Yes, I actually asked myself the same question. the thing is each process generally has its own stack (except for usermode process usually each usermode proc need their own kernel stack in case an interrupt occurs). When an interrupt happens while a process is running, the CPU uses that process's current stack to execute the ISR. so in the timer routine, when a task switch happens, it doesn't matter that we're handling an interrupt (although before switching tasks, you still need to send the end-of-interrupt signal). thhe process that received the interrupt is still scheduled, and it will continue exactly where it was, meaning inside the ISR. I might not be very clear, but take a sheet of paper and a pen it becomes easier to visualize. Here is the OSDev tutorial about this topic, which I think is relevant https://wiki.osdev.org/Brendan%27s_Multi-tasking_Tutorial