devices (e.g., keyboard input, mouse clicks, or printer completion).
  • Software Interrupts: Triggered by software conditions (e.g., errors in program execution).
  • Timer Interrupts: Generated by the system timer to control process scheduling and time-sharing between tasks.

Role of Interrupts in the CPU

• When an interrupt occurs, the CPU:
  1. Stops the Current Process: The CPU saves the state of the current process, including the program counter (which keeps track of where the CPU is in its current task).
  2. Identifies the Source of the Interrupt: The CPU uses an interrupt vector table to identify the ISR associated with the specific interrupt.
  3. Transfers Control to the ISR: The CPU begins executing the ISR, which is a specific routine designed to address the interrupt.
• Once the ISR completes, the CPU resumes the original process from where it left off, restoring the saved state and program counter.

Interrupt Service Routine (ISR)

• Definition: An ISR is a specialised set of instructions that the CPU executes in response to an interrupt. Each type of interrupt has its own ISR, designed to handle that specific event.
• Function:
  • The ISR's job is to quickly resolve the interrupting event. For example, if a key is pressed, the ISR for the keyboard interrupt will read and store the key input.
• Process:
  • The CPU switches to the ISR as soon as an interrupt is detected.
  • After executing the ISR, the CPU returns to the previously paused task.

Role of Interrupts in the Fetch-Decode-Execute Cycle

• Fetch-Decode-Execute Cycle: This is the fundamental cycle the CPU uses to process instructions.
  • Fetch: The CPU retrieves an instruction from memory.
  • Decode: The CPU decodes the instruction to understand what needs to be done.
  • Execute: The CPU performs the required operation.
• Interrupt Handling in the Cycle:
  • At the end of each fetch-decode-execute cycle, the CPU checks for any pending interrupts.
  • If an interrupt is detected, the CPU:
  • Pauses the cycle to process the interrupt.
  • Saves the current state to allow resuming later.
  • Executes the ISR, addressing the interrupt.
  • Once the ISR completes, the CPU resumes the fetch-decode-execute cycle from where it left off.

Why an Interrupt Might Be Generated

• User Actions: When a user presses a key or clicks a mouse, the hardware sends an interrupt to inform the CPU of the action.
• Hardware Signals: Devices like printers or storage drives send interrupts to indicate task completion or errors (e.g., a paper jam in a printer).
• Errors: When a program encounters an error, a software interrupt may be triggered for error handling.
• Timing Events: The system timer generates periodic interrupts to allow the OS to manage task scheduling, ensuring fair CPU time distribution across processes.

Examples

1. Keyboard Interrupt:

• When a user presses a key, a hardware interrupt is generated.
• The CPU temporarily stops its current process to execute the ISR for keyboard input, which reads and stores the key press.

2. Timer Interrupt:

• The system timer sends an interrupt at regular intervals.
• This allows the OS to perform tasks like switching between running processes, which is essential for multitasking.
• The ISR for the timer interrupt ensures that each process gets adequate CPU time.

3. Printer Interrupt:

• When a document is sent to print, a printer interrupt may signal the CPU when the printer is ready to receive data or when printing is complete.
• The ISR handles this communication to manage the printing job.

Note Summary