Objectives of the Unit

Today, we'll start by discussing interrupts in the context of instruction execution. Can anyone tell me what happens when an interrupt occurs?

I think it means that the CPU has to pause what it's doing to handle a specific request from hardware or I/O devices.

Exactly right! When a hardware device needs urgent attention, the current instruction sequence is interrupted. This allows the system to handle the interrupt effectively. Can anyone explain what we need to do with the Program Counter during an interrupt?

The value of the Program Counter needs to be saved so that we can return to it after handling the interrupt.

That's correct! We save the PC in a stack to remember where to continue from after the interrupt is addressed.

Is the interrupt service routine where we actually process the interrupt?

Yes, that's right! After saving the PC, we jump to the Interrupt Service Routine, or ISR, where we run the necessary code to handle the interrupt. Let's summarize: interrupts pause code execution and require saving the PC value so we can return to it later.

Next, we will discuss the instruction cycle. Who can tell me the stages of the instruction cycle?

I think there are fetching, decoding, and executing phases.

Correct! The instruction cycle consists of three main phases: fetch, decode, and execute. Let's break this down further. What happens during the fetch phase?

The instruction is retrieved from memory and loaded into the instruction register.

Precisely! During the fetch phase, the instruction is fetched based on the address stored in the Program Counter. Once fetched, we move to the decode phase. What happens here?

In the decode phase, the CPU interprets the instruction and prepares to execute it.

Exactly. After decoding, we advance to the execute phase. What do we do here?

The CPU performs the actions required by the instruction, like performing calculations or accessing memory.

Spot on! At the end of the execute phase, we then check for any interrupts before we move on to fetch the next instruction, ensuring a smooth operation. In summary, the instruction cycle is crucial in executing instructions effectively.

Now let's dive deeper into how interrupts are handled. Who remembers what we do with the Program Status Word (PSW) during an interrupt?

Do we also save that before jumping to the interrupt service routine?

Correct! Along with the PC value, we also save the PSW to preserve the state of the processor. Can anyone describe the significance of the instruction cycle code (ICC)?

The ICC indicates which phase the instruction is currently in, like fetching or executing.

Exactly! The ICC helps the CPU keep track of its current activity, especially critical during interruptions. So, let's summarize: We save both the PC and PSW before addressing interrupts and use the ICC for phase tracking.

To solidify our understanding, let’s consider a practical example. If we had a user program running and an interrupt occurred, what would happen?

First, the CPU would check if the current instruction is complete before handling the interrupt.

Correct! The CPU only checks for interrupts after completing an instruction to avoid complications. Can anyone explain how the flow proceeds from there?

If there is no interrupt, the process continues as usual. But if there is an interrupt, we save PC and jump to the ISR.

Exactly! After servicing the interrupt, the PC value is restored, allowing the program to continue seamlessly. A very important point worth noting! Let's review, once an interrupt is detected, it’s important to finish the current instruction before addressing it.