15.3. Different Addressing Modes

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.

The indirect addressing mode is a method where the operand's address is not contained directly within the instruction itself. Instead, the instruction specifies a memory location (let's call it M), which holds another memory address (let's refer to this new address as x). To find the actual operand, the CPU first retrieves the address x from M, then goes to memory location x to get the operand needed for further processing.

Think of it like looking for a book in a library. The first step is to check the catalog (M), which gives you the location of another shelf (x). You then go to that shelf to find the specific book (the operand) you need for your research.

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 initial stages of executing an instruction in the indirect mode involve three main steps: 1) The Program Counter (PC) outputs the address of the next instruction, 2) This address is stored in the Memory Address Register (MAR), and 3) A control signal is issued to begin fetching the instruction from memory. These stages are crucial because they ensure the CPU knows which instruction to execute next.

Imagine a waiter in a restaurant taking down customer orders. The waiter (PC) notes down the order (address of the instruction) on a notepad (MAR) and then goes to the kitchen to get that specific dish (fetching the instruction).

Next one is what I have to do. So, this is the instruction load R into memory from indirect memory location.

Once we have the instruction loaded, it indicates that we need to load data into a register (R) from the indirect memory location associated with M. In essence, the CPU will access memory, retrieve the address indicated by M, and then use that address to fetch the actual value that needs to be loaded into the register.

Continuing with the book example, once the waiter knows the order includes a specific dish, he doesn't just put any random food on the plate (loading R). Instead, he goes back to the kitchen to fetch that specific dish (the value from the indirect location) which was already prepared.

So, now the after executing the third instruction the third control step register instruction register IR is having the value of LOAD R, M. So, 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.

When the instruction to load data into the register is ready, the Instruction Register (IR) reflects this command, which tells the CPU to take the stored address in M and prepare to fetch the data from that memory location. This value of M is loaded into the Memory Address Register (MAR), setting the stage for the actual memory access needed to retrieve the operand.

This is similar to the waiter identifying the specific dish on the menu (LOAD R, M) and then going to the exact kitchen area (MAR) where that dish is stored (loading the address to fetch the operand).

So, in stage 5 we have to wait till everything is ready. Once it is ready basically we know that the value of memory location M that is x in the example is now loaded into the memory data register.

At this stage, after waiting for the memory fetch cycle to complete, the CPU ensures the value from the memory location indicated by M (x) is now available in the Memory Data Register (MDR). This is crucial as it prepares the operand to be moved to the designated register.

Think of the waiter waiting for the chef to finish preparing the dish (fetching x). Once the dish is ready, it is placed on a serving tray (MDR) so the waiter can deliver it to the customer (load into R).

After you have to wait for some amount of time and then in this case now exactly you are going to have the operand which is present in the memory location x that the content we are assuming l for the time being will be taken from MDR it will be dump to register R.

Once the operand is prepared and available in the Memory Data Register, the next step is to load this data into the designated register (R). This is done by transferring the value from the MDR into R, completing the process of loading the operand through the indirect addressing method.

Finally, the waiter delivers the specific dish (l) from the tray (MDR) to the customer’s table (R). This completes the cycle of retrieving and delivering the requested item.

So, in the indirect mode we can get the values. So, now, again another mode we are taking which is called registering indirect in this case it was a memory indirect it is a register indirect.

The discussion shifts to another addressing mode known as register indirect mode. Unlike indirect mode, where the address is stored in memory, in register indirect mode, the address of the operand is stored directly within a CPU register. This typically allows for faster access as there is one less memory fetch involved.

If we return to our restaurant example, instead of fetching a dish from memory (the kitchen), the waiter now simply retrieves it from a platter (the register) already at the table. It’s quicker and more efficient.

Depending on different type of addressing modes the sequence of control or the number of steps will all differ.

Finally, the section wraps up by summarizing that different addressing modes, such as immediate, direct, indirect, and register indirect, each have their unique processes and steps involved. Understanding these differences is vital for optimizing instruction execution and understanding control signals within a CPU.

It's like understanding various routes to reach a destination: some take longer and have more steps (indirect mode), while others are direct and quicker (register indirect mode). Knowing your options helps you to plan your journey effectively.

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