22.5.1. Fetch, Decode, and Operand Fetch

So, as I told you that for a pedagogical perspective. So, we are first going to see what is the performance or what is the functions which are performed by a CPU for executing an instruction.

So, basically as it is a Von Neumann architecture so the instruction as well as data are in the memory. So, first what happens? So, whenever you want to execute an instruction, first you have to calculate the address of the instruction and in which memory location and at what address the instruction is there.

In computing, particularly in the context of a CPU built on Von Neumann architecture, both instructions and data are stored in memory. To execute an instruction, the first step a CPU takes is to determine the address where the required instruction is stored. This is crucial because each instruction must be fetched from a specific memory location before it can be processed.

Think of it like looking for a book in a large library. Before you can read a book (execute an instruction), you must first know exactly where it's located on the shelves (calculating the address).

Secondly, the instruction will be fetched, now it will be fetched and loaded into a special register called instruction register IR.

After determining the instruction’s address, the CPU fetches the instruction from that memory location. This instruction is then loaded into a special register, known as the Instruction Register (IR). The IR temporarily holds the instruction so that it can be decoded and executed. This is an essential step in the instruction execution cycle.

Imagine a chef retrieving a recipe (the instruction) from their cookbook (memory). The chef holds this recipe in a special place (Instruction Register) while they prepare the dish.

So, now, the instructions will be decoded that what it has to do, sometimes it may be very simple like shift 2 numbers or add 2 numbers or do bitwise shifting and sometimes it can be very complicated like a matrix multiplication.

Once the instruction is loaded into the IR, it needs to be decoded. This means interpreting the instruction to understand what operation needs to be performed. Instructions can be simple, such as adding two numbers or shifting bits, or more complex, like performing matrix multiplication. Decoding is vital because the CPU must know what action to take before executing.

Think of decoding like understanding the steps of a recipe. Some recipes might require simple steps like chopping vegetables, while others might involve elaborate techniques like sous vide cooking.

Then based on that it may have it may operate on 2 numbers, it may operate on 2 operands it may operate on single operand.

After decoding the instruction, the CPU identifies the operands it will use. An operand can be a single value (like checking if a number is greater than 0) or multiple values (like adding two numbers together). This step establishes what data the CPU will need to perform the operation dictated by the instruction.

Returning to our kitchen analogy, it’s like gathering the ingredients you need for the dish. For a simple salad (single operand), you just need lettuce. For a sandwich (two operands), you might need bread and meat.

So, next you have to find out that what we you find out all those things when you decode an instruction, after decoding you have to find out whether you want one operand to be fetched, whether you want 2 operands to be fetched or whether the operand is given itself with the instruction.

Once the operands are identified, the CPU must determine how to fetch them. It may need to retrieve data from memory or may find the necessary data included in the instruction itself (like with immediate addressing). This step can involve calculating the address where the operands reside if they're not directly given, leading to efficient execution of the instruction.

Continuing with the recipe metaphor, if you need to fetch lettuce and tomatoes from the fridge (retrieving from memory) or if the recipe states you need two eggs right on the page (immediate addressing).

For example, I may have an instruction that is called immediate addressing mode where the data is given in the instruction itself.

In an immediate addressing mode, the operand is included directly within the instruction. This means there’s no need for the CPU to go to memory to retrieve the operand. It's a fast way to access the required data since it reduces the overall number of steps needed to execute the instruction.

When a recipe states you need '2 tablespoons of sugar' right in the instructions, you can quickly grab it from your cabinet. This is like immediate addressing where the value is immediately available.

So, after that is done you have to do the operation of the data and then you have to store back the data in the memory, if it is required.

Once the operands are fetched and prepared, the CPU performs the operation dictated by the instruction. This could range from addition, subtraction, logical operations, or data transfer. Depending on the operation, the result may need to be stored back in memory, completing the instruction cycle.

It's the moment where the chef actually combines all the ingredients in preparation (executing the operation) and then puts the finished dish (result) back on the table or in the fridge (memory) if it needs to be stored.

In a nutshell basically we store fetch the instruction, decode the instruction and then find out whether some operands has to be fetched from the memory and then fetch the operands do the operation and store it in a memory location.

The entire process of executing an instruction can be summarized in a series of steps: fetch the instruction from memory, decode it to determine what operation to perform, fetch any necessary operands, execute the operation, and then store the result if needed. This cycle of fetching, decoding, and executing is fundamental to how CPUs operate.

Think of the instruction cycle as cooking a meal step-by-step: gathering your ingredients, understanding the steps, preparing the meal, and finally serving or storing it.

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