Interrupt Service Routine

An Interrupt Service Routine (ISR), also known as an interrupt handler, is a crucial component in computer systems that manages the execution of tasks in response to interrupts. Interrupts are signals emitted by hardware or software indicating an event that needs immediate attention. The ISR is a special type of subroutine that the Central Processing Unit (CPU) executes in response to an interrupt signal.

Mechanism of Interrupts

When an interrupt occurs, the CPU stops executing the current instructions and jumps to the ISR. The address of the ISR is typically stored in an Interrupt Vector Table (IVT). An IVT is a data structure that the processor uses to determine the correct ISR to execute for each type of interrupt. Each entry in the IVT is called an interrupt vector, which points to the starting address of the corresponding ISR.

Interrupt Vector Table

The Interrupt Vector Table (IVT) is essential in managing interrupts efficiently. It ensures that each type of interrupt is handled by the correct ISR. The table is generally located at a predetermined location in memory and contains pointers to ISRs for various interrupt types. The Interrupt Descriptor Table (IDT) is a similar concept used by the x86 architecture to implement the IVT.

Types of Interrupts

Interrupts can be broadly categorized into hardware interrupts and software interrupts:

• Hardware Interrupts: These are signals generated by hardware devices like keyboards, mice, and network cards to notify the CPU about an event. For example, pressing a key on the keyboard generates a hardware interrupt that prompts the CPU to execute the corresponding ISR.

• Software Interrupts: These are generated by software programs to signal the CPU to execute a particular task. For instance, system calls in an Operating System (OS) are often implemented using software interrupts.

ISR Execution and Latency

The time taken by the CPU to start executing the ISR after an interrupt is known as interrupt latency. Minimizing this latency is crucial for real-time systems where timely response to events is essential. Techniques such as fast interrupt requests (FIRQs) help in reducing interrupt latency by allowing the ISR to be executed directly without additional delay.

Reentrancy and Nested Interrupts

An important consideration in ISR design is reentrancy. An ISR is reentrant if it can be interrupted and safely called again ("re-entered") before its previous executions are complete. Handling nested interrupts, where an ISR itself can be interrupted by another ISR, requires careful management of the CPU state and resources.

Interaction with the Operating System

In a multitasking OS, the ISR interacts with various system components. The OS may temporarily halt a running process to handle the interrupt, and then resume the process once the ISR is complete. Context switching, managed by the OS kernel, is often involved in this process.

Applications in Energy Harvesting

Interestingly, ISRs also find applications in systems involving the thermoelectric effect and atomic batteries. Devices like thermoelectric generators rely on the direct conversion of temperature differences into electrical voltage, which can generate interrupts for power management. Atomic batteries, which use energy from radioactive decay, can similarly produce interrupts for monitoring and controlling energy output.

Related Topics

• Operating System
• Central Processing Unit
• Interrupt Vector Table
• Thermoelectric Effect
• Atomic Batteries
• Interrupt Latency
• Fast Interrupt Request
• Reentrancy (computing)

Understanding ISRs is essential for developing efficient and responsive computing systems across various applications, from everyday computing to specialized energy management systems.