13.1.4. Stage 4: Instruction Execution

Let us again clean it up, because we will have to revisit this figure many times. So, again I am cleaning it up. So, next is what? Now what now actually next stage is till now we have seen, that the output of this 𝑃𝐶 = 𝑃𝐶 + 1 is memory is in register 𝐼𝑅 and memory has you have given the command to read the memory. In second stage what we do? so whatever I told you about the first one is written over here, you can read it now what is it says 𝑍 𝑃𝐶 . So now, what 𝑜𝑢𝑡 𝑖𝑛 this 𝐼𝑅 has, if you look at the initial last slide then 𝐼𝑅 had the value of 𝑃𝐶 = 𝑃𝐶 + 1, but at that time it was 𝑍 .

In this stage, we prepare to execute an instruction by cleaning up and clarifying the previous information. The program counter (PC) is incremented to point to the next instruction, and this value is held in the instruction register (IR). We also recognize that the instruction is ready to be fetched based on the command to read memory.

Think of it like preparing to cook a new recipe. You gather your ingredients (the instruction) and check that you have everything ready before starting (clearing up the workspace and making sure the tools are available). The program counter is like knowing what recipe you're going to cook next.

Now, I am making as 𝑍 and 𝑃𝐶; that means, the value of 𝐼𝑅 will go to 𝑃𝐶 program counter, via the bus because 𝑍 and 𝑃𝐶 and we are waiting for 𝑊𝐹𝑀𝐶 so are waiting till the memory 𝑜𝑢𝑡 𝑖𝑛 says that, I am ready and whatever you asked in the first stage it has been dumped to the memory buffer register in fact, again revisiting.

In this step, the value now present in the IR is transferred to the program counter (PC), which is responsible for tracking the instruction currently being executed. We depend on control signals to signify when the memory is ready for the next step, establishing a communication flow between the memory and registers.

Picture this as delivering a message from one colleague (IR) to the main office (PC) where updates are recorded. The delivery relies on certainty that the office is ready to accept new information, similar to when we make sure the printer is ready before sending a document.

So, this is 𝑍 and 𝑃𝐶 is now becoming 𝑃𝐶. So, the incremented value of 𝑃𝐶 is going to this 𝑜𝑢𝑡 1 by this path. So, 𝑃𝐶 = 𝑃𝐶 + 1 or the constant, is loaded into the 𝑃𝐶 and also I am waiting for 𝑊𝐹𝑀𝐶; that means, if the signal is one; that means, what the value of the memory location, where the instruction was there is loaded into the memory data register or the memory buffer register, and now you can read the instruction to the instruction register.

Here, the incremented value of the program counter (PC) is prepared for fetching the instruction from memory. We have to wait for a signal (WFMC) which indicates that the targeted memory location is ready, ensuring that the instruction can now be read into the Instruction Register (IR) without any errors.

It's akin to waiting for your microwave to beep, indicating that your food (instruction) is heated and ready to be served (fetched into the IR). You have to wait until the signal confirms that you can safely open the door without any surprises.

So, in the second stage 𝑃𝐶, this data will be read from this memory this 𝐼𝑅, which actually nothing but 𝑃𝐶 = 𝑃𝐶 + 1 and it will be read to the 𝑃𝐶 by this bus. So, the 2 signal 𝑍 and 𝑃𝐶 accomplishes that and we are waiting till basically, our memory is ready. So, that is over here.

At this point, the data read from the memory is successfully stored in the Instruction Register (IR). This means we have accurately fetched the instruction to be executed, while still relying on the signal to indicate that everything is functioning as it should.

Imagine you're photocopying a document. You press 'copy' but you have to wait until the machine beeps to let you know it has finished copying (the instruction is stored in IR). Only when it's ready can you take the document from the output tray (execute the instruction).

After that what happens? Now the memory is ready, now what you have to do you have to load it into load it, load the value of this instruction into the instruction register, very simple you will make memory data register out and register in as simple as that just have a very quick look.

Once the memory signals readiness, we proceed to load the value of the fetched instruction into the Instruction Register (IR). This signifies the preparation for execution, where control signals are generated to coordinate the operation of the data registers.

This phase is like preparing to read a book—once it’s in your hands, you’re set to start reading (executing the instruction) after ensuring it's the correct book (the instruction is ready).

Once the instruction has been fetched, it is in the instruction register now you have to tell what I have to do. So, what was the instruction the instruction was basically, load 𝑅1, 𝑀 that is whatever is present in the memory location that is 𝐿𝑂𝐴𝐷 𝑅1, 𝑀; that means, in memory location 𝑀 whatever value is, there data is there you have to load it into 𝑅1.

Now that the instruction is in the IR, the system identifies the operation to perform based on this instruction. In this case, it specifies loading the value located at memory address M into register R1 for processing.

Think of this as receiving a shopping list (instruction) that directs you to retrieve specific items from the store (memory) and bring them back to your kitchen (register R1) for use. You know exactly what you need to fetch!

So now, what? So, you have to take this value 𝑀 and loaded it into the memory address register, because that part is going to tell where the operand exists so obviously, first instruction will be 𝐼𝑅 because the value of the instruction, which is present in the instruction register has to be given into the bus, and then your memory address register basically, we read the value from 𝑀.

To execute the load operation, the system places the value of memory address M into the Memory Address Register (MAR). This ensures that the system knows exactly where to find the data. Following this, we must engage the bus to communicate this information effectively.

It’s like marking a specific aisle number (M) on a shopping list to locate where a particular item (data) is stored in the store (memory). Directing yourself to the correct location is crucial for efficiency!

Now what there is some subtle thing over here, we are not and memory has to be in read mode of course, but there is slight thing which we have skipped over here, that is the whole instruction register will have load then 𝑅1 and 𝑀.

Here it's important to ensure that the memory is in the proper 'read' mode to retrieve the data successfully. Not all parts of the instruction in IR are necessary for this operation—only the needed components will be directed to read from memory.

Imagine a checkout counter where you can only purchase certain items. You have to ensure only the necessary items from your cart (the instruction) are presented for checkout; the rest are not needed and can slow down the process.

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