15.3.2. Control Signals and Instruction Execution

Now, we will go to the another mode, which is more complex in nature which is the indirect mode. By indirect mode already we mean that whenever it’s the indirect mode of 𝑀; that means, we say this is the memory here is 𝑀, at the address of 𝑀 here there may be some addresses called x is some content over here, then again you have to look at the content in x and basically this is your operand, this is what is the idea we all know about it.

In the indirect addressing mode, when referring to a memory location (let's call it 'M'), it doesn't point directly to a value. Instead, it points to an address (let's call it 'x'). To retrieve the operand, the system first fetches the content at M (which is x) and then retrieves the actual value located at x. Hence, the operand is not directly fetched from M but rather from another address found inside M.

Imagine looking up a friend's phone number in a contact book. The contact book's entry (let's say the page number) points to another notebook where the actual number is stored. In this analogy, the contact book is like M, the page number is like x, and the actual phone number is the operand you want to retrieve.

So, if you now look at basically your first 3 stages. So, first 3 stages; as I already were discussing like 𝑃𝐶 𝑜𝑢𝑡 𝑀𝐴𝑅 and this is control stage basically are only for fetching the instruction.

The execution of an instruction in indirect addressing consists of several stages. In the initial stages, the Process Control (PC) is used to output the address, which is then stored in the Memory Address Register (MAR). This primarily involves the fetching of the instruction from memory, indicating that these three stages set the groundwork necessary for the actual execution of the instruction.

Think of this like preparing to cook a recipe. First, you gather all your ingredients (fetching the instruction). Understanding where each ingredient is located is crucial before you can start cooking (accessing the actual operand).

In the fourth stage what I am going to do I am going to take the value of 𝑀 and I am going to feed it into the memory address register so that you can read the value of memory location 𝑀 that is x in this example.

During the fourth stage of the execution process, the value found at memory address M is input into the Memory Address Register (MAR). This step is crucial as it allows the system to access memory location x, which contains the operand. Essentially, it enables the system to prepare itself to read the actual value that will be used in the computation.

Continuing from our cooking analogy, this stage is like actually retrieving all your ingredients from storage based on the list you prepared earlier. It's not enough to have a list; you must also gather them to start cooking.

Of course, in stage 5 we have to wait till everything is ready.

In this stage, a brief delay occurs as the system waits for the data from the memory location to be ready to be read. This involves not only waiting for the data but also for the system to ensure that all previous operations have completed successfully before proceeding. Timing and control signals play a vital role here to synchronize different parts of the computing system.

This step can be compared to waiting for water to boil before adding pasta to a pot. You can't just throw the pasta in; you have to ensure the water is boiling to get perfect results.

So, if you quickly look at what it happens in the bus it will be more clear. So, look at the bus. So, many times you have to refer to this diagram.

Once the data is confirmed to be ready, it is transferred from the memory location (x) to the Memory Data Register (MDR). From here, the operand can now be loaded into the intended register (e.g., R). This is a key step in ensuring the operand is now in a usable form for further processing.

This is similar to dumping the cooked pasta into a bowl after boiling it. The pasta (operand) is now prepared and ready to be served or used in your dish (further processing).

So, in the indirect mode we can get the values. So, now, again another mode we are taking which is called registering indirect...

After discussing indirect addressing mode, the next logical topic is register indirect addressing. This method is slightly different as it uses a register as the pointer to the memory location rather than a memory address directly. The contents of the register will point to the location where the data can be found, simplifying the access process somewhat.

Imagine if instead of looking up your friend's phone number in a book, you had it stored in your phone's notes app. When you want to call them, you simply access your notes app directly. This is more efficient than flipping through pages.

In this case if you can see. So, your indirect mode if it’s the memory indirect mode you require 8 stages to do it, but if in your register indirect basically, you can solve it in 6, basically 6 steps of time.

The key difference between memory indirect addressing and register indirect addressing is the number of operations (or stages) it takes to complete the instruction execution. While memory indirect may take up to 8 stages to complete, register indirect can execute in as little as 6 stages because it simplifies the fetching process and minimizes access times.

Going back to our cooking example, think of it as using a microwave (register indirect) to heat food versus using an oven (memory indirect). The microwave cooks food faster because it takes less time to reach readiness.

So, this unit basically gave you a spectrum of different type of addressing modes...

Throughout this section, we have covered various addressing modes, including immediate, direct, indirect, and register indirect modes. Each addressing mode has a different sequence of control signals and execution steps that dictate how instructions are processed. Understanding these differences is critical for designing and utilizing effective instruction sets within a CPU.

If we think about different ways of ordering food - dine-in, takeout, delivery, or drive-thru - each method serves the same purpose but varies in efficiency and process. Knowing the fastest or most effective method for a context helps improve execution, akin to understanding addressing modes in processors.

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