12.6.4. Steps in Fetch Cycle

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.

A fetch cycle is the process through which a computer retrieves instructions from memory to execute them. It starts with the processor needing to know where an instruction is located in memory. This location is identified using a special register called the Program Counter (PC). The PC holds the address of the next instruction that the CPU is supposed to execute.

Think of the fetch cycle like a librarian who needs to retrieve a book from a storage room. The librarian knows the section and shelf number (this is like the address in the PC) where the book is located. They go to that location, get the book (fetching the instruction), and bring it back to the front desk to read (process).

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, we note that address is say 50, then program counter will have the value fifty over here.

The Program Counter (PC) keeps track of the memory addresses of instructions. For example, if the current instruction is stored at memory address 50, the PC will contain the value 50. This way, the CPU knows exactly where to look for the next instruction to execute. After fetching that instruction, the PC is incremented to point to the next instruction in sequence automatically.

Imagine a train traveling along a railway track. The train moves from one station to the next, and each station is like a memory location with a specific number (address). The conductor keeps a note of the current station (the current address) so they know where to go next. When they depart from a station, they update their note with the next station's number.

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.

Once the CPU fetches an instruction, it needs to prepare for the next instruction that follows. This requires updating the PC to point to the next instruction's address. By incrementing the PC, the CPU ensures that it will fetch instructions in the correct order, allowing programs to run smoothly as each instruction is executed sequentially.

Consider a recipe book. As you complete one recipe (fetch and execute an instruction), you move on to the next recipe in the book. To do so, you turn to the next page (incrementing the PC) to ensure you follow the recipes in the correct order.

After that whenever we are getting this particular instruction, this instruction will be loaded to instruction register.

After fetching an instruction from memory, the CPU needs to temporarily hold that instruction so it can process it. This is done by loading the fetched instruction into the Instruction Register (IR). The IR acts as a storage area where the instruction is held until the CPU is ready to interpret and execute it.

Think of the Instruction Register like a work desk where you place important documents. Once you receive a document (the fetched instruction), you place it on your desk (the IR) for review before taking any further actions on it.

Once we have the instruction in the instruction register, then the 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.

The control unit is a vital component of the CPU that directs operations. After placing the instruction in the Instruction Register, the control unit determines what action needs to be taken based on that instruction. It sends signals to various parts of the computer to carry out the required operations, ensuring that everything happens in the correct sequence.

Imagine a traffic manager at an intersection. Once they see a set of instructions on their monitor (the instruction), they issue commands (signals) to the traffic lights and signs to either allow cars to go or stop, ensuring the flow of traffic is managed smoothly.

So, after getting the instruction, we know that we have to perform say addition operation, then processor is going to perform the required actions; that means, control unit is going to generate the correct signal at correct time, so that operation can be performed in correct way.

The fetch cycle ends when the instruction has been interpreted and the control unit has generated the necessary signals for execution. The CPU can then carry out the operation dictated by the instruction—whether it's an addition, subtraction, or other action—using the Arithmetic Logic Unit (ALU) or related components.

Think of the fetch cycle as a project kickoff meeting. After gathering all necessary information (fetching the instruction), the project manager (control unit) determines the steps to complete the project (perform actions) and delegates tasks to team members (the rest of the CPU).

First that address we have to put it into the MAR. After that this address will go to this particular memory unit through this system bus ok then we are going to read the information, so that’s why we are saying that first I am going to place the information from PC what we have in PC.

During the fetch cycle, the memory address where the instruction is located is placed into the Memory Address Register (MAR). This address is then sent to the memory through what is known as the system bus. The system bus functions as a communication pathway that allows data transfer between the CPU and memory. Once the instruction's address is loaded into the MAR, the CPU can read the required information from that memory location.

Envision a delivery system where an order (the instruction address) is placed into a delivery vehicle (MAR). The vehicle then travels to the warehouse (memory) using the designated roads (system bus) to pick up the order (fetching the instruction).

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