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Interactive Audio Lesson
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Introduction to Interrupts
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Today, we'll explore what happens when an interrupt occurs during instruction execution. Can anyone tell me what an interrupt is?
Isn't it when something interrupts the current process, like an input from a keyboard?
Exactly! Interrupts can come from hardware or I/O devices that need immediate attention. Now, what do we do when an interrupt happens during code execution?
Do we save where we were in the program?
Correct! We save the value of the Program Counter or PC to a stack, so we can return to that point later.
Saving State and Handling ISR
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Once we save the PC, we need to load the address of the ISR into the PC. What happens next?
We execute the ISR?
Yes! The ISR is responsible for handling the interrupt. After executing the instructions in the ISR, what do we do?
We pop the PC value from the stack to go back to the original program!
Exactly! And we continue from where we were interrupted. This process ensures that we can handle unexpected events without losing our place in the program.
Understanding Instruction Cycle Code
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Now, how do we keep track of which part of the instruction cycle we are in? This is where the Instruction Cycle Code or ICC comes in. Can anyone recall the states defined by the ICC?
It's 00 for fetch, 01 for decode, 10 for execute, and 11 for interrupt!
Spot on! The ICC helps us understand our current position in the execution cycle. If an interrupt occurs, we switch to state 11. Once the interrupt is serviced, we can return to fetching the next instruction.
So, it’s like a way of keeping track of everything?
Exactly! It provides clarity on which operation is currently taking place and helps manage the execution flow.
Practical Example of Interrupt Handling
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Let’s look at a practical example. Imagine we have an instruction that adds two numbers stored in memory. If an interrupt happens after executing the first instruction, what will we do?
We save the value of PC right after that instruction.
Right! So if the PC was pointing to the next instruction after the addition, where do we store that value?
We push it onto the stack!
Exactly! After executing the ISR, we pop the PC off the stack and continue with the addition process. This allows us to maintain smooth execution.
Flow of Code Execution
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Let’s summarize our journey today. What happens during a typical code execution flow where an interrupt occurs?
First, we execute the instruction, then save the PC if there's an interrupt.
Correct! We then load the ISR into the PC and handle the interrupt. Once done, we restore the PC and continue where we left off. Why is saving the PC important?
It helps us to return to our previous instruction without losing track!
Perfect! Understanding this flow helps us appreciate how computers manage multiple tasks and interruptions efficiently.
Introduction & Overview
Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.
Quick Overview
Standard
The section covers how interrupts occur during code execution, how they affect the program counter, and the mechanism through which instruction service routines (ISR) are handled. It discusses saving the state of the program before servicing an interrupt and resuming execution after the ISR is completed.
Detailed
Step-by-Step Code Execution
The section delves deep into the interrupt handling process that occurs during instruction execution in a computer program. An interrupt occurs when the normal flow of code execution is interrupted by an external event, such as an I/O operation. The key steps involved in handling interrupts are as follows:
- Interrupt Occurrence: During the execution of instructions (e.g., instruction 10), if an interrupt is triggered, the current state of the program must be preserved.
- State Preservation: The value of the Program Counter (PC), which indicates the next instruction to execute, is saved onto a stack. This is crucial because it allows the system to resume execution from the point of interruption once the interrupt has been serviced.
- Going to ISR: The address of the Interrupt Service Routine (ISR) is loaded into the PC. The ISR is a separate block of code designed to address the specific interrupt, allowing the computer to handle events.
- Execution of ISR: The system executes the instructions within the ISR.
- Restoration of State: Once the ISR is completed, the previously saved PC value is popped from the stack, allowing the program to resume execution exactly where it left off.
Additionally, this section introduces the Instruction Cycle Code (ICC) which indicates the current state of the instruction cycle: fetching (00), decoding (01), executing (10), and interrupt service (11). Understanding these processes is crucial for comprehending how programs effectively manage unexpected events during execution.
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Audio Book
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Understanding Interrupts
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Now, we are coming to the indirect phase of instruction execution that is an interrupt. An interrupt is basically when the normal flow of code is going on, then some hardware or some IO devices interrupt, which has to be serviced in an urgent manner.
Detailed Explanation
An interrupt occurs when the current instruction execution is disrupted by a request from hardware or IO devices, indicating that it needs immediate attention. This interrupts the normal sequence of operations within the CPU, prompting it to address these requests.
Examples & Analogies
Think of it like a teacher in a classroom. If a student raises a hand urgently to ask a question, the teacher must pause the lesson to address that student's need, even though they were in the middle of explaining a concept.
Saving Program Counter and Registers
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After executing an instruction, the CPU checks whether an interrupt has occurred. If an interrupt is detected, the current value of the Program Counter (PC) is saved onto a stack. This action is necessary because the current position in the code (indicated by the PC) needs to be remembered for later.
Detailed Explanation
Once an interrupt is detected, the CPU saves the value of the Program Counter (PC) onto a stack, which is a temporary storage area. The PC represents the next instruction that should be executed. Saving this value ensures that the CPU can return to this point after serving the interrupt.
Examples & Analogies
Imagine you are reading a book, and someone calls you away for an important matter. You would likely place a bookmark at your current page to easily return to where you left off once you're done. Similarly, the CPU uses a stack to 'bookmark' its current position.
Transition to Interrupt Service Routine (ISR)
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After saving the necessary values, the address of the Interrupt Service Routine (ISR) is loaded into the Program Counter. This causes the CPU to 'jump' to the ISR code, which will handle the interrupt.
Detailed Explanation
The PC is updated to point to the Interrupt Service Routine (ISR), which is a special set of instructions designed to deal with the specific interrupt. The execution flow of the program is temporarily diverted to execute this routine until the interrupt is serviced.
Examples & Analogies
Continuing with the classroom analogy, after addressing the student's urgent question, the teacher may go to a different part of the lesson to help resolve the issue effectively before returning to the original topic they were teaching.
Returning to Normal Execution
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Once the ISR is complete, the CPU retrieves the saved Program Counter value from the stack. This action allows the CPU to resume execution from where it left off before the interrupt occurred.
Detailed Explanation
After the ISR finishes its operations, the CPU 'pops' the stored value of the PC from the stack, which tells it where to continue execution in the original program. This mechanism ensures a smooth transition back to the previous task.
Examples & Analogies
Referring back to our reading example, after handling the interruption by putting the book down to address a friend's call, you can easily pick up your bookmark and continue reading where you left off.
Instruction Cycle Code (ICC)
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There is a special code called Instruction Cycle Code (ICC) used to determine which phase of instruction execution is currently running: fetch, decode, execute, or interrupt.
Detailed Explanation
The Instruction Cycle Code helps track the execution phase. For instance, 00 represents fetch, 01 indicates decode, 10 indicates execute, and 11 means interrupt phase. This allows the CPU to manage and switch between instruction phases effectively.
Examples & Analogies
Think of the ICC as the agenda of a meeting. It tells participants whether the meeting is currently in the presentation phase, discussion phase, or Q&A phase, helping to keep the meeting organized and on track.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Interrupts: Events that interrupt the flow of code execution.
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Program Counter (PC): Register marking the next instruction to execute.
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Interrupt Service Routine (ISR): Code module for handling the interrupt.
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Instruction Cycle Code (ICC): Indicates current instruction processing phase.
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State Saving: Storing the PC value before handling an interrupt.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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When a keyboard is pressed, the CPU interrupts the current task to process the key input immediately.
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If code execution is paused to handle a printer request, the PC saves its current state to continue later.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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When an interrupt goes BING, save your PC, it’s a smart thing!
📖 Fascinating Stories
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Imagine a chef in a kitchen who is preparing a meal, but the doorbell rings. The chef saves the recipe (PC), attends to the door, and then comes back to finish cooking.
🧠 Other Memory Gems
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Remember 'S-L-R' for interrupts: Save state, Load ISR, Return back!
🎯 Super Acronyms
I.C.C = Instruction Cycle Code
- 'I' for Interrupt
- 'C' for Cycle
- 'C' for Code.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: Interrupt
Definition:
An event that temporarily halts the normal flow of execution to give attention to a different process or event.
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Term: Program Counter (PC)
Definition:
A register that keeps track of the address of the next instruction to be executed.
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Term: Interrupt Service Routine (ISR)
Definition:
A special code that handles the logic and processing when an interrupt occurs.
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Term: Instruction Cycle Code (ICC)
Definition:
A code that signals the current phase of instruction processing (fetch, decode, execute, or interrupt).
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Term: Stack
Definition:
A data structure that stores data in a Last In First Out (LIFO) manner, commonly used for saving the PC value.