An interrupt is a signal that causes the CPU to stop what it's currently doing and handle a specific event. It's a crucial mechanism for efficient system operation, allowing the CPU to respond to events from hardware or software in a timely manner. Without interrupts, the CPU would have to constantly poll devices to check for activity, wasting valuable processing time.
Types of Interrupts
Interrupts can be broadly classified into two main types:
Hardware Interrupts: These are generated by external hardware devices, such as a keyboard, mouse, or disk drive, to signal that they require attention.
Software Interrupts: These are generated by software instructions, often used to request services from the operating system or to handle exceptional conditions like division by zero.
How Interrupts Work
The process of handling an interrupt involves several steps:
Interrupt Request: A hardware or software device sends an interrupt request signal to the CPU.
Interrupt Acknowledge: The CPU acknowledges the interrupt request.
Context Saving: The CPU saves the current state of the program (its context) onto the stack. This includes information like the program counter (PC), registers, and flags.
Interrupt Service Routine (ISR) Execution: The CPU jumps to a pre-defined routine called the Interrupt Service Routine (ISR) associated with the specific interrupt.
Interrupt Handling: The ISR executes to handle the event that caused the interrupt. This might involve reading data from a device, updating system state, or performing other necessary actions.
Context Restoration: After the ISR completes, the CPU restores the saved context from the stack.
Return to Previous Program: The CPU returns to the point in the interrupted program where it left off.
Interrupt Priority
When multiple interrupts occur simultaneously, the CPU needs a way to determine which interrupt to handle first. This is achieved through interrupt priority. Each interrupt is assigned a priority level, and the CPU handles the interrupt with the highest priority first. The operating system manages interrupt priorities.
Interrupt
Priority
Description
Keyboard Interrupt
Medium
Signals key presses.
Disk Drive Interrupt
Low
Signals data transfer completion.
Timer Interrupt
High
Used for timekeeping and scheduling.
Advantages of Using Interrupts
Efficiency: The CPU doesn't waste time constantly checking for events.
Responsiveness: The system can quickly respond to events from hardware and software.
Multitasking: Interrupts enable the operating system to switch between different tasks, creating the illusion of parallel execution.
Disadvantages of Using Interrupts
Context Switching Overhead: Saving and restoring the context takes time.
Priority Inversion: A low-priority interrupt can block a high-priority interrupt if they are both waiting for the same resource. (This is usually handled by the OS).
Suggested diagram:
Suggested diagram: A diagram illustrating the interrupt process, showing a hardware device sending an interrupt signal to the CPU, the CPU saving context, executing the ISR, and then returning to the interrupted program.