15.4.1. Objectives Satisfied

Now, we will go to another mode, which is more complex in nature—indirect mode. By indirect mode, we mean that whenever it’s the indirect mode of M; this means, we say this is the memory here is M, at the address of M there may be some addresses called x with some content over here...

In indirect mode, the operand is not given directly in the instruction. Instead, the instruction points to a memory location that contains the address of the actual operand. For example, if M is an address in memory, the content at that address (let's call it x) must be accessed to get to the real operand. The process begins with loading the instruction, which involves fetching the first stages: PC, MAR, and Control Stage for instruction fetching.

Consider a treasure map. The map leads you to a location where another clue is placed (x), which then guides you to the treasure (the actual operand). You cannot find the treasure until you first go to the map's specified location.

So, the next stage is IR memory register in and read if we are fetching. After executing the third instruction, the instruction register IR has the value of LOAD R, M. In the fourth stage, I am going to take the value of M and feed it into the memory address register to read the value of memory location M...

After the instruction is fetched into the Instruction Register (IR), the CPU takes the value stored at the memory location M and places it into the Memory Address Register (MAR). This action allows the CPU to access the content stored at memory location x. Therefore, IR becomes critical as it holds the instruction needed to know which register to load and where to find the data.

Imagine if the map (IR) not only points to a location but also tells you the exact combination to open a safe (MAR) at that location to get the treasure inside (the operand). Without this combination, you can't access the treasure.

Of course, in stage 5 we have to wait till everything is ready. So, once it is ready we know that the value of memory location M (x) is now loaded into the memory data register...

After the MAR is populated with the address M, the system needs to wait for the Memory Function Complete (MFC) signal. This signal indicates that data from the specified memory location has been fetched and is now available in the Memory Data Register (MDR). The indirect nature of this mode means extra steps are needed to successfully load the value into the designated register.

Think of this step as ordering a pizza. You make the order (send the address to the MAR), but you must wait for the delivery team to arrive with your pizza (the MFC signal). Only when the pizza is delivered (data is available in MDR), can you enjoy your meal (use the operand).

Now, we are taking the value of x, which is an actual address to load the register R that is assumed to contain the operand l. This is done after the data is passed through the MAR and MDR.

After the instruction is executed and the correct data is fetched, the final step is to transfer this operand into the destination register (R). This operation concludes the indirect addressing sequence, where the operands retrieved from memory may now be manipulated or used in arithmetic operations.

Continuing with the pizza analogy, once your pizza is delivered (the operand loaded into MDR), you can finally eat it (use the operand in computations). The whole process illustrates how intricate addressing schemes can be, just like the effort put into ensuring perfect pizza delivery.

In this case, we see how indirect addressing takes more steps compared to register indirect mode, where the content of a register points directly to the data's memory location...

Indirect addressing involves multiple stages, often requiring at least two accesses to memory. In contrast, register indirect mode reduces the number of steps as the register directly points to the memory location where the data resides. As a result, this mode is generally faster and more efficient, streamlining the data access process.

Imagine again needing to go to a second location to collect something (indirect mode) versus having the item right next to you (register indirect mode). The latter saves time and effort, making it a preferred method for data handling in computing.

So, this unit basically gave you a spectrum of different types of addressing modes and functionalities like load and add. If you can answer the simple question of designing and controlling these instructions, this course's objectives will be satisfied.

By the end of this unit, the learner should be capable of demonstrating their understanding of various addressing modes and how they integrate with internal CPU structures. The ability to articulate how programming instructions execute through control steps reflects a solid grasp of CPU operation.

Think of it like learning to navigate various routes in a city. Once you understand different paths (addressing modes) and how they connect to your destination (CPU operations), you’ll be able to efficiently travel through the city (efficiently execute instructions).

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