Lecture Notes: Interrupts and Interrupt Service Routine (ISR)

Overview

• Discusses the role of interrupts and the interrupt service routine (ISR) within the fetch-decode-execute cycle.
• Examines how a processor handles interrupts while executing programs.

Key Concepts

Fetch-Decode-Execute Cycle

• Normal Process: Processor fetches, decodes, and executes instructions repeatedly to run programs.
• Interrupts: Events that signal the processor to divert attention from the current task.

Interrupt

• Analogy: Similar to a student raising a hand to ask a question, indicating they need attention.
• Process: After fetch-decode-execute, the processor checks for interrupts.

Interrupt Handling

• Problem: Processor needs to stop the current task to run the interrupt's code (ISR).
• Register Content: The CPU's registers contain crucial data, including:
  • Program counter
  • Current instruction register
  • Other registers like MAR and MDR
• ISR: A set of instructions that need to be fetched, decoded, and executed.
• Saving State: Registers' contents are copied to memory (the stack) when an interrupt occurs.

Stack

• Data Structure: Used to save and retrieve the state of the processor.
• Stack Frame: Stores the registers' values for the suspended program.
• Execution: Allows resuming of the previous program post-interrupt by retrieving values from the stack.

Nested Interrupts

• Higher Priority Interrupts: Possible for an interrupt to occur during another interrupt's execution.
• Handling:
  • Suspend current ISR (e.g., ISR A)
  • Push current state to the top of the stack.
  • Execute higher priority ISR (e.g., ISR B).
  • Post ISR B, resume ISR A using stack values.

Priority of Interrupts

• Priority Levels: Interrupts have priorities; they take precedence over standard programs.
• Example Scenario: Opening Task Manager or shutting down a PC during a freeze requires interrupt.

Exam Preparation

• Understand different types of interrupts and their handling.
• Key Question: What causes an interrupt to the CPU, and how is it handled?

Conclusion

• Summarizes the function and handling of interrupts within computer systems.
• Highlights complexities and variations in PC architectures related to interrupts.

These notes capture the essential elements of the video lecture on interrupts and the interrupt service routine, providing a comprehensive study guide on the topic.