26.7. Context Switching During Interrupts

When we are going to give service to the interrupt then what does it mean? We have to perform some operation. Again those operations can be treated as a collection of instructions, which is 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.

Context switching occurs during an interrupt when the processor's current task is interrupted to execute a different task – the interrupt service routine (ISR). This ISR manages the request generated by the interrupting device. Essentially, context switching involves pausing the currently running program, saving its state, and loading the state of the ISR for execution.

Imagine you're cooking dinner (your main program) when someone knocks on the door (the interrupt). You momentarily stop cooking, write down the recipe's current step (saving the context), and tend to the door. After dealing with the interruption, you return to the kitchen, pick up where you left off in the recipe (restoring context), and continue cooking. The door knocking is akin to an interrupt in a computer process.

When we are going to process the interrupt, depending on the interrupt service routine, we are going to fetch data or we are going to store it. If it is input devices, we are going to fetch it and keep it in our memory. If it is output device, then we are going to take the information from memory and put it in the output device.

When an interrupt occurs, the current state of the processor's registers, including the program counter (PC) and the program status word (PSW), must be saved. The processor stores these values in a stack so that it can later revert to its previous state. This saving of status enables the system to pause absent tasks without losing track of what it was working on.

Think of this like a bookmark in a book. If you're reading and need to take a call (interrupt), you place a bookmark on the page (saving state) to remember where you left off. After the call, you can return to the exact spot in your book, just like the processor resumes its previous task.

We are going to execute the interrupt service routine to give service for that interrupt, and after completion of that particular interrupt service routine, when data transfer is over, then we are going to restore back the processor status.

After saving the current processor states, the ISR is executed. This routine performs the necessary actions related to the interrupt, such as data retrieval or transmitting data to memory. Once the ISR has completed its operations, the processor needs to restore its previously saved state to continue the initial task.

Continuing the cooking analogy, after attending to the door (processing the interrupt), you return to the kitchen (restore previous state) and pick up your cooking where you left off. You follow the steps from your recipe, having remembered just how to get back into the flow seamlessly.

Again, we are going to bring the information from the system state to the processor; that means, we are going to restore all the registers' values, we are going to restore the program status word, and along with that, we are going to bring the program counter values also.

During this process, the previously saved values from the stack are restored back into the processor's registers. The processor's state is fully restored so that it can continue executing the original task seamlessly without any loss of information or context. This involves not just restoring the values but ensuring that the integrity of the entire process is maintained.

It's like finishing a conversation at the door and coming back to the kitchen. You recall the last ingredient you were working with (restoring registers) and check the ongoing cooking process (restoring program counter). Only then can you continue to cook as if you never left!

This is basically context switching; we are switching the context from the main program to the interrupt service routine. Then we are going to execute those contexts respectively, ensuring the logical flow of program execution.

Context switching is a vital process in modern computing that facilitates multitasking by allowing the CPU to switch between several programs with the least disruption. This capability is essential for improving system responsiveness and efficiency, especially in environments where multiple interrupts can occur.

Consider the scenario of a multitasking parent: handling homework help (main program), then switching to answer a phone call (interrupt service routine), and finally returning to help with homework. This multitasking approach reflects how contexts in computing systems enable smooth and efficient operations.

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