12.2.7. Execution of Instructions

So, if you look at flow, in last basically two units we mainly covered about what is a single bus organization? How different components are connected? and then we have looked into that, for a given instruction what are the very broad kind of control signals required and what are the timing diagrams required involved in generating the control signals, to execute the instruction.

In the previous units, we learned about the structure of a single bus organization and how components are linked. We also explored the basic control signals needed to execute instructions. This understanding sets the foundation for diving deeper into how these signals work and interact with each other during instruction execution.

Think of it like planning a road trip. Before starting the journey, you need to understand the routes and maps (the bus organization) and what stops you need to make for gas or food (control signals). Just as these preparations help ensure a successful trip, understanding the structure and signals helps in executing instructions correctly in a computer.

Basically, in a nutshell, what we will see? Any first step of the instruction basic instruction flow that is, basically your the fetch. So, fetch basically what happens? You take instruction from the memory and basically bring it to the instruction register that is the first part of the instruction.

The first step in executing an instruction is the fetch phase. During this phase, the processor retrieves the instruction from memory and loads it into the instruction register. This is crucial because the instruction register holds the command that needs to be executed next, setting up the next steps in the processor's workflow.

Imagine you are a chef. The first step in preparing a meal (an instruction) is to get the recipe (fetch). You look it up in the cookbooks (memory) and write it down (load into instruction register) so you can refer to it while cooking.

In fact, also in this instruction you will also have to increment the program counter to point to the next instruction. So, what we do? We also instruction the ALU to perform addition, in this case it will add the value of program counter which is now in the bus, with the increment.

After fetching an instruction, the next step involves updating the program counter (PC). The PC keeps track of the location of the next instruction to be executed. By instructing the Arithmetic Logic Unit (ALU) to add a constant (usually 1 or the size of the instruction) to the current value of the PC, we prepare the CPU to fetch the subsequent instruction during the next cycle.

Think of a treasure hunt where each clue leads you to a new location (instruction). Once you find a clue (execute an instruction), you need to remember where you just were, so you can go to the next one (increment the PC).

Now you have to wait till the memory signal is ready, basically what happens, whenever we are giving a read command and you have given the data that is the PC value to the memory address register, you have to wait for some amount of time, till the memory says that I am ready.

After sending a read command to the memory, the system must wait for the memory to respond indicating that it has finished processing the request. This ensures that the instruction or data being fetched is indeed available before proceeding further in the instruction cycle.

Consider ordering food at a restaurant. After you've placed your order (sent a read command), you have to wait until the kitchen has prepared your meal and signals you that it is ready to be served.

In the third stage, what happens? Outputs the memory data register to the bus, because in second step you already know that, the memory has is now ready after the memory says that it has it is ready, that it has dumped the content of the memory value, which was pointed by the memory address register to the memory data register and whenever it say that I am ready, the second stage starts.

Once the memory signals that the instruction is ready, the instruction is transferred from the memory data register to the instruction register. This step completes the fetch phase and prepares the CPU to execute the instruction. The instruction register now contains the command that will be executed next.

Returning to our restaurant analogy, this step is when the waiter brings your food to your table (outputting content to instruction register). Now, you're ready to enjoy your meal (execute the instruction)!

Now you have to take the instruction from your memory data register to the instruction register, and try to find out what happens?

After the instruction is loaded into the instruction register, the CPU must decode it to understand what operation it needs to perform. This involves breaking down the instruction into its components, such as the operation code (opcode) and the operands, so the CPU knows how to proceed.

This is similar to reading the recipe you fetched earlier. You look at the ingredients (operands) and cooking methods (operation types) to know what steps to follow next.

In fact, if it a 2 word instruction we will add 2 and so, forth. So, basically the first step of the instruction basically loads the instruction from the memory to the instruction register, it initiates and it will also increment value of program counter to point to the next location.

Once the instruction is fully decoded, the CPU enters the execution phase, where it performs the operation specified by the instruction. Depending on the instruction type, this may involve performing calculations, transferring data to or from memory, or changing the state of registers.

Imagine completing a step in the recipe. If the recipe calls for chopping vegetables (execution), you would now grab your knife and cutting board and get to work on that task, until it's done.

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