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A short class with Sarah and her students. The other ways are under More.
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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.
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.
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.
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.
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.
Events that interrupt the flow of code execution.
Register marking the next instruction to execute.
Code module for handling the interrupt.
Indicates current instruction processing phase.
Storing the PC value before handling an interrupt.
When an interrupt goes BING, save your PC, it’s a smart thing!
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.
Remember 'S-L-R' for interrupts: Save state, Load ISR, Return back!
I.C.C = Instruction Cycle Code: 'I' for Interrupt, 'C' for Cycle, 'C' for Code.
Term
What is an interrupt?
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Term
What does the Program Counter do?
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Term
What does '11' mean in ICC?
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An event that temporarily halts the normal flow of execution to give attention to a different process or event.
A register that keeps track of the address of the next instruction to be executed.
A special code that handles the logic and processing when an interrupt occurs.
A code that signals the current phase of instruction processing (fetch, decode, execute, or interrupt).
A data structure that stores data in a Last In First Out (LIFO) manner, commonly used for saving the PC value.
This section explains the concept of interrupts in instruction execution and the process of managing program counters during these events.
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.
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:
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.