Interrupts

 

Introduction to Interrupts

An interrupt is a signal that temporarily stops the execution of the current program, allowing the processor to handle a more urgent task. Once the interrupt is serviced, the processor resumes its previous task. Interrupts enable efficient multitasking and real-time response to critical events in a computer system.

Key Characteristics of Interrupts

• Asynchronous or Synchronous: Hardware interrupts are asynchronous (can occur at any time), while software interrupts are synchronous (occur at specific instruction execution points).

• Prioritized Execution: Higher-priority interrupts are handled before lower-priority ones.

• Context Switching: The processor saves its current execution state before handling the interrupt.

• Real-Time Response: Crucial for embedded systems, operating systems, and real-time applications.

Types of Interrupts

Interrupts are broadly classified into hardware and software interrupts.

1. Hardware Interrupts

Hardware devices generate interrupts to request the processor’s attention. These can be further classified as:

(a) Maskable Interrupts

• Can be enabled or disabled by the processor using an interrupt mask.

• Example: Keyboard input, mouse movement, and printer signals.

(b) Non-Maskable Interrupts (NMI)

• Cannot be ignored or disabled.

• Used for critical events like power failure alerts or memory corruption.

• Example: The processor receiving a signal from the power supply indicating imminent shutdown.

(c) External Interrupts

• Generated by peripheral devices such as timers, I/O devices, and network adapters.

• Example: A network interface card (NIC) generating an interrupt when a new data packet arrives.

2. Software Interrupts

Software interrupts are generated by programs or the operating system to request services from the CPU. These include:

(a) System Calls

• Also called "supervisor calls" or "traps."

• Used by programs to request OS functions such as file operations, memory allocation, or process scheduling.

• Example: int 0x80 in Linux for system calls.

(b) Exception Handling

• Occurs when a program encounters an error during execution.

• The processor invokes an interrupt to handle the error.

• Example: Division by zero, accessing restricted memory.

(c) Breakpoints & Debugging Interrupts

• Used in debugging to halt program execution at a specified instruction.

• Example: A debugger inserting breakpoints to analyze program flow.

Interrupt Handling Process

When an interrupt occurs, the system follows a structured process to handle it efficiently.

Steps in Interrupt Handling

1. Interrupt Request (IRQ) Generation – The device or software signals the processor.

2. Interrupt Acknowledgment – The CPU recognizes the interrupt and pauses the current execution.

3. Saving Context – The processor saves the state (registers, program counter, etc.).

4. Identifying the Interrupt Source – The processor checks which device or process triggered the interrupt.

5. Executing the Interrupt Service Routine (ISR) – The ISR performs the necessary operation to handle the interrupt.

6. Restoring Context – The CPU restores the saved state.

7. Resuming Execution – The processor continues executing the previous program.

Interrupt Priorities

When multiple interrupts occur simultaneously, the processor must decide which one to handle first. This is managed through interrupt prioritization mechanisms.

Methods of Interrupt Prioritization

1. Fixed Priority Scheme
   
   

• Each interrupt source is assigned a fixed priority.

• Example: Power failure (highest), disk failure (high), keyboard input (low).

2. Dynamic Priority Scheme
   
   

• The priority of interrupts changes based on system conditions.

• Example: A real-time operating system (RTOS) adjusting priority dynamically.

3. Daisy Chaining Method
   
   

• Devices are connected in a series.

• The highest-priority device gets serviced first.

• Example: Peripheral devices like printers and hard drives using chained priority.

4. Polling Method
   
   

• The CPU checks each interrupt source sequentially.

• Used in simpler systems with fewer devices.

• Example: Microcontrollers checking I/O devices.

Interrupt Service Routine (ISR)

An Interrupt Service Routine (ISR) is a special function executed when an interrupt occurs. It contains the code needed to handle the interrupt.

Key Features of ISRs

• Short and Efficient: ISRs must be quick to avoid long system delays.

• No Return Values: ISRs typically do not return values since they handle specific hardware/software requests.

• Minimal Resource Use: Should avoid complex operations like memory allocation.

Nested Interrupts

• Occur when an interrupt is interrupted by a higher-priority interrupt.

• The processor must handle multiple interrupts in the correct order.

• Example: A disk I/O interrupt being preempted by a power failure interrupt.

Interrupt Vector Table (IVT)

• A table that stores memory addresses of ISRs.

• When an interrupt occurs, the CPU looks up the IVT to find the ISR address.

• Example: Intel x86 systems have a 256-entry IVT.

Interrupt-Driven vs. Polling Systems

Feature	Interrupt-Driven	Polling
CPU Utilization	Efficient, CPU executes other tasks while waiting	Inefficient, CPU continuously checks for events
Response Time	Fast, immediate response to events	Slow, depends on polling frequency
Complexity	Requires ISR and interrupt controller	Simple, no need for complex ISR
Power Consumption	Low (processor sleeps when idle)	High (CPU remains active even when no event occurs)
Example	Modern OS, real-time systems	Simple microcontrollers

Applications of Interrupts

1. Real-Time Systems: Used in embedded systems, medical devices, and industrial automation.

2. Operating Systems: Manages tasks like process scheduling, I/O handling, and memory management.

3. Networking: Network adapters generate interrupts to signal packet arrival.

4. Multitasking: Enables concurrent execution of multiple processes.

5. Embedded Systems: Used in microcontrollers for sensor data acquisition.