Polling & Interrupts
What are Polling and Interrupt Handling?
Polling and interrupt handling are two mechanisms used by a CPU to manage input and output (I/O) events efficiently.
These methods determine how a computer reacts to external devices, such as keyboards, mice, and network connections.
What is an Event?
In computing, an event refers to a signal or action that requires the CPU's attention. It is typically generated by hardware devices (like keyboards, mice, or network interfaces) or software processes to indicate that something has occurred - such as user input, data arrival, or a completed task.
Polling vs. Interrupt Handling
| Method | Description | Pros | Cons |
|---|---|---|---|
| Polling | The CPU continuously checks whether a device requires attention. | Simple to implement, predictable timing. | Wastes CPU cycles, high processing overhead. |
| Interrupt Handling | The CPU responds only when an external device signals an event. | Efficient, allows multitasking. | More complex to implement, introduces latency. |
Deep Dive: Interrupts
An interrupt is a signal sent to the CPU by a hardware or software component, requesting immediate attention. It temporarily halts the current execution so the CPU can address the event, allowing efficient and timely responses to critical tasks.
When an interrupt occurs, control is transferred to a special block of code called the Interrupt Service Routine (ISR), which is responsible for handling the specific interrupt.
Steps in an Interrupt Service Routine (ISR):
- Interrupt Received: The device or process signals the CPU.
- Current State Saved: The CPU saves the values of the Program Counter (PC) and relevant registers.
- ISR Executed: The CPU jumps to the address of the ISR and executes the interrupt-handling instructions.
- State Restored: After the ISR finishes, the CPU restores the saved state from before the interrupt.
- Resume Execution: The CPU resumes normal operation from where it was interrupted.
Deep Dive: Polling
Polling is a technique where the CPU continuously checks (or "polls") a device or memory location to see if it requires attention. This method allows the CPU to maintain control over when and how I/O operations are performed, but can lead to inefficient use of system resources.
Polling is often used in systems where:
- Events are frequent and predictable.
- Response times must be consistent and easily managed.
- Simplicity is more important than performance.
Polling Workflow:
- Check Device Status: The CPU repeatedly checks if a device is ready (e.g. has data to read).
- Wait if Not Ready: If the device is not ready, the CPU loops back and checks again.
- Process When Ready: When the device signals readiness, the CPU proceeds with data transfer or processing.
While polling is simple to implement and predictable, it can consume unnecessary CPU cycles, especially when devices are idle. This can reduce overall system efficiency-particularly in multitasking environments.
Summary Comparison
| Criteria | Polling | Interrupt Handling |
|---|---|---|
| Event Frequency | Works well for frequent events | Best for sporadic events |
| CPU Processing Overhead | High (constantly checking devices) | Low (only reacts when needed) |
| Power Consumption | Higher (CPU remains active) | Lower (CPU remains idle when not needed) |
| Event Predictability | Good for regularly occurring tasks | Better for unpredictable tasks |
| Security Concerns | Less secure; can be manipulated | Interrupt storms may cause issues |
Real-World Scenarios
| Scenario | Approach | Details |
|---|---|---|
| Keyboard & Mouse Inputs | Interrupts | Detects user actions in real time using interrupt signals. |
| Network Communication | Interrupts | Handles data packets as they arrive without constant checking. |
| Control Systems | Polling | Continuously monitors sensors like temperature or light levels. |
| Disk I/O Operations | Interrupts | CPU is notified when data is ready for processing. |
Choosing Between Polling and Interrupts
- Use Polling when events are frequent and predictable, such as in real-time monitoring systems.
- Use Interrupts when events are sporadic or require quick responses, such as user input or network activity.
Key Takeaways
- Polling and interrupt handling are CPU techniques for event detection.
- Polling is simple but inefficient; interrupts are efficient but more complex.
- Interrupts are common in inputs, networking, and time-sensitive systems.
- Choose the approach based on CPU load, power usage, and event type.