12.1.3. Incrementing Program Counter

Now, basically what we are going to do in a fetch cycle. So, it is a fetching and information from memory to the processor. Now, what we must know when we are going to fetch an instruction, at least we have to know the memory location where we have the instruction.

In a fetch cycle, the main purpose is to retrieve instructions from memory for the processor to execute. Before fetching an instruction, the processor needs to know the memory address of the instruction it wants to access. This address is crucial as it guides the processor to the correct location in memory.

Think of it as looking up a book in a library. Before you can read a book, you must know its shelf number. Just like the processor needs to know the memory location to find the instruction, you need to know where to look for the book.

So, already I have mentioned that we are having a special purpose register are called program counter, 𝑃𝐶 - program counter. So, in that particular case, what will happen I am having a call register called program counter, and program counter will have the address of this particular memory location.

The Program Counter (PC) is a special register in the CPU that holds the address of the next instruction to be executed. Each time an instruction is fetched, the PC is used to indicate where the next instruction can be found in memory. By managing these addresses, the PC ensures the processor executes instructions in the correct sequence.

Imagine the PC as a bookmark in a book. It tells you where you left off so you can continue reading from the right page, ensuring you don't miss any information.

So, after fetching one instruction then what will happen we have to after completion of this particular instruction, we have to fetch the instruction from next memory location, because it is in the sequence so that’s how you can say that sometimes we have to increment the 𝑃𝐶 also.

Once an instruction is fetched, the Program Counter must be incremented to point to the next instruction in memory. This incrementing ensures that the CPU processes instructions in a sequential manner, moving from one instruction to the next without skipping any.

Think of moving through a checklist. For every task you complete, you check it off and move to the next one. Just as you wouldn't skip tasks, the CPU uses the incremented PC to ensure it processes every instruction in the right order.

So, after fetching the information, generally we update this particular program counter, we just increment it. After that whenever we are getting this particular instruction, this instruction will be loaded to instruction register.

Once the instruction is fetched from memory, it is loaded into a different register called the Instruction Register (IR). This is where the instruction resides before it is executed. The CPU uses the content of the IR to determine what actions to take next.

This process is like copying a recipe from a cookbook onto a countertop. Once the recipe is laid out, you can follow the steps without flipping through the book. The IR holds the instruction for the CPU, enabling it to perform the required operation.

Once we have the instruction in the instruction register, then processor will be knowing what operation we need to perform, so that information will be given to the control unit, and control unit is going to generate the appropriate signals.

After loading the instruction into the Instruction Register, the next step involves the CPU's Control Unit. The Control Unit interprets the instruction and generates control signals necessary for executing the operation. This coordination is crucial for the processor to know how to utilize its different components effectively.

It's similar to having a conductor lead an orchestra. The conductor interprets the music notes (instructions) and signals the musicians (components of the CPU) to play their parts at the right time, ensuring a cohesive performance.

So, this is the fetch cycle. And now, in a symbolic way, now I can say how we are going to do the fetch cycle...

The fetch cycle involves multiple steps: first, the Program Counter provides the memory address to the Memory Address Register (MAR), which initiates the read process. After reading the instruction from memory into the Memory Buffer Register (MBR), the instruction is then moved to the Instruction Register (IR), and the Program Counter is incremented to prepare for the next instruction. This cycle is systematic and ensures an organized retrieval of instructions.

Think about a teacher passing out assignments. The PC writes the list of student names (addresses) on the board. The MAR calls out each student to retrieve their assignment from the desk (memory). Once obtained, it is placed on the teacher's desk (in the IR), and the teacher checks off the name before moving to the next student.

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