What is isr?

An ISR (Interrupt Service Routine) is a small piece of code that a computer or microcontroller runs automatically whenever an interrupt signal occurs. An interrupt is a brief request that tells the processor to pause what it’s doing and handle something important right away, and the ISR is the routine that handles that request.

Let's break it down

  • Interrupt: A signal from hardware (like a button press, timer, or sensor) or software that says “stop what you’re doing for a moment.”
  • Service: The processor “services” the interrupt by saving its current state, so it can return to it later.
  • Routine: A short function written by the programmer that tells the processor exactly what to do when the interrupt happens (e.g., read a sensor, send data, clear a flag). When the interrupt occurs, the CPU jumps to the ISR, runs the code inside, then goes back to the original task.

Why does it matter?

Interrupts let a system react instantly to real‑world events without constantly checking (polling) for them. This makes devices faster, more power‑efficient, and able to handle many tasks at once-crucial for things like keyboards, network cards, and real‑time control loops.

Where is it used?

  • Microcontrollers in toys, appliances, and IoT devices
  • Embedded systems such as automotive engine controllers, drones, and medical equipment
  • Operating system kernels for handling hardware like keyboards, mice, timers, and network packets
  • Real‑time audio/video processing where timing is critical

Good things about it

  • Low latency: Responds to events almost instantly.
  • Efficient CPU use: No need for constant polling, saving processing power.
  • Modular design: Keeps event‑handling code separate from the main program logic.
  • Scalable: Multiple interrupts can be prioritized, allowing critical tasks to pre‑empt less important ones.

Not-so-good things

  • Complexity: Writing correct ISRs requires careful handling of shared data and timing.
  • Debugging difficulty: Interrupts can happen at unpredictable moments, making bugs hard to reproduce.
  • Priority issues: Poorly designed priorities can cause important tasks to be delayed (priority inversion).
  • Limited execution time: ISRs should be short; long processing inside an ISR can block other interrupts and degrade system performance.