13.2.1. ADD Operation: Register Mode

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 the beginning of the ADD operation, the Program Counter (PC) is incremented by 1, which prepares the CPU for the next instruction. This process keeps track of where the next instruction is located in memory and is critical for maintaining the correct order of operations. The incremented value of PC is then loaded into the Instruction Register (IR), which temporarily holds the instruction that will be executed next.

Think of the PC as a librarian who always needs to know the next book to fetch from the shelf. When the librarian finds a book (instruction), they note down the next book's location so they can read it after the current one.

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 the second stage, the value stored in the Instruction Register (IR) is analyzed. This value corresponds to the instruction that needs to be executed next. The current instruction in IR is derived from the memory location indicated by the incremented PC. The system waits for the memory to confirm it is ready to read before moving forward.

Imagine the librarian has fetched the specified book. However, before reading, the librarian waits for the book to be processed and verified to ensure that it is the correct one. Only once confirmed does the librarian start reading from that book.

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.

Once the memory confirms readiness, the instruction is transferred from the Memory Data Register (MDR) to the Instruction Register (IR). This step is essential as it facilitates the execution of the instruction. It is a straightforward operation, involving control signals that direct the transfer of the instruction.

Consider the librarian who successfully verifies a book's availability and then places it on their reading desk. This step is necessary before the librarian can begin to delve into the book’s contents.

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.

At this stage, the instruction to be executed is decoded. In this scenario, the instruction is to load data from memory location M into register R1. The Control Unit interprets the instruction and prepares to execute it by determining the necessary operations and data involved.

Think of decoding as the librarian understanding the book's title and determining what needs to be done - whether to simply read it, summarize it, or perform other actions related to it.

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.

Next, the value of the memory location where the data resides is transferred to the Memory Address Register (MAR). This operation is crucial since it directs the system to the specific location from where data will be fetched. The IR's content is utilized in the bus system to facilitate this transfer.

Imagine the librarian marking the exact shelf where the next book is located, ensuring everyone knows where to find the needed information.

Then at 5th stage, we wait till the memory says that, I am done with it. So, once the memory says that I am done with it; that means, the data this 𝑀 data is now loaded in to the memory data register.

In the fifth stage, the system pauses to let the memory complete the read operation. Once the data has been successfully loaded into the Memory Data Register (MDR), the system can continue to process the instruction. This confirmation step prevents data corruption and ensures that the right information is fetched.

The librarian waits until the book is fully checked out and processed before they begin reading it. This checking ensures that the book is ready and free of mistakes or problems.

So now, what you will have to do you have to just dump the memory data register value that is 𝑀𝐷𝑅 to 𝑅 that is 𝑅1. So, in 6 stages I complete the instruction.

Finally, the last step involves transferring the value from the Memory Data Register (MDR) into register R1. This operation completes the instruction cycle, and R1 now holds the data that was originally in memory location M, ready for use in subsequent operations.

The librarian, after all the preparations, finally reads from the book, extracting knowledge and insights directly into their notebook for future reference.

--