An interrupt service routine (ISR) is a piece of code that is used when an event interrupts the normal execution flow of a microcontroller. An ISR handles hardware-generated interrupts. It examines any interrupt and decides how to proceed before executing the appropriate action and providing a logical interrupt value back to the hardware. If no further processing is needed, it will return back to the kernel with an appropriate response, successfully ending its role in handling that interruption. ISRs play a vital role in making sure microcontrollers run efficiently and smoothly regardless of what events might occur.  A basic example of this is the event-handling routine for when keys on a keyboard are pressed or released. When a key is pressed down, the ISR will recognize the input and send the relevant data to the active program. For instance, if you press and hold down the right arrow key while editing a text file, the ISR will tell the CPU that this key is depressed.   The CPU then instructs the word processor or text editor to move the cursor right until you let go of it again, whereupon an "end" signal is sent through to terminate that operation and inform any active programs that they may continue as before. Such real-time communication between input devices and programs helps user interactivity remain fluid and immediate–an important feature in today's computing world.