22.4.1. Interrupts and Their Management

An interrupt is there. So, basically it suspends the normal code flow and immediately it has to service the interrupt for example, the code is executing I am moving the mouse. So, the mouse has been changed.

An interrupt is a signal that temporarily stops the CPU from executing its current instructions to allow for the handling of an event. This can happen, for example, when the user interacts with input devices like a mouse. The CPU must pause its current task to manage the input from the mouse, ensuring that the user interface remains responsive.

Think of an interrupt like a teacher in a classroom who must pause the lesson to address a student's raised hand. The lesson is important, but the teacher needs to ensure that students feel their concerns are acknowledged and addressed.

After executing the current instruction, the system checks whether there is an interrupt, for example, the mouse has been moved.

Once an interrupt occurs, the CPU must handle it through an Interrupt Service Routine (ISR). This is a dedicated function that executes specific tasks to manage the interrupt. For instance, if the mouse is moved while another program is running, the ISR allows the system to update the display according to the mouse movement.

Imagine a customer service hotline. When you call in with an issue (the interrupt), a representative (ISR) quickly addresses your problem while other general calls may still be on hold. Just like the representative handles your specific request, the ISR handles specific tasks required by the interrupt.

Before servicing the interrupt, you have to store the present state of the code like the program counter, the status register, etc., which is in a stack.

To ensure that the CPU can return to the interrupted task, it must save its current state. This is done by storing the values of registers and the program counter in a stack. This allows the CPU to remember where it left off when it gets back to the normal processing after handling the interrupt.

This is similar to bookmarking a page in a book. When you need to pause reading (handle an interrupt), you mark your place (save state) so you can return to it later and continue exactly where you left off.

After the ISR has serviced the interrupt, you have to recall the previous status by popping the stored values from the stack.

Once the interrupt has been handled, the CPU retrieves the saved state from the stack. It restores the values to the registers and continues executing from where it paused. This efficient process ensures seamless transitions between handling interrupts and normal tasks.

You can think of this as resuming your activity after a distraction. If you were writing an essay but got interrupted by a phone call, once the call is over, you would look back at where you left off (reading your notes, or even the last sentence you wrote) and continue from there.

After every instruction is executed, you check whether an interrupt has come. If an interrupt has come, save all the stuff like the program counter, instruction register values...

This chunk highlights the regularity of interrupt checks after each instruction completion. It emphasizes that the CPU is continuously monitoring for interrupts to handle tasks in real-time, maintaining system responsiveness and efficiency.

Imagine a cashier at a grocery store. After scanning each item, they glance at the line to see if there are any questions or if assistance is needed (checking for interrupts). If someone raises a hand for help, they pause scanning, assist the customer, and then return to scanning.

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