13.1. Instruction Fetch and 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.

This chunk discusses the initial steps in the instruction fetch process. The Program Counter (𝑃𝐶) is responsible for keeping track of the address of the next instruction to be executed. When the Program Counter is incremented (𝑃𝐶 = 𝑃𝐶 + 1), it points to the next instruction in memory. The instruction at that address is then fetched and stored in the Instruction Register (𝐼𝑅). This is a crucial step in the execution of a program as it prepares the CPU to execute the next instruction.

You can think of the Program Counter as a reader following a book. Each time they finish a page (execute an instruction), they turn the page to find out what to read next. Just like the Program Counter keeps track of which instruction to execute next, a reader keeps track of the next page they'll read.

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 says that, I am ready.

This chunk discusses the transfer of the instruction stored in the Instruction Register (𝐼𝑅) to the Program Counter (𝑃𝐶). The signal 𝑊𝐹𝑀𝐶 indicates that the memory is ready to send or receive data. Data is routed via a communication pathway called the bus. This step is essential for ensuring that the CPU can efficiently access and execute the next instruction.

Imagine sending a document from one office to another via a postal service. The document (instruction) must wait until the postal service (memory) confirms it's ready to accept or deliver it. Similarly, the instruction needs to wait until the memory is ready to complete data transfer.

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.

At this point, the memory has signaled that it’s ready to send the instruction. The next task is to transfer this instruction from the Memory Data Register (MDR) to the Instruction Register (𝐼𝑅). This step marks the completion of the instruction fetching process, allowing the CPU to prepare for executing the fetched instruction.

Think of this step as receiving a package that contains a book. Once the package is at your doorstep (memory is ready), you take the book (instruction) out of the package (MDR) and place it onto your shelf (Instruction Register), ready to read (execute).

What will you do very simple 𝑀𝐷𝑅 and 𝐼𝑅 it will serve the purpose basically, what is being done in the 4th instruction. Now, what is you instruction?

This portion emphasizes that once the instruction is loaded into the Instruction Register, the CPU can begin its execution. The execution of instructions is the processing phase where the actual command, such as loading data or performing calculations, takes place. The details of what the instruction does are critical for the CPU to understand and execute correctly.

Think of this as a chef receiving a recipe (instruction) in the kitchen. Once the recipe is read and understood, the chef can begin cooking (executing the instruction). Just like a recipe provides specific steps to create a dish, an instruction provides specific operations for the CPU to perform.

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.

In order to execute certain instructions, the CPU may need to access data stored in memory. This chunk explains that to find the data (operand), the value (address) from the instruction is loaded into the Memory Address Register (MAR). This step is essential for locating where the data resides in memory, so it can be fetched and used in operations.

It's like a librarian looking for a specific book in a library. The librarian (CPU) first needs the location of the book (address). Once they have this information, they can head to the right shelf (memory) and retrieve the book (data) needed for the task.

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.

After the Memory Address Register (MAR) has received the address, the CPU must wait for the memory to fetch the corresponding data. The fetched data is stored back in the Memory Data Register (MDR). This stage signifies the transition from accessing the instruction to actually using operands needed for instruction execution.

Consider the librarian analogy again. After finding the book on the shelf, the librarian now needs to wait until they can take the book (data) back to their desk (MDR) where they can read and utilize the information (execute the instruction).

So now you will 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.

In the final stage of the instruction execution process, the data stored in the Memory Data Register (MDR) is transferred to the appropriate register, such as 𝑅1. This marks the completion of the instruction; the CPU has now executed the instruction and stored the result. This entire process is a cycle of instruction fetch and execution, critical for the functioning of any program.

Using the chef and recipe analogy again, this final step is akin to the chef plating the finished dish (data) and serving it (storing the result in a register). The recipe is now complete, and the result is ready to be enjoyed (used by the CPU).

--