12.2.1. Control Unit Involvement

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. Now, where I am going to get this particular information. 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 fetch cycle is the initial phase in the process of executing a program. During this cycle, the processor retrieves instructions from memory. To properly fetch an instruction, the processor must know the specific memory location where the instruction is stored. This is achieved through the Program Counter (PC), a special purpose register that holds the address of the instruction to be fetched. Essentially, the PC acts as a pointer to the next instruction that the processor will execute.

Think of the Program Counter as the page number in a book. Just as you would look at the page number to find out where the next part of the story is, the Program Counter tells the processor which 'page' in memory to look at next.

Thus say that it is a number, address you just think as a number say this is the zeroth address first location, second location like that we are having total n - one location. So, 0 to n - 1 total n location. So, we note that address is say 50, then program counter will have the value fifty over here that means, program counter is going to give us the information from where we need to fetch the instruction.

Memory is organized in sequential locations with each location assigned a unique address, starting from 0. For instance, if the address of the instruction needed is 50, the Program Counter will hold the value 50 to indicate that the processor should fetch the instruction from that specific location. This way, the CPU knows precisely where to go to retrieve the right instruction needed for execution.

Imagine you are at a library, looking for a specific book. The book's location is known by a specific shelf number (the address), and the Program Counter serves as your guide that tells you which shelf to go to next for the book (the instruction) you need.

Now, 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. First it is going to have the address of an instruction processor fetch this information from memory to the processor, and along with that it will increment 𝑃𝐶 because after completion of this particular instruction, what will happen we have to go to fetch of the next instruction, and next instruction will be available in the next memory location.

After fetching an instruction, the processor needs to prepare to fetch the next instruction. This is accomplished by incrementing the value of the Program Counter, so it points to the next memory address sequentially. If the PC starts at 50 for the first instruction, after executing the instruction, it increments to 51, indicating to the CPU to fetch the next instruction from this new address.

Consider reading a series of instructions from a recipe book. After you finish a step (or instruction), you inevitably turn the page to the next step. Incrementing the Program Counter is like flipping the page in the book to proceed to the next instruction.

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. We have said that we are having a special register called instruction register.

Once the instruction is fetched from memory, it is then stored in another special purpose register called the Instruction Register (IR). This registers the instruction that is ready to be executed by the processor. The IR holds this instruction until it is processed, ensuring that the instruction is effectively 'ready' for execution.

Imagine that you have a stack of flashcards with different tasks written on them. Once you pick a card (fetch the instruction), you set it down on your desk to read and work on it. The Instruction Register acts like your desk where you keep the current task card in sight until you complete it.

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.

With the instruction now in the Instruction Register, the CPU's control unit interprets what needs to be done next and generates the necessary control signals. These signals control the various components of the processor, orchestrating the execution of the instruction, which might involve arithmetic operations, data movement, or other tasks.

Think of the control unit as a conductor of an orchestra. The conductor cues different musicians (components) in the orchestra (processor) to play their parts at the right time, ensuring that the performance (execution of the instruction) goes smoothly.

And now, in a symbolic way, now I can say how we are going to do the fetch cycle. Now, already I have talked about 𝑃𝐶 that program counter that we are having. And we know 𝐼𝑅 instruction register after fetching it we are going to put it into the 𝐼𝑅.

In summary, the fetch cycle is vital for instruction execution in a CPU. The cycle involves the Program Counter pointing to the right memory location, fetching the instruction into the Instruction Register, and then using the Control Unit to execute that instruction. Understanding this cycle helps in grasping how a CPU processes information step-by-step.

Visualize making a call based on a contact's number stored in your phone book. First, you look up the name (PC), then you dial the number (fetching), and finally, you communicate (execute). This stepwise approach explains how a CPU performs its fetching and executing tasks.

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