Summary
Highlights
The video discusses the role of interrupts and the interrupt service routine within the fetch, decode, and execute cycle. Programs are executed by the processor continuously fetching, decoding, and executing instructions. However, other devices or applications may require the processor's attention, and they signal this need through an interrupt, similar to a student raising a hand in a classroom.
The fetch-decode-execute cycle is expanded to include an additional step: checking for new interrupts to be handled after execution. When an interrupt occurs, the processor must stop its current program to run an Interrupt Service Routine (ISR). This presents a problem because CPU registers hold data for the current program, including the program counter and current instruction register, which will be overwritten by the ISR.
To resolve this, when an interrupt is received, the current values in the registers are copied onto a data structure called a stack, specifically pushed onto a stack frame. This saves the program's state. The ISR then executes, and once complete, its stack frame is popped off, restoring the original program's register values, allowing it to resume execution from where it left off.
The stack also elegantly handles nested interrupts. If a higher priority interrupt occurs while one ISR is executing, the current ISR's state is pushed onto the stack. The new higher priority ISR executes, and upon completion, its frame is popped, allowing the previous ISR to resume. Once the first ISR finishes, its frame is popped, and the original program resumes. Interrupts generally have higher priority than standard programs.
Interrupt priorities are crucial. For example, if a computer freezes, pressing Ctrl+Alt+Delete generates a high-priority interrupt to open Task Manager or shut down, preventing a complete system crash. Without interrupts, the only way to recover from such a situation might be to forcefully power down the device.
The video summarizes typical interrupts to remember for exams, acknowledging that real-world interrupt systems are more complex. It concludes with a key question for viewers: "What causes an interrupt to the CPU and how is it handled?"