13.2. Arithmetic Operations

As I told you, we will look at different instructions. So, that was just a load instruction. So, in this case, we are going to see another arithmetic operation. So, in this case we are saying ADD R1, R2; in this case the value of R1 will be added to R2 and stored in R2.

In this section, we are transitioning from basic load operations to more complex arithmetic operations, particularly focusing on the ADD instruction. The ADD instruction takes the values from two registers, R1 and R2, adds them together, and stores the result back into R2. This operation highlights how the CPU utilizes its registers to perform calculations efficiently.

You can think of this operation like two friends (R1 and R2) pooling their money together. If Friend R1 has $10 and Friend R2 has$15, when they combine their money (ADD operation), the total amount they now have is $25, which reflects the new balance in R2.

Now what is going to happen? ... you will say, Z = R1; that is, now the value of IR has to be fed to register R1. So, what you will do you will say, Z and R.

The process of executing the ADD instruction can be broken down into specific stages. Initially, the program counter (PC) indicates where the instruction is. In the first stage, the instruction is fetched and decoded, primarily using the instruction register (IR) and the memory address register (MAR). Subsequently, the operands from both R1 and R2 are prepared for addition, leading to a final step where the result is stored back into R1. Each stage is crucial for ensuring the correct execution of the instruction.

Imagine a cook (CPU) taking a recipe (ADD instruction). First, they find the recipe in a cookbook (fetching), then gather the ingredients (R1 and R2), mix them together (execution of addition), and finally place the mixed ingredients in a bowl (storing the result in R1). Each step is essential to create a successful dish.

First three stages basically correspond to fetch, whether it is add R1, R2, ... the first 3 stage, basically this one this one and this one will remain constant for everything.

The control signals govern how the CPU operates during the execution of the ADD instruction. In the initial three stages (fetching), there are consistent operations that occur, regardless of the type of instruction being processed. The CPU must accurately synchronize these operations to ensure the instruction is correctly executed. This means controlling when to read from memory, when to load registers, and when to perform arithmetic operations.

Think of a train system. The initial signals for a train to start moving (fetching state) are the same whether it is an express train or a local train. The control signals function similarly in a CPU, ensuring every instruction has a standard way to be processed before any specific actions related to that instruction occur.

In 6 micro instructions and the corresponding control signals, what we have done we have shown how a complete instruction is fetched, decoded and executed.

This section summarizes the entire process of executing the ADD operation in just six micro instructions. The steps encapsulate fetching an instruction from memory, decoding it, and executing the addition by relying on control signals to manage the flow and timing of operations. Understanding this concise version of operations helps in grasping more complex sequences in computer architecture.

Consider it like a film set where every scene is shot in specific takes. Each take is like a micro instruction, and the director ensures each take flows into the next. By the end of the shooting (the operation), the film is complete, illustrating how a sequence of actions builds up to achieve a larger goal.

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