26.8.2. Handling Interrupts During Execution

Now when we are going to give service to the interrupt then what does it means? We have to perform some operation. Again those operation can be treated as a collection of instructions which is a basically nothing but again another separate computer program. So, we are going to execute that program and that program is basically known as your interrupt service routine. For different devices, we are having different interrupt service routine.

When an interrupt occurs, the processor stops its current tasks to handle the request. This involves triggering a specialized program called the Interrupt Service Routine (ISR). Each device may have its own ISR which tells the processor how to respond to the interrupt. Essentially, it's like pausing a game to pay attention to an urgent notification — once you handle the notification, you can go back to your game.

Imagine you're studying at home, but your phone buzzes with a text message from a friend. You pause your studying (current tasks) to read the message (interrupt service routine). Once you reply, you return to your studies — just like a processor returning to its previous work after handling the interrupt.

So, that’s why we are going to have run the interrupt service routine. Now I think you can correlate this situation with a function call. I think we have discussed that issues when we are going to discuss about the instruction set design and how we are going to implement it. So, when we are going to execute a sub function or a sub routine then what will happen, we temporarily suspend the execution of the main program.

To ensure that the main program can resume where it left off, the processor must save its context, which includes data about the current instruction, the program counter, and other registers. This is akin to noting down your page number in a book before taking a phone call. Once the call is over, you can pick up exactly where you left off.

Think about how you write something down in a notebook so you don't forget it. When you get an important phone call, you might write down what you were doing before the call, so when the call ends, you can easily go back to your notes and continue. Similarly, the processor notes its state so it can resume later.

So, once the transfer is over then what will happen, again we will come back to the main program, a program from where we have given the interrupt or given the service to the interrupted devices. So, again we have to restore the information; that means, again we are going to bring the information from system state to the processor.

After an ISR has completed its tasks, the processor retrieves the saved context to return to the previous state of execution. It ensures all previous parameters, including the program counter, registers, and status flags, are restored correctly so that the program resumes seamlessly. This process is parallel to checking your notes and getting back to where you left off in your book.

Once you finish your phone call, you look back at your notes to see where you were before the interruption. You can quickly jump back into your studying, just like the processor can quickly resume its operation after the interrupt handling is complete.

So, this is basically a context switching. We are switching the context from main program to the interrupt service routine. We are saving the context of the main program or the currently executing program in the system stack, then we are going to load the program counter with the starting address of the interrupt service routine and we are going to give the service to the interrupt.

The Program Status Word (PSW) is a critical component in managing the processor's status during context switching. It contains crucial information like flag bits, which indicate the outcome of operations. When servicing an interrupt, it's essential to save and restore this information to ensure that computations continue accurately post-interrupt.

Imagine that you're a chef who has to pause cooking to answer the door. You jot down your current recipe steps (the PSW) before stepping away. After you finish greeting your guest, you quickly refer to your notes to continue cooking without forgetting where you left off, ensuring the dish turns out perfectly.

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